chapter 8-3 nutrient relations: nitrogen - Bryophyte Ecology ...

CHAPTER 8-3 

NUTRIENT RELATIONS: NITROGEN 

Figure 1. Physcomitrella patens growing on previously flooded soil. Note the nitrogen-fixing blue-green bacterium, Nostoc, at the 

arrow. Photo by Michael Lüth. 

N Forms 

Nitrogen is available in many forms. The most 

abundant of these, N2 gas, cannot be used by plants or 

animals and must be converted by Cyanobacteria or 

bacteria before plants can use it. Animals can only obtain 

it by eating other organisms that have already placed the N 

into amino acids. Other forms of N that plants can absorb 

include ammonium (NH4 + ), nitrite (NO2 - ), nitrate (NO3 - 

), and organic forms such as amino acids and urea. As we 

shall soon see, not all plants have the same ability to use 

these forms and some are toxic to most taxa. 

Nitrate and Ammonium 

Plants, including bryophytes, can take in and use both 

NO3 - (nitrate) and NH4 + (ammonium). The form of 

nitrogen needed by bryophytes varies with species and 

habitat. Aquatic higher plants use nitrogen in three 

inorganic forms: NO2 - (nitrite) (Schwoerbel & Tillmanns 

1964, 1977), NO3 - , NH4 + (Schwoerbel & Tillmanns 1972; 

Rudolph & Voigt 1986). Bryophytes usually absorb NH4 + 

more easily than they absorb NO3 - (Schwoerbel & 

Tillmanns 1974; Simola 1975; Miyazaki & Satake 1985; 

Schuurkes et al. 1986). Vanderpoorten (2000) reported 

that NH4 + N is one of the best factors to explain differences 

in aquatic Amblystegium distributions in river systems. 

Frahm (1975) found that the brook moss Fontinalis 

antipyretica var. gigantea had a low tolerance for NH4 + , 

but Schwoerbel and Tillmanns (1974, 1977) found 

conflicting evidence showing that this species uses NO3 - 

and NH4 + , with NH4 + being taken up first if provided 

together with NO3 - . In fact, it is unable to uptake NO3 - in 

the dark (Schwoerbel & Tillmanns 1974). It is possible 

that various strains have developed within species that have 

different tolerance levels for some of their nutrients. The 

report of amino acid utilization in the aquatic Java moss 

(Taxiphyllum barbieri; Alghamdi 2003) seems unusual 

among the aquatic mosses and may somehow relate to its 

ability to live in aquaria and tropical streams where most 

15

16 Chapter 8-3: Nutrient Relations: Nitrogen 

other bryophytes seem unable to survive. Could this in 

some way relate to the higher annual temperatures of its 

tropical habitat? 

Assuming that bryophytes operate as do tracheophytes, 

NO3 - , once in the plant, is converted to NH4 + . In leaves, 

the intermediate product, NO2 - , is reduced by nitrite 

reductase (an enzyme that facilitates the addition of 

hydrogen and loss of oxygen from NO2 - during the 

photosynthetic electron transport process). No intermediate 

product is released and the final product is NH4 + . Since 

photosynthesis provides the NADH (nicotinamide adenine 

dinucleotide, the active coenzyme form of vitamin B3) and 

ferredoxin needed for conversion of nitrogen oxides to 

NH4 + , the conversion process is enhanced by the same 

things that enhance photosynthesis – high light and warm 

temperatures (Salisbury & Ross 1978). Thus, more 

ammonium is produced. 

nitrite reductase 

NO3 - → NO2 - → NH4 + 

nitrate nitrite ammonium 

Brown (1982) suggested that the pH or alkalinity 

affects availability of N for plants, with NO3 - being more 

available in neutral or alkaline soils and NH4 + in acidic 

soils and water. But NH4 + is usually toxic to plants in any 

appreciable quantity. Sironval (1947) found that NH4 + ions 

caused degeneration of the caulonema of Funaria 

hygrometrica and Southorn (1977) found they caused 

morphological abnormalities in the same species. Killian 

(1923) likewise found morphological abnormalities in the 

leafy liverwort Scapania. On the other hand, Burkholder 

(1959) found that cultured bryophytes did equally well on 

both NO3 - and NH4 + salts. An interesting consequence of 

pH differences was suggested by Machlis (1962). In 

Sphaerocarpos texanus, male plants are smaller than 

females in the field. Machlis attributed this to the ability of 

male plants to absorb NH4 + ions more readily than females, 

causing them to have a lower pH, which could suppress 

growth. He supported this suggestion by growing the 

plants on potassium, which caused no pH change, and 

likewise no reduction in the size of male plants. 

In a study designed to determine the effects of various 

forms of N on bryophyte function, Alghamdi (2003) 

studied the popular, fast-growing aquarium moss 

Taxiphyllum barbieri (Java moss). He found that the 

benefit to the moss depends on what parameter you 

measure (Figure 2). For example, dry biomass increase 

was greatest in high NO3 - concentrations (30 mg/L N), 

whereas the greatest increase in length occurred in high 

NH4 + concentrations (30 mg/L N). This difference resulted 

in the least biomass increase per stem length in high NH4 + 

concentrations, despite the relatively high increase in 

length in that treatment. The overall appearance of the 

mosses in high NH4 + , then, was to appear long and thin 

compared to those in other treatments, but not dissimilar to 

the plants in the control (standard nutrient solution but with 

no N source). Based on the lower growth in the NH4NO3 

media, Alghamdi reasoned that in the presence of NH4 + , the 

NO3 - became unusable because of the inhibition of nitrate 

reductase by NH4 + . At the same time, the lower 

concentration of NH4 + (15 mg/L N) in combination 

compared to NH4 + alone (30 mg/L N) reduced the growth. 

This relationship was consistent with much greater growth 

at 30 mg/L N than at 10 mg/L N as NH4 + (Figure 2). 

Figure 2. Effects of various forms of inorganic N (control = 

no N, NO2 - = nitrite, NO3 - = nitrate, NH4 + = ammonium) on 

growth in length and biomass of Taxiphyllum barbieri. Box mean 

(dot) and median (horizontal line); bottom of box is first quartile 

and top is third quartile. Whiskers represent lowest and highest 

observations still inside region defined by lower limit Q1 - 1.5 

(Q3 - Q1) and upper limit Q3 + 1.5 (Q3 - Q1); * represents 

outliers that extend beyond the whiskers; n = 15 sets of 3 stems. 

Means with same letters are not significantly different from each 

other (DNMRT, α = 0.05). Based on Alghamdi (2003). 

NO2 - caused only modest improvements in biomass 

and length over the N-free controls (Figure 2), but caused 

considerable increase in chlorophyll a (Alghamdi 2003; 

Figure 3). The chlorophyll a:b ratio was highest in the high 

NO3 - treatment, due to mosses in that treatment having the 

least chlorophyll b per biomass of moss, a concentration 

even lower than that of the controls (Figure 3). In fact, the 

effects of inorganic N form on chlorophyll b resulted in 

either no improvement over N-free controls, or depressed 

levels of chlorophyll b. Chlorophyll a, on the other hand, 

was higher in nearly all the nitrogen treatments than in the 

controls. Baxter et al. (1992) found a similar but slight 

decrease in total chlorophyll concentration in Sphagnum 

cuspidatum, typically a submersed species, with increasing 

levels of NH4 + , but in Alghamdi's experiments,

Taxiphyllum barbieri actually had a total chlorophyll 

increase, although not statistically significant, with an 

increase from 1 to 30 mg/L N as NH4 + (Figure 3). 

Protein concentrations in Taxiphyllum barbieri showed 

a very different picture from other measurements, with 

little difference among treatments except at 10 and 30 mg/L 

NO3 - (Figure 3; Alghamdi 2003). In Sphagnum cuspidatum 

the addition of NH4 + (as NH4Cl) generally caused an 

increase in amino acids, at least within the first 15 days, in 

both locations studied, with arginine increasing the most. at 

the unpolluted site and actually decreasing at the NH4 + - 

polluted site (Baxter et al. 1992). The latter study suggests 

that Sphagnum cuspidatum may acclimate to a higher level 

of NH4 + in a way that it eventually requires higher levels 

than populations not continuously exposed to such high 

levels. Clearly the uses of the various forms of N in 

bryophytes are complex and one cannot give a simple 

answer as to which form is best. 

Figure 3. Effects of various forms of inorganic N (control = 

no N, NO2 - = nitrite, NO 3 - = nitrate, NH4 + = ammonium) on 

chlorophyll a and protein concentrations of Taxiphyllum barbieri. 

Notation as in Figure 2; n = 15 sets of 3 stems. Based on 

Alghamdi (2003). 

In Sphagnum, differences exist among the species. 

Sphagnum flexuosum is apparently unable to utilize NO3 - 

(Schuurkes et al. 1986), and Touffet (1971) found that 

NO3 - actually reduced the growth of Sphagnum and was 

less effectively utilized than NH4 + when it was the only N 

resource. Nevertheless, in many Sphagnum species nitrate 

reductase, an inducible enzyme (Deising 1987), permits use 

of NO3 - . High levels of NH4 + inhibit nitrate reductase, and 

hence reduce growth, by inhibiting NO3 - uptake (Rudolph 

et al. 1987). Rudolph and Voigt (1986) demonstrated that 

322 µM was a favorable concentration of NO3 - in 

Sphagnum magellanicum, whereas at 225 µM NH4 + the 

chlorophyll content decreased. At 600 µM NH4 + , the 

nitrate reductase activity was reduced up to 20%. These 

factors most likely contribute to the location of mosses in 

their particular habitats by defining their nutrient niches. 

Chapter 8-3: Nutrient Relations: Nitrogen 17 

Organic Nitrogen 

Most agricultural plants seem to absorb their nitrogen 

in the form of NH4 + or NO3 - , but it seems that bryophytes 

have more options. Sphagnum is able to use urea (along 

with phosphate) in the Alaskan wetlands, resulting in an 

increase in biomass compared to controls (Sanville 1988). 

In nature, amino acids likewise can be abundant, present as 

breakdown products of plant and animal wastes and litter. 

Yet few culture studies or field tracer studies have included 

these organic forms. Is it possible that bryophytes can use 

this organic N as their primary source? If so, they may 

benefit from organic leachates in early stages of litter 

decomposition of a soil environment. 

In bogs and poor fens, NH4 + seems to be the 

predominant form of available N (Rosswall & Granhall 

1980). NO3 - is often lost through denitrification (Hemond 

1983). Not surprisingly, some studies show that Sphagnum 

seems to require most of its inorganic N as NH4 + 

(Schuurkes et al. 1986). But Simola (1975, 1979) showed 

that Sphagnum nemoreum and S. fimbriatum both could use 

amino acids. Sphagnum nemoreum grows almost as well 

on the amino acids arginine and alanine as on NH4 + salts, 

but it is unable to use leucine, lysine, or methionine 

(Simola 1975). More recently, McKane (1993), using 

tracer studies, found that for Sphagnum, Aulacomnium 

palustre, and Hylocomium splendens, the amino acid 

glycine was actually the preferred form of nitrogen over 

NH4 + and NO3 - . 

It appears that in Arctic ecosystems, organic nitrogen 

(amino acids, especially glycine) may actually be the 

preferred source of N for some bryophytes, including 

Sphagnum rubellum (Kielland 1997). Even amino acids 

with higher molecular weights, such as aspartate and 

glutamate, can be absorbed at higher rates than inorganic 

N. Kielland suggested that the high capacity for absorbing 

amino acids might be an adaptation to the low inorganic N 

availability in the Arctic. 

Even floodplain bryophytes can use amino acids. 

Schuler et al. (1955) found that in culture the thallose 

liverwort Sphaerocarpos texanus grew more typically on a 

mix of amino acids than it did on NH4NO3 alone. 

Brown (1982) suggested that in low N environments 

the mosses may be able to move organic molecules 

containing N from dying and dead cells to the growing 

apex. It is very likely that these molecules would be amino 

acids, as well as dipeptides and other organic compounds. 

Burkholder (1959) examined the effects of 20 amino 

acids (0.0001 M AA to 0.0016 M AA with and without the 

addition of NH4NO3 on the color and growth of Atrichum 

undulatum. Glycine, L-cystine, L-cysteine, and Ltyrosine 

were the only treatments with amino acids alone 

in which the moss retained its green color. Others were 

yellow-green, brown-green, or brown (DL-serine and DLtryptophan). 

When grown in combination of each of these 

20 amino acids with NH4NO3, plants in all treatments grew 

more than in any of the amino acids alone except in the 

highest concentration (0.0016 M) of DL-tryptophan. 

Growth was generally greatest in the lower concentration 

of amino acid (0.0001 M) plus NH4NO3. 

Alghamdi (2003) chose common soil water-soluble 

amino acids (glycine, methionine, serine, arginine, and 

alanine) to compare their effects on growth, branching, 

chlorophyll, and protein on Taxiphyllum barbieri. He


found that four of these amino acids induced branching, 

relative to the controls, but no branching appeared in any of 

the methionine treatments (Figure 4). 

Figure 4. Effects of water soluble amino acids on number of 

branches in the Java moss, Taxiphyllum barbieri. cont = control, 

gly = glycine, meth = methionine, ser = serine, arg = arginine, 

ala = alanine. From Alghamdi (2003). 

Figure 5. Effect of water soluble amino acids on the 

biomass, length, and robustness (wt:length) of the Java moss, 

Taxiphyllum barbieri. Cont = control, Gly = glycine, Meth = 

methionine, Ser = serine, Arg = arginine, Ala = alanine. Length 

and biomass represent sum of 3 stems; n = 10 sets of 3 stems. 

Notation as in Figure 2. Based on Alghamdi (2003). 

Methionine proved to be inhibitory to growth in 

length whereas serine caused an increase in both dry 

biomass and length relative to controls (Figure 5; Alghamdi 

2003). Arginine as the only N source at 1, 10, and 30 

mg/L caused a striking increase in the biomass and ratio of 

dry biomass to length, but maintained a length somewhat 

less than that of the N-free controls (Figure 5). This 

resulted in unusually short, wide plants, combined with 

high protein concentrations but below normal chlorophyll 

concentrations at the lowest level applied (1 mg/L; Figure 

6). 

Methionine likewise caused an increase in biomass 

and decrease in length growth with concentration increase 

(1, 10, 30 mg/L). Alanine caused an increase in both 

length and biomass with concentration, with the overall 

effect being one of a more robust plant at higher 

concentrations, having a higher biomass to length ratio than 

that of the controls. The mosses responded to 1 mg/L 

glycine much as they did to the N-free medium, but at 

higher concentrations (20 and 30 mg/L) their length and 

biomass both increased considerably over that of controls. 

