Geoscience Reference
In-Depth Information
unlike normal cyanobacterial cells, no oxygen production takes place in
the heterocyst. Instead, they contain special proteins to consume oxy-
gen from solution, providing the perfect oxygen-free environment for
nitrogen fixation.
Cyanobacteria without heterocysts must find other ways to shield ni-
trogenase from oxygen, and there are lots of different approaches. Some
cyanobacteria living in microbial mats, for example, only fix nitrogen at
night when the mat environment becomes anoxic. Other cyanobacteria
also restrict nitrogen fixation to the night, but they fix nitrogen in oxy-
genated waters. In this case, the cyanobacteria respire rapidly enough
to remove oxygen from within the cell so nitrogen fixation can proceed.
The most fantastic solution, I think, comes from
Trichodesmium
, which
may be the most important nitrogen fixer in the oceans.
Trichodesmium
is a filamentous cyanobacterium lacking heterocysts, and it often forms
impressive bundles observable by the naked eye. Indeed, large blooms
of
Trichodesmium
are easily seen by satellite. Completely counter to intu-
ition,
Trichodesmium
displays its highest rates of nitrogen fixation at mid-
day, when light intensity is at a maximum. This would normally also
be the time of day when rates of oxygen production by photosynthesis
are highest because rates of photosynthesis usually increase with light
intensity. Not so with
Trichodesmium
. At high light, these cyanobacteria
turn photosynthesis down and use the light energy to drive oxygen-
utilizing reactions in the cell.
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In contrast, in the dim light of the morn-
ing and evening, photosynthesis is switched on, and nitrogen fixation
is turned down. This strategy works when light is plentiful, which may
explain why
Trichodesmium
prefers clear water and tropical latitudes (but
who doesn't?).
Up to now we have explored various aspects of cyanobacterial ecol-
ogy and physiology, but we have purposefully ignored plants and algae.
hat about them, since they also make oxygen? Indeed, could there be
some relationship between the diminutive cyanobacteria that we often
must struggle to see and the plants and algae that nearly define our
modern world? As it turns out, the relationship is strong, and demon-
strates a beautiful happenstance in the history of life. Sometime long,
long ago, a cyanobacterium took up residence in a eukaryotic cell. This
was a mutually beneficial (usually known as symbiotic) relationship
where the eukaryote likely gained food from the cyanobacterium, and
