Geoscience Reference
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without the added dye. Some of the photosynthetic pigments in oxygen-
producing phototrophs will do this, and because these fluorescing cells
were tiny, John suspected they were cyanobacteria. This turned out to
be the case, and marine
Synechococcus
was discovered.
But this was not the last word on important cyanobacterial popula-
tions in the sea. At around the same time that
Synechococcus
cells were
identified as an abundant species, other similar-looking cyanobacterial
populations were also observed. These organisms contained a somewhat
different internal structure than
Synechococcus
, and the chlorophyll pig-
ments were also a little different; in the absence of more detailed studies,
these organisms were initially classified as
Synechococcus
cells. This changed
in the late 1980s when Penny Chisholm from MIT was using a rela-
tively new technology called flow cytometry to explore the nature of
photosynthetic populations in the sea. Using this technique, one can
distinguish populations based on their size and on their ability to fluo-
resce under different wavelengths of light. Many populations, such as
Synechococcus,
generate distinctive signals, yielding an excellent tool for
quantifying their population size. In applying flow cytometry, Penny
was indeed able to find and quantify populations of
Synechococcus.
An-
other population of fluorescent cells also appeared, however, and these
differed from
Synechococcus
in many important ways. First, with diameters
of a mere 0.6 to 0.8 microns, these tiny cells were much smaller than typi-
cal
Synechococcus
cells. hile they were clearly cyanobacteria, they also
had unique pigments not found among
Synechococcus
, or any other pho-
totrophic organism for that matter. They were also abundant, even more
abundant than
Synechococcus
, at least in some places; and while
Synechococ-
cus
tended to prefer the upper, well-lit regions of the water column, this
new population preferred the deeper, darker regions. These were, indeed,
the same
Synechococcus
-like cells reported earlier. After noting similarities
with a group of symbiotic cyanobacteria of the genus
Prochloron
, Penny
named these new cyanobacteria
Prochlorococcus
.
So, within about 10 years, our understanding of marine phototrophic
communities completely changed. But there is even more to the story.
These tiny cyanobacteria, both
Synechococcus
and
Prochlorococcus,
are not
only abundant, but they can contribute up to 50% or more of the pri-
mary production in some places of the ocean. So, even though they are
hard to see, cyanobacteria are a big deal in the carbon cycle.