Figure 6. Effect of water soluble amino acids on the protein 

content and total chlorophyll concentration of the Java moss, 

Taxiphyllum barbieri. cont = control, gly = glycine, meth = 

methionine, ser = serine, arg = arginine, ala = alanine. n = 10 sets 

of 3 stems. Notation as in Figure 2. From Alghamdi (2003). 

Alghamdi (2003) then compared the effects of glycine, 

which seemed to produce the "healthiest" plants, to those of 

the inorganic forms of N. This aquatic moss did less well 

on the inorganic forms NH4NO3 or NO3 - than on NH4 + 

alone or NH4 + + the amino acid glycine and did best on 

glycine alone, producing more biomass, longer stems, and 

more branches (Figure 7, Figure 8). In fact, glycine 

seemed to induce branching (Table 1). 

In the same series of experiments, Alghamdi (2003) 

examined the effects of inorganic N and glycine on the 

chlorophyll and protein content. Glycine, both alone and in 

combination with NH4 + , resulted in the highest protein 

concentrations (Figure 9). The effects on chlorophyll were 

less clear, but the highest total chlorophyll occurred in the 

highest glycine concentration. NH4 + at 20 mg/L, however, 

produced similar chlorophyll concentrations, but at 30 

mg/L the chlorophyll content decreased.

Figure 7. Effect of N source as nitrate (NO 3 + ), ammonium 

(NH4 + ), glycine (gly), and combinations at two concentrations on 

number of branches in Taxiphyllum barbieri. The combinations 

have half the total N from each source. From Alghamdi (2003). 

Figure 8. Effect of nitrate (NO 3 + ), ammonium (NH4 + ), 

glycine (gly), and combinations on the increase in biomass and 

length and robustness (wt:length) of the Java moss, Taxiphyllum 

barbieri. Notation as in Figure 2; n = 10 sets of 3 stems. From 

Alghamdi (2003). 

Other organic compounds, such as nucleic acids, are 

also released from organism tissues as they decay. Based 

his data showing that Atrichum undulatum had good 


growth in a medium with yeast nucleic acids as its N 

source, Burkholder (1959) tested growth of this species on 

the nucleic acid bases. Growth of leafy shoots was good in 

adenine and guanine, but there was no growth in uracil or 

thymine. Growth in xanthine, uric acid, and cytosine was 

less than that in NH4NO3. Both uracil (in the presence of 

NH4NO3) and aspartic acid caused Sphagnum squarrosum 

to become thalloid (resembling its protonema), as did 

hydroxyproline + glycine, occasionally (Burkholder 1959). 

Not all mosses responded in the same way. Growth of 

Leptobryum pyriforme and Splachnum sphaericum and 

others was "excellent" on a medium with NH4NO3 plus 

uracil, but was poor in Sphagnum squarrosum. On the 

other hand, while growth of Leptobryum pyriforme was 

good with uric acid and cytosine, Splachnum sphaericum 

had poor growth. The ability to use nucleic acids, amino 

acids, and other organic N compounds could permit 

bryophytes to take advantage of partially decomposed litter 

in which these nitrogen sources leak from the dead tissues. 

Table 1. Effect of various N forms on moss branching in 

Taxiphyllum barbieri. From Alghamdi (2003). 

Treatment Moss Branching 

glycine long with many short branches 

NO3 - short and no branches 

NH4 + long and few short branches 

glycine + 

NH4 + 

NH4NO3 

long with many short branches and 

slightly thin 

short, thin and few short branches 

Figure 9. Effects of inorganic N compared to glycine on the 

protein and chlorophyll content of Java moss (Taxiphyllum 

barbieri). Notation as in Figure 2; n = 10 sets of 3 stems. From 

Alghamdi (2003).


Amino acids, leaking into the environment, could 

cause developmental anomalies leading to abnormal 

growth forms in bryophytes. Some amino acids, such as 

hydroxyproline, can cause desuppression in the 

development of underleaves in liverworts (Basile & Basile 

1980; Basile et al. 1988), causing them to look like normal 

leaves. In Atrichum, amino acids inhibited leafy shoot 

development (Burkholder 1959). This might be another 

example of the Gaia hypothesis (Lovelock, 1988), wherein 

the ecosystem behaves like a superorganism and species 

depend on other species for their biochemical needs during 

development. 

Nitrogen Fixation 

With 78% of our atmosphere being composed of 

nitrogen and only about 5% of biomass being nitrogen, one 

would expect this element to be no problem for living 

systems to obtain. But unlike phosphorus, it cannot 

normally be obtained from bedrock. And just as you and I 

can make no use of the free, gaseous nitrogen we breathe, 

most plants can't either. Instead, plants require their 

nitrogen fixed into ammonium (NH4 + ) or nitrate (NO3 - ) 

salts (or converted to amino acids) before they can obtain 

and convert it to specific amino acids and proteins they 

need. Nitrogen fixation is the process of trapping 

atmospheric nitrogen and converting it to NH4 + and in 

some cases, converting it to NO3 - . 

Nitrogen fixation is a major source of usable nitrogen, 

particularly in bogs and fens. Like many tracheophytes, 

bryophytes can use N released by N fixation from 

associated bacteria and Cyanobacteria. The heterocysts 

(Figure 10) of Cyanobacteria make them a rich source of 

amino acids as a result of their nitrogen-fixing activity. 

That is, they are able to convert atmospheric N to a form 

usable by other living organisms. 

Figure 10. Anabaena (Cyanobacteria) showing heterocyst in 

middle lower part of picture. Photo by Janice Glime. 

In the process of nitrogen fixation in Cyanobacteria, 

the simple CH2O group from sugars, fixed by cells adjacent 

to the heterocyst, is moved into the heterocyst (Figure 11). 

Atmospheric nitrogen (N2) enters adjacent cells and is 

passed to the heterocyst. In the heterocyst nitrogen 

reductase (enzyme that catalyzes addition of H + to N to 

form NH4 + ) catalyzes the transformation of N2 to the 

reduced NH4 + with H + obtained from the CH2O group. 

Figure 11. Nitrogen fixation in Cyanobacteria, with 

atmospheric nitrogen entering an adjacent cell and being 

transferred to the heterocyst, where it is converted to ammonium 

(NH4 + ). The ammonium is then moved to the adjacent cell where 

it is converted into organic compounds, typically amino acids. 

Diagram by Janice Glime. 

Many studies have shown that some bryophytes, 

especially peatland bryophytes, obtain N through N 

fixation processes of surface-dwelling Cyanobacteria as 

well as other bacteria (Cullimore & McCann 1972; 

Granhall & Selander 1973; Alexander et al. 1974; Basilier 

et al. 1978; Smith & Ashton 1981; Smith 1984; Nakatsubo 

& Ino 1986, 1987; Bentley 1987; Given 1987; Bergman et 

al. 1993; Madhusoodanan & Dominic 1996). In the 

Cyanobacteria, the most significant contributions come 

from taxa such as Nostoc (Figure 1), Anabaena, and 

Calothrix (Figure 12) that have special cells called 

heterocysts. These cells provide a "safe" environment for 

nitrogen fixation because they lack the oxygen-generating 

reactions of photosystem II. The enzyme nitrogen 

reductase is unable to make the conversion in an aerobic 

environment, hence requiring a location where 

photosynthetic oxygen is not available. Since only the 

Cyanobacteria and some true bacteria are able to use the 

abundant atmospheric nitrogen, this conversion makes a 

significant contribution to usable nitrogen in the ecosystem. 

Figure 12. Upper: Algae on Campylopus at geothermal 

vent in New Zealand. Lower: Nostoc, a typical N-fixing 

Cyanobacterium that can be found associated with bryophytes. 

Photos by Janice Glime

These Cyanobacteria fix more nitrogen than is 

essential for their own needs and release the excess to their 

environment. Significant contributions of N through N 

fixation by Cyanobacteria occur in grasslands (Vlassak et 

al. 1973), boulder communities (Snyder & Wullstein 

1973a, Jones & Wilson 1978), tropical forests, especially in 

epiphyllous communities (those growing on a leaf) 

(Bentley 1987), poor Sphagnum mires (Basilier 1979), and 

polar turfs (Alexander et al. 1978). 

In the terrestrial moss Hymenostylium recurvirostre, 

association with Nostoc is common. Labelled 15 N from N 

gas, converted by Nostoc, resulted in the highest 

concentrations in the new rhizoids, then new shoots, then 

old shoots and old rhizoids (Jones & Wilson 1978). Jones 

and Wilson suggest that these locations indicate the 

nitrogen is being translocated from old to young tissues. 

Not only is free NH4 + available, but also large quantities of 

extracellular amino acid leakage is associated with this 

Nostoc. In view of the discussion above on bryophyte use 

of amino acids, it is likely that the moss and its neighbors 

might be using these amino acids as part of their N source. 

In some of the liverworts and hornworts, 

Cyanobacteria seem to behave symbiotically (Saxena 

1981), but more frequently it seems to be only a matter of 

suitable habitat. For example, in the moist Pacific 

northwest, approximately 85% of the sampled epiphytic 

leafy liverwort Porella navicularis (Figure 13) harbors 

Nostoc (Cyanobacteria) in distinct colonies under the leaf 

margins and in other plant crevices (Dalton & Chatfield 

1985). Nitrogen fixation is measured by the acetylene 

reduction method, and the product C2H2 is used as the 

measure of fixation. The production of fixed N on P. 

navicularis resulted in a mean of 53.5 nmol C2H2 g -1 d m h - 

1 and reached up to 316 nmol C2H2 g -1 d m h -1 . Dalton and 

Chatfield (1985) at first thought the Porella association 

was symbiotic, but the low number of heterocysts (3-7%) is 

typical of free-living Nostoc; symbiotic ones typically have 

a frequency of 30-40%. In either case, the effect is the 

same; by providing a suitable habitat for Cyanobacteria, the 

mosses facilitate an increase of available N in the system. 

Nevertheless, at least some micro-organisms living in 

association with epiphyllous liverworts are able to transfer 

this fixed nitrogen directly to their host plants (Figure 14; 

Bentley & Carpenter 1984), thus constituting a loose 

arrangement that benefits the tracheophyte as well as the 

bryophyte. In the palm Welfia georgii, 10-25% of the N in 

the leaf was derived from the micro-organisms harbored 

there among the leafy liverwort cover. 

In bryophyte-Cyanobacteria associations in the 

Antarctic (Smith & Russell 1982; Smith 1984; Nakatsubo 

& Ino 1987, Line 1992; Pandey et al. 1992), Arctic 

(Alexander et al. 1978) and alpine/subalpine zones 

(Lambert & Reiners 1979), N fixation may be a very 

important contribution of this limiting nutrient to the 

ecosystem (Smith & Ashton 1981). Although Smith and 

Ashton failed to show much acetylene reduction to indicate 

fixation activity in the field at ~0°C, they considered that 

during the warm summer, fixation by Cyanobacterial flora 

of bryophytes could approach that exhibited in the lab at 

~20°C, thus contributing significantly to the available N in 

the ecosystem. In a 48-hour field incubation with an air 

temperature of –1.7°C and moss moisture of 300-1500%, 

only the moss Ditrichum strictum associations had any 


positive acetylene reduction (1.17 & 1.21 µg g -1 48h -1 ). 

The more protected, but nevertheless very cold, 

Clasmatocolea humilis and Jamesoniella grandiflora 

associations failed to demonstrate any fixation. 

Figure 13. Porella navicularis growing epiphytically on a 

branch. Photo by Kent Brothers with permission from 

www.botany.ubc.ca/ bryophyte/LAB8.htm. 

Figure 14. Means and standard errors of 5 hrs of production 

of fixed nitrogen in leaves of the palm Welfia georgii incubated 

alone (with epiphylls removed) and leaves with intact epiphylls, 

indicating a much greater transfer of new N to the leaf when 

epiphylls are present. Redrawn from Bentley & Carpenter (1984). 

In support of the suggestion that contributions in the 

summer may be significant, Nakatsubo and Ino (1987) 

found that approximately 330 mg N m -2 was fixed per 

growing season in some areas of the Antarctic. Fogg and


Stewart (1968) found that most N fixation occurs at 

temperatures above 10°C, thus explaining the lack of 

activity in the Smith and Ashton study. Temperatures in 

the moss-Cyanobacterial associations in summer in the 

maritime Antarctic typically are in excess of 10°C, often 

reaching 20°C during midday (Huntley 1971). Smith 

(1984) found that the fixation rate increased at 

temperatures from –5°C to a maximum at 25-27°C, 

decreasing sharply after that. Saturation occurred at ~1000 

µmol m -2 s -1 photon flux density, decreasing at higher 

levels. Once suitable temperatures were available, 

moisture seemed to be the most important criterion, causing 

an increase in fixation up to the highest water content 

measured: 3,405%! The chemical conditions suitable for 

fixation seem to be restrictive, with an optimum pH in this 

system of 5.9-6.2 and a negative response to the addition of 

P, Co, or Mo (Smith 1984). Hence, under warmer 

conditions, fourteen out of nineteen bryophyte associations 

did indeed exhibit fixation, with values increasing as 

moisture content increased (Smith & Russell 1982). Rates 

ranged from 0.36 to 310.57 nmol C2H2 g -1 d m h -1 among 

the fourteen with measurable fixation. 

In the Arctic soils of Svalbard, Norway, N fixation by 

both free-living and bryophyte associations of 

Cyanobacteria is the only significant source of N input to 

the soil ecosystem (Solheim et al. 1996). The most 

important bryophytes for harboring such associations were 

Calliergon richardsonii and Sanionia uncinata. An 

interesting factor in the fixation was grazing by geese. 

Grazed areas had a 10-fold maximum fixation (693.6±1.5 

nmol C2H4 h -1 gdm -1 ) compared to ungrazed areas 

(65.3±16.6 nmol C2H4 h -1 gdm -1 ), perhaps because in these 

areas the Cyanobacteria also occurred on the grass. The 

transfer of fixed N to the plants supported high plant 

productivity. On the other hand, where birds harbored 

under cliffs, the concentration of bird droppings inhibited N 

fixation. 

The alpine zone likewise is nitrogen limited due to the 

slow decay rate and limited organic layer. Cyanobacteria 

are important in binding the soil and in providing reduced 

N. In the subalpine zone of the White Mountains of New 

Hampshire, USA, the moss Plagiomnium cuspidatum 

provides a suitable habitat for Cyanobacteria (Lambert & 

Reiners 1979). On Mt. Fuji, the moss communities of the 

dry SW slope are nearly devoid of N-fixing activity, but on 

the moist NE-facing cliffs they exhibit high activity, 

especially with Nostoc colonies (Nakatsubo & Ohtani 

1991), again demonstrating the importance of moisture. In 

the somewhat less severe climate of the Alaskan blue 

spruce taiga system, feather mosses such as Pleurozium 

schreberi and Hylocomium splendens are important 

substrates for N-fixing aerobic and facultative anaerobic 

bacteria (Billington & Alexander 1983). 

