from an early, intracellular symbiosis of cyanobacterial cells in the macrophytes (Plate
2.1).
(^) One genus of filamentous cyanophytes, Trichodesmium, lives in tropical seas.
“Tricho”, as it is known to students of its biology, can generate gas vacuoles and float
at the sea surface. It may also regulate buoyancy to move up and down, harvesting
nutrients at depth then rising back toward brighter light. Like some genera of
freshwater cyanophytes, it can fix gaseous nitrogen, N 2 , into ammonium and
nitrogenous organic molecules. Thus, given some phosphorus and trace elements,
nitrogen limitation is alleviated, and the filaments can develop into extensive near-
surface blooms. Trichodesmium blooms occur occasionally in all tropical waters, and
are particularly common in the Arabian Sea and Red Sea probably due to abundant
dust input (supplying iron), very warm waters and prolonged calm spells. When the
Red Sea is red, the color usually comes from Trichodesmium mats. Another nitrogen-
fixing, filamentous cyanophyte, Richelia intracellularis, is a symbiont of several
diatom genera: Rhizosolenia, Chaetoceros and Hemiaulus. Nitrogen fixation by
Richelia allows occasional blooms of these diatoms in the oligotrophic, central gyres.
Moisander et al. (2010) studied the distribution of two unicellular N 2 -fixing
cyanobacteria, Crocosphaera watsonii and UCYN-A, by measuring the presence of
the gene encoding the iron-protein in the nitrogenase enzyme. Results showed that
UCYN-A has a broader latitudinal distribution than Trichodesmium, implying that
significant N 2 fixation may occur in temperate, as well as, tropical waters.
Interestingly, UCYN-A lacks the enzyme for assimilating CO 2 and releasing O 2
(Bothe et al. 2010). This protects the O 2 -sensitive nitrogenase, but as a consequence
UCYN-A is dependent on extra-cellular sources of organic carbon.
Prochlorococcus
(^) The organisms most recently recognized as an important component of the
phytoplankton are the picoplanktonic (<2 μm) cells of the genus Prochlorococcus
(Chisholm et al. 1988). Genetic analysis revealed that Prochlorococcus is a
cyanobacterium closely related to Synechococcus. Prochlorococcus can be the most
abundant phytoