By examining only the biomarker inventory, however, we can't say
for certain which groups of cyanobacteria are actually responsible
for this large change. Shown here are a few examples of important
groups of known N2 fixing microorganisms:
(1) Trichodesmium, a very large
N2-fixing organism that's present
in several different morphologies ranging from free trichomes (chains
of cells) to the large (> 1 mm) aggregates or "bundles"
shown in the slide, compared to the much smaller (3-5 mm) Synechococcus-like
organisms that have recently been isolated by Dr. John Waterbury.
The latter is a small unicellular N2-fixer,
and this new organism was just discovered as an abundant component
of the plankton at Station ALOHA (Zehr et al. 2001, Nature 412,
635-638). Both groups may contribute to the N2-primed
prokaryote carbon pump discussed previously, but with fundamentally
different ecological consequences due to variations in size.
key group of microorganisms, the diatoms, also share a common biomarker
pigment -- fucoxanthin. As with the cyanobacteria, not all diatoms
are created equal; some are single-celled while others grow into
long chains or large aggregates. Still others have endosymbiotic
N2-fixing cyanobacteria living with
them. These latter groups may, therefore, contribute to both the
event driven and the N2-primed carbon
pumps. As seen in this data set, there have been alternating 3-4
year periods of higher than average (the 12-yr climatology) and
lower than average euphotic zone depth integrated inventories of
fucoxanthin, again suggesting non steady-state population dynamics.
One major implication of this time variable population of diatoms
is that if you fertilize at one point in time versus another point
in time, you're likely to get a different result due to variations
in the "seed" populations.