But the importance of the Cyanobacteria and other Nfixing 

micro-organisms is not limited to such cold 

environments. N fixation by Cyanobacterial associations 

with bryophytes may be important in many ecosystems 

where it has hardly been recognized (Cullimore & McCann 

1972; Madhusoodanan & Dominic 1996). In rainforests, 

epiphyllous liverworts provide the moist microhabitat 

needed for high rates of nitrogen fixation by associated 

bacteria and Cyanobacteria (Bentley & Carpenter 1980; 

Bentley 1987; Carpenter 1992), which may be transferred 

to the host leaves (Bentley & Carpenter 1984). In 

cryptogamic crusts (i.e. soil crusts of algae, lichens, 

bryophytes, and micro-organisms; Figure 15) of prairies, 

deserts, and grasslands, Cyanobacteria are able to maintain 

an active state longer when water is held by the bryophytes. 

This increases their contribution to the usable N in the soil 

(Vlassak et al. 1973; Giddens 1982; Belknap et al. 2001). 

The crust itself is vital to maintaining both water and 

nutrients in the soil during and following heavy storms. In 

geothermal fields and following fires, bryophytes again 

provide the moist environment needed to maintain N-fixing 

micro-organisms (Brasell et al. 1986). Hence, we must ask 

if the bryophytes are net users of nitrogen, or do they 

facilitate a net gain to the system. At least in some habitats 

they definitely facilitate a gain by providing the right 

habitat for fixation to occur. 

Figure 15. Cryptogamic crust with the moss Syntrichia 

inermis. Photo by Lloyd Stark. 

Although moss associates are responsible for most N 

fixation in Arctic and subarctic ecosystems, legume 

associations are considered the predominant N fixers in 

temperate ecosystems (Stewart 1967). Nevertheless, in 

some temperate habitats bryophytes are the only plants able 

to occupy the habitat. For example, on granite outcrops, 

bryophytes, especially Grimmia/Schistidium, are well 

known for their role in accumulating soil and nutrients and 

holding the moisture needed for tracheophyte 

establishment. Microbial nitrogen fixation on these 

bryophytes is part of this successional story (Snyder & 

Wullstein 1973a; Jones & Wilson 1978). Likewise, 

bryophyte-Cyanobacteria associations are important in the 

colonization of volcanic lava. Cyanobacteria are common 

on bryophytes of dry lava fields (Englund 1976) as well as 

on the moist, warm bryophyte surfaces near steam vents 

(Broady et al 1987). Both Anabaena variabilis and Nostoc 

muscorum were associated with Funaria hygrometrica on 

the newly formed volcano Surtsey off the Icelandic coast 

(Rodgers & Henriksson 1976). Although the Funaria did 

not directly affect the fixation rate, growth of both the 

Funaria and the Cyanobacteria benefitted by the 

association, and the N content of Funaria also increased as 

a result of the cyanobacterial N fixation. 

Thus, as in the Arctic, temperate bryophytes often have 

associated Cyanobacteria, especially Nostoc. Soil 

associations with bryophytes can benefit the ecosystem in 

several ways. Not only do they provide additional usable N 

to the ecosystem, as in the Gymnostomum recurvirostrum

association in Upper Teesdale (Wilson 1975), but they also 

provide a buffer against erosion and leaching of nutrients 

already in the upper soil layers. 

Few studies have quantitatively addressed the role of 

micro-organisms in bryophyte communities, particularly in 


peatlands where their role is significant (Gilbert et al. 

1999). Nevertheless, these micro-organisms are 

undoubtedly key players in nutrient cycling through the 

microbial loop. 

Table 2. Comparison of N fixation rates by Cyanobacteria associated with bryophytes in various habitats. Rates converted to nmol 

N using the 3:1 ratio of reduced acetylene to fixed N given by Nakatsubo and Ino (1987) and Vlassak et al. (1973). gfm = grams fresh 

mass; gdm = grams dry mass. Table compiled by Medora Burke-Scoll. 

Location Habitat 

Bryophyte and Cyanobacteria 

partner 

Tropical Lava and on volcanic island Funaria hygrometrica + Nostoc & Anabaena 0.42 nmol N cm -2 hr -1 

Rate Reference 

Rodgers & Henriksson 

1976 

Tropical Undisturbed forest floor Chiloscyphus coalitus + Anabaena &/or Nostoc 1.87 nmol N gdm -1 hr -1 Brasell et al. 1986 

Tropical Undisturbed forest floor 

Chiloscyphus fissistipus + Anabaena &/or 

Nostoc 

Tropical Undisturbed forest floor Bazzania adnexa + Anabaena &/or Nostoc 1.23 nmol N gdm -1 hr -1 

8.2 nmol N gdm -1 hr -1 Brasell et al. 1986 

Brasell et al. 1986 

Tropical Undisturbed forest floor Hypnum chrysogaster + Anabaena &/or Nostoc 3.1 nmol N gdm -1 hr -1 Brasell et al. 1986 

Tropical Undisturbed forest floor Pohlia nutans + Anabaena &/or Nostoc 3.27 nmol N gdm -1 hr -1 Brasell et al. 1986 

Tropical Undisturbed forest floor Tortella calycina + Anabaena &/or Nostoc 2.57 nmol N gdm -1 hr -1 Brasell et al. 1986 

Tropical Undisturbed forest floor Pohlia nutans + Anabaena &/or Nostoc 3.27 nmol N gdm -1 hr -1 Brasell et al. 1986 

Temperate Grassland Ceratodon purpureus + Nostoc 10.4 nmol N gdm -1 hr -1 Vlassak et al. 1973 

Temperate 

Japan 

Aquatic 

Sphagnum capillaceum + Stigonema, 

Hapalosiphon, Scytonema, & Nodularia 

0.13 nmol N gfm -1 hr -1 Morimoto & Maruyama 

1982 

Temperate Peatland 

Sphagnum + Stigonema, Hapalosiphon, 

Scytonema, & Nodularia 

0.13 nmol N gfm -1 hr -1 Morimoto & Maruyama 

1982 

Temperate 

Coniferous forest floor 

(Bilberry-spruce forest) 

Sphagnum girgensohnii + Anabaenopsis 

None detected *included 

only plant apex. 

