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

20 Chapter 8-3: Nutrient Relations: Nitrogen
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