Basilier 1979 

Temperate Forest margin Sphagnum papillosum + endophytic Nostoc 

0.033 nmol N gdm -1 hr -1 

(only plant apex) 

Basilier 1979 

Temperate Fen Sphagnum angustifolium + endophytic Nostoc 

43.3 nmol N gdm -1 hr -1 


Basilier 1979 

Temperate Fen 

Drepanocladus aduncus + unidentified epiphytic 

Cyanobacteria 

25.67 nmol N gdm -1 

hr -1 (only plant apex) 

Basilier 1979 

Temperate Fen Sphagnum riparium + epiphytic Hapalosiphon 

26.67 nmol N gdm -1 

hr -1 (only plant apex) 

Basilier 1979 

Temperate Lakeside Sphagnum annulatum + Nostoc 

15.3 nmol N gdm -1 hr -1 


Basilier 1979 

Temperate Desert Grimmia + Azotobacter 0.065 nmol N gdm -1 -1 

hr 

Snyder & Wullstein 

1973b 

Temperate Desert Syntrichia ruralis + Azotobacter 0.061 nmol N gdm -1 hr -1 Snyder & Wullstein 

1973b 

Boreal 

Iceland 

Iceland Lava field Grimmia + Anabaena & Nostoc 

0.13 nmol N/20 cm 

plant · hr -1 

Englund 1976 

Boreal Iceland Lava field Racomitrium + Anabaena & Nostoc 

0.1 nmol N/20 cm plant 

· hr -1 

Englund 1976 

Subalpine Forest floor Sphagnum + Cyanobacteria 0.743 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Peatland Sphagnum lindbergii + Nostoc & Scytonema 1.3 nmol N gdm -1 hr -1 

Granhall & Selander 

1973 

Subalpine Peatland Sphagnum + Cyanobacteria 0.29 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Aquatic Sphagnum + Cyanobacteria 0.13 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Atrichum + Cyanobacteria 0.053 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Dicranum + Cyanobacteria 0.023 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Pleurozium schreberi + Cyanobacteria 0.026 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Plagiomnium cuspidatum + Cyanobacteria 0.15 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Polytrichum + Cyanobacteria 0.011 nmol N gdm -1 hr -1 Lambert & Reiners 1979 

Subalpine Forest floor Bazzania trilobata + Cyanobacteria 0.033 nmol N gdm -1 hr -1 Lambert & Reiners 1979


Subalpine Coniferous forest floor Feather mosses 0.23 nmol N gdm -1 hr -1 

Subalpine Coniferous forest floor Sphagnum 7.47 nmol N gdm -1 hr -1 

Alpine zone 

of Mt. Fuji 

Antarctic 

Antarctic 

Antarctic 

Mountain summit 

East Ongul Island, 

Antarctica. Sand near a 

rocky peak. 

Marion Island (highly 

minerotrophic receiving 

nutrient-rich mire runoff) 

Marion Island (exposed 

wind-swept rocky ridges) 

Antarctic Marion Island (submerged) 

Antarctic 

Fumaroles near summit of 

Mt. Melbourne 

Aongstroemia fuji-alpina, Ceratodon purpureus, 

& Bryum + Nostoc 

Ceratodon purpureus & Bryum pseudotriquetrum 

+ Nostoc 

Brachythecium subplicatum + Anabaena, 

Calothrix, Hapalosiphon, Nostoc, Sphaerocystis, 

Stigonema, & Tolypothrix 

Ditrichum strictum (balls) + Anabaena, 

Calothrix, Hapalosiphon, Nostoc, Sphaerocystis, 

Stigonema, & Tolypothrix 

Grimmia falcate + Anabaena, Calothrix, 

Hapalosiphon, Nostoc, Sphaerocystis, Stigonema, 

& Tolypothrix 

Campylopus pyriformis & Cephaloziella 

exiliflora + Mastigocladus laminosus 

Peatland Associations 

Sphagnum is highly colonized by a variety of 

Cyanobacteria, both on its surface (Hooper 1982), and in its 

hyaline cells (Figure 16; Granhall & Hofsten 1976; 

Granhall & Lindberg 1978), especially by Nostoc and 

Hapalosiphon (Sheridan 1975). In bogs and fens, 

Cyanobacteria on bryophyte surfaces can contribute 

considerable usable N to the ecosystem (Alexander et al. 

1974; Basilier et al. 1978, Basilier 1979; Lambert & 

Reiners 1979; Rosswall & Granhall 1980, Hooper 1982). 

Chapman and Hemond (1982) determined that the 

contribution was greater than that from the only other 

known input, bulk precipitation (as NO3 - ). Three types of 

Sphagnum associations fix N: epiphytic Cyanobacteria, 

intracellular Cyanobacteria, and N-fixing bacteria 

(Granhall & Selander 1973, Granhall & Hofsten 1976). 

Basilier (1979) reported N-fixation activity by 

Cyanobacteria on Sphagnum, Drepanocladus, and 

Calliergon in phosphorus-rich environments. Basilier and 

coworkers (1978), as well as Granhall and Selander (1973), 

found that the highest N fixation rates in their studies 

occurred on the mosses Sphagnum and Drepanocladus 

(s.l.), with a mean value of 9.4 g m -2 yr -1 . In fact, 

Cyanobacteria associated with Sphagnum can have higher 

N fixation per heterocyst than do free-living Cyanobacteria 

in the same condition (Basilier 1980). Granhall and 

Lindberg (1978) reported a total rate of 0.8-3.8 g fixed N 

m -2 yr -1 in wet Sphagnum communities in a mixed pine and 

spruce forest in central Sweden. Zimicki (1976) and 

Basilier et al. (1978) have estimated N fixation in various 

sites for Sphagnum riparium to be 0.5-6.4 g m -2 yr -1 . 

Basilier et al. (1978) found that the fixation rate in the 

Sphagnum riparium association was strongly light 

dependent, but that pH in the range of 4.3 to 6.8 had little 

effect. Maximum fixation occurred around noon with the 

middle of the growing season exhibiting the highest rates. 

Interestingly, they found that rates on the apical portions 

and non-green portions of the Sphagnum were lower, and 

that the highest rates occurred on the periphery of the moss 

community. On the other hand, using 15 N as a tracer, 

Basilier (1980) later found that enrichment of N from 

Cyanobacteria fixation appeared within two hours in the 

apex of Sphagnum. It appears that habitat comparisons 

need to be made to determine where the highest rates might 

occur. 

3.4 nmol N cm -2 hr -1 

2.37 nmol N cm -2 hr -1 

Granhall & Lindberg 

1978 

Granhall & Lindberg 

1978 

Nakatsubo & Ohtani 

1991 

Nakatsubo & Ino 1987 

103.5 nmol N gdm -1 hr -1 Smith & Russell 1982 



11 nmol N gdm -1 d -1 Broady et al. 1987 

Figure 16. Potential interactions of micro-organisms within 

the hyaline cell of Sphagnum. Redrawn from Granhall & Hofsten 

(1976). 

Once the Cyanobacteria convert the N to NH4 + and 

amino acids, these are available not only for the bryophytes 

they occupy, but also for the tracheophytes rooted among 

them. In Thoreau's Bog in Massachusetts, N fixation 

exceeded atmospheric N deposition (Hemond 1983), and 

Hemond concluded that microbial N fixation provides 

sufficient quantity of N that N may never be limiting to 

primary productivity in a bog (or poor fen) ecosystem. 

Liverwort Symbiosis 

Several attempts have been made to explain the high 

degree of N fixation in liverwort associations. In an early 

attempt, Griggs (1937) grew liverworts from Katmai 

volcanic ash on N-free sand for three years to determine 

their success compared to that of liverworts on the same 

medium, but with the addition of 4 mg/L NH4NO3. During 

that three-year period, the ones with the additive grew no 

better, but toward the end of the three years, the N-free 

cultures became pale and unhealthy. When 4 mg/L 

NH4NO3 was added to the N-free cultures, they promptly 

revived. Griggs took this as evidence that no N fixation 

had occurred. 

Nevertheless, at least the thallose liverwort Blasia 

pusilla has symbiotic Cyanobacteria that do perform N

fixation (Rodgers 1978; Peters 1991; Figure 17). In fact, 

there are many genetic strains of Nostoc associated with 

Blasia (West & Adams 1997; Costa et al. 2001). The 

presence of Nostoc induces both structural and metabolic 

changes within the Blasia thallus (Kimura & Nakano 1990; 

Meeks 1990). 

Figure 17. Blasia pusilla. Arrow indicates Nostoc colony. 

Photo by Paul Davison, University of North Alabama. 

Nostoc is only capable of invading the liverwort when 

the Nostoc is in its mobile stage (Kimura & Nakano 1990). 

That is, when the segments (called hormogonia) of a 

filament separate, they are mobile by a gelatinous sol-gel 

transformation that permits them to slither and glide. In 

this stage they are able to invade the thallus of B. pusilla 

and induce the morphological changes that permit the 

partnership to work. At the same time, the B. pusilla 

signals the Nostoc by producing two auricles (earlike 

lobes), each with an enclosed chamber housing a slime 

papilla that fills the chamber with mucilage (Renzaglia 

1982a). The mucilage attracts the Nostoc, which then takes 

up residence in the chamber. Once the Nostoc arrives, the 

auricle increases in size and closes its opening. Following 

the invasion, the surrounding cells of the Blasia thallus 

have attenuated growth and produce branched filaments 

from hyaline cells that penetrate the Nostoc colonies 

(Kimura & Nakano 1990). These filaments form a 

labyrinth of wall ingrowths into the Nostoc cells, 

suggesting that they may have the role of transfer cells for 

exchanging metabolites (Ridgway 1967; Duckett et al. 

1977). Once it has settled into its thallus home, the Nostoc 

produces numerous heterocysts, which are essential for the 

N fixation. 

When the Nostoc grows deeply embedded within the 

liverwort thallus, it no longer has access to dissolved CO2. 

Stewart and Rodgers (1977; 1978) determined that the 

Nostoc obtains its carbon through transfer from the Blasia 

thallus to Nostoc, suggesting that this is really a mutualistic 

relationship. Within the thallus the Nostoc requires a 

higher light intensity and higher temperature (above 17ºC) 

for maximal activity than when living alone (max activity 

above 12ºC) (Rodgers 1978). Hence, the liverwort 

provides a safe compartment that will remain moist much 

longer than the external environment, and even provides 

the needed carbon source for its symbiont. 

The presence of these Nostoc symbionts in liverworts 

seems to be restricted to taxa that are pioneers (Schuster 

1992a, b)., living in temporary habitats that are likely to be 


low in usable N. The ability to colonize rapidly, symbiont 

intact, is facilitated in Blasia pusilla by the production of 

two types of gemmae. These gemmae permit the symbiont 

to travel with the gemma and easily renew the partnership 

arrangement upon germination (Renzaglia 1982b; Duckett 

& Renzaglia 1993). Taxa that depend on spores for their 

dispersal would not benefit from this convenience. 

Hornwort Associations 

Hornworts (Anthocerotophyta) are well known for 

their symbiotic associations with Cyanobacteria, especially 

Nostoc in association with Anthoceros and Phaeoceros 

(Peirce 1906; Ridgway 1967; Enderlin & Meeks 1983; 

Steinberg & Meeks 1987). A wide diversity of Nostoc 

strains infect these hornworts (West & Adams 1997), and, 

it appears that Anthoceros harbors a Nostoc that is unique 

from that of Blasia (Leizerovich et al. 1990). But 

Phaeoceros (Figure 18) also hosts the filamentous 

Calothrix (Cyanobacteria) (West & Adams 1997); this 

multiplicity of symbiotic genera is apparently unusual; Rai 

et al. (2000) indicate that typically only one genus will 

infect a particular taxonomic group of plants. 

Figure 18. Phaeoceros carolinianus showing bluish green 

color typical of plants with Nostoc inhabitants. Photo by Michael 

Lüth. 

For the association to begin, the Nostoc must form 

hormogonia that can break away and move through the 

environment to reach the hornwort (Wong & Meeks 2002), 

just as in Blasia. But it seems that the hornwort makes 

certain that this occurs, if there is Nostoc in the vicinity. 

Free-living Nostoc rapidly forms hormogonia when in the 

presence of Anthoceros punctatus, or even in the presence 

of agar preconditioned with A. punctatus (Campbell & 

Meeks 1989), indicating a diffusable substance from A. 

punctatus that stimulates this response. 

Both Nostoc and the hornwort seem to be modified 

physiologically once joining in symbiosis (Joseph & Meeks 

1987; Campbell & Meeks 1992). Before the partnership 

can work, the Nostoc must form heterocysts (Wong & 

Meeks 2002). This is where the enzyme nitrogenase, 

needed for the N fixation, is located in both free-living and 

symbiotic strains (Rai et al. 1989). When mutants of 

Nostoc punctiforme, unable to form heterocysts, were 

introduced to Anthoceros punctatus, the partnership 

formed, but no N fixation occurred; the mutants did not 

produce any nitrogenase. 

As in the Blasia symbionts, the nitrogenase of the 

Nostoc must have an anaerobic environment in which to fix


nitrogen. Campbell and Meeks (1992) demonstrated this 

by showing that the symbiont could produce fixed N only 

under anaerobic conditions when grown outside its host. 

However, when it grew in its Anthoceros punctatus host, it 

could be grown aerobically; the special cavities where it 

grew on the host provided the anaerobic conditions needed. 

Perhaps one explanation for the success of N fixation 

within the host lies in the structure of the symbiont 

heterocyst, contrasting with that of the free-living Nostoc 

strains. When growing inside the host, the Nostoc 

heterocyst lacks the outer polysaccharide layer typical of 

free-living Nostoc (Campbell & Meeks 1992). Rather, it 

appears that when the Nostoc grows in the cavities of 

Anthoceros punctatus, the cavities replace that wall 

function. Anthoceros also mediates the nitrogenase 

activity, supressing it in the presence of NO3 - (Campbell & 

Meeks 1992) and NH4 + (Steinberg & Meeks 1991). The 

end product of the Nostoc fixation is NH4 + , accounting for 

75% of the introduced radioactive N after 0.5 min, but only 

14% after 10 minutes of incubation (Meeks et al. 1985), 

indicating a rapid transformation to something else. 

Glutamine and glutamate are quickly synthesized via the 

glutamine synthetase-glutamate synthase pathway, 

preventing the toxic buildup of NH4 + . Thus one end result 

of the symbiosis is that the intracellular levels of NH4 + are 

low compared to those of symbiont-free Anthoceros. 

Only 10% of the NH4 + is assimilated into the Nostoc; 

1% is lost to the medium; Anthoceros incorporates the 

remainder. Prakasham and Rai (1991) demonstrated that 

there is a specific methylammonium transport system in the 

symbiotic Nostoc, which may account for the reduced NH4 + 

levels and rapid transfer to the host. In symbiont-free 

Anthoceros supplied with high levels of NH4 + , the 

glutamate dehydrogenase system is functional, permitting 

an NH4 + buildup (Meeks et al. 1983). Therefore, it appears 

that the Nostoc partner provides a very effective and safe 

source of NH4 + for the Anthoceros host (Meeks et al. 

1985). 

As in the Blasia partnership, Nostoc living within the 

hornwort gets its carbon primarily from its host plant 

(Stewart & Rodgers 1977). In fact, Nostoc isolated from 

Anthoceros punctatus had only 12% of the Rubisco activity 

of free-living strains, with an equal reduction in CO2 

fixation (Steinberg & Meeks 1989; Rai et al. 1989). 

However, the distribution and levels of Rubisco were 

similar in the two strains (Rai et al. 1989), with 4.3% and 

5.2% of the protein as Rubisco in symbionts and free-living 

Nostoc, respectively (Steinberg & Meeks 1989), suggesting 

that there is regulation of the Rubisco activity and not an 

alteration at the gene transcription level. This could be 

related to the fact that the structure of the chloroplast 

differs somewhat; the Nostoc contains the typical 

cyanophycean granules, but it lacks phycobilisomes, the 

cellular organelle located on the surface of the thylakoids 

of the chloroplasts and in which the biliprotein pigments 

(phycocyanin, phycoerythrin) are present (Honegger 

1980). 

Because the Nostoc has reduced ability to fix its own 

carbon, this transfer of fixed carbon from A. punctatus to 

Nostoc is necessary for the fixation of N2. When the 

Nostoc-hornwort association was deprived of light for 28 

hours, the rate of acetylene reduction (as a measure of N 

fixation) declined by 99%, but resumed up to 64% of its 

illuminated activity when supplied with glucose in the dark 

(Steinberg & Meeks 1991), indicating the need for light 

and photosynthetic activity for the partnership to work. 

When gametophytes were deprived of light, but 

sporophytes were provided with light, nitrogenase activity 

continued (Stewart & Rodgers 1977), suggesting a transfer 

of sugar from the sporophyte to the gametophyte, then to 

the Nostoc. These factors suggest that the Nostoc, living in 

the reduced light of the interior of the hornwort thallus, 

may be dependent upon the hornwort for glucose or similar 

carbohydrate as an energy source in order to continue its N 

fixation, thus completing a true mutualistic relationship 

with its host. 

The local sites of the host plants act as islands that 

effectively keep the Nostoc strains in isolation. Even 

within a single host plant there may be a great diversity of 

cyanobacterial strains, and these strains seem to be 

restricted to one site (Costa et al. 2001). Nevertheless, 

some host plants shared strains of Nostoc that could be 

found growing 2000 m away. Furthermore, strains found 

in Blasia could also be found in the lichen Peltigera 

neopolydactyla. Although different cavities can easily host 

different strains in both Blasia and the Anthocerotophyta, a 

single cavity seems only to host one strain. 

Lunar Rocks 

Liverworts were among the few organisms to grow 

successfully on lunar rocks. But why? Marchantia 

polymorpha exhibited a tremendous increase in growth 

following being sprinkled with Apollo 11 or 12 lunar rock 

material. Hoffman (1974) followed up on this observation 

by testing the effects of basalt from Minnesota and Chorizon 

substrate from the Valley of Ten Thousand 

Smokes, Alaska. In both cases, the growth of M. 

polymorpha was significantly increased. But what caused 

this surge of growth? Nitrogen was absent in any form in 

both the lunar material and the basalt, and neither P nor K 

was abundant, so the three typical fertilizer nutrients seem 

not to be the cause. The macronutrients Ca, Mg, and S 

were all more abundant in basalt than in the C-horizon soil, 

but the C-horizon soil caused the greater stimulation. Iron 

remains a possibility, being abundant in all three substrata. 

We already know that it stimulates the growth of Funaria 

hygrometrica (Hoffman 1966). On the other hand, no data 

were gathered on the pH, which could affect the solubility, 

and therefore availability, of all the nutrients. Some have 

speculated that survival of the liverwort was possible due to 

partnering Cyanobacteria that could trap and convert the 

atmospheric nitrogen. Perhaps we need to look for soil and 

rock components that foster the N fixation reaction. 

N Enrichment 

Many studies in peatlands have included enrichment of 

N to determine effects on bryophyte productivity. In the 

high Arctic heath, Gordon et al. (2001) found that 

applications of N (0, 10 & 50 kg ha -1 yr -1 ) and P (0 & 

5 kg ha -1 yr -1 ) caused a decrease in lichen cover; 

applications of 10 kg ha -1 yr -1 resulted in a higher 

proportion of physiologically active bryophyte shoots. 

Nevertheless, individual bryophyte species responded 

differently, suggesting that we cannot draw generalizations 

from limited fertilization experiments.

Summary 

Nitrogen is available to bryophytes as ammonium 

(NH4 + ), nitrite (NO2 - ), nitrate (NO3 - ), and organic 

forms such as amino acids and urea. Nitrite, however, 

is generally toxic. Ammonium can lower internal pH 

and suppress growth. Nitrite can cause an increase in 

chlorophyll a, whereas nitrate can cause a decrease in 

chlorophyll b, both causing an increase in the a/b ratio. 

But effects on amino acid and protein concentration 

vary among species and among habitats. In the Arctic, 

amino acids and urea are utilized by both bryophytes 

and tracheophytes. Sphagnum often seems to benefit 

more from amino acids than from ammonium. 

N deficiency, or the wrong form of N (e.g. NH4 + ), 

can cause bryophytes to become long and thin, 

appearing etiolated. Glycine, serine, arginine, and 

alanine can induce branching. Methionine not only 

did not induce branching, but it also inhibited growth. 

Glycine caused the greatest weight and length gain of 

these amino acids in Java moss. Even nucleic acids are 

usable N sources, with good leafy shoot growth in 

adenine and guanine, but no growth in uracil or 

thymine in some species and good growth in others. In 

Sphagnum squarrosum uric acid and cytosine caused 

the plant to be come thalloid. 

Some, perhaps many, bryophytes solve the nitrogen 

problem through symbiotic partners, especially 

Cyanobacteria, that carry out nitrogen fixation. This 

process seems to be especially important in the polar 

and alpine regions under warmer summer conditions up 

to ~25ºC. But more xeric conditions such as among 

epiphyllous tropical bryophytes and associated with 

prairie and grassland cryptogamic crusts also benefit 

from N fixation. In all of these habitats, bryophytes 

have an important role in maintaining the moisture 

necessary for the fixation to occur. 

Peatlands have a high N fixation rate, and 

Cyanobacteria are common in association with 

Sphagnum. They have a wider pH tolerance range (4.3- 

6.8) than the Cyanobacteria in the cold habitats (5.9- 

6.2). 

The liverwort Blasia pusilla provides a special 

chamber in each auricle where it is moist with 

mucilage and the Cyanobacteria enter and grow. It then 

seals the chamber and produces filaments that penetrate 

the Nostoc colonies. Finally the Nostoc produces 

numerous heterocysts. The Nostoc even travels with 

the gemmae. 

Anthoceros punctatus forms a similar partnership, 

as do most of the hornworts, but it even stimulates the 

Nostoc to form hormogonia, permitting it to slither 

toward the hornwort. In both liverwort and hornwort 

partnerships, the ammonium produced by the 

Cyanobacterial heterocyst is quickly converted to 

glutamine and glutamate to avoid the buildup of toxic 

ammonium. The Anthoceros gets almost 90% of the 

fixed N and provides fixed C to its Cyanobacteria 

partner. 

Moon rock, and rock taken from volcanic areas on 

Earth, stimulate the growth of bryophytes, but we don't 

know why. One possibility is the high concentration of 

iron; another is that symbionts thrived on these rocks, 

providing N fixation. 


It appears that bryophytes play a major role as a 

substrate for N fixation in many nutrient-poor habitats, 

making than essential component of those ecosystems. 

Acknowledgments 

I appreciate the contributions of undergraduate Phil 

Gaudette and M. S. student Jennifer Jermalowicz Jones for 

their critical reading of the manuscript from the 

perspectives of students interested in nutrient relationships 

of bryophytes. Medora Burke-Scoll prepared the table on 

N contributions by Cyanobacteria. 

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influence of abiotic factors on nitrogen fixation rates in the 

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Alghamdi, A. A. 2003. The Effect of Inorganic and Organic 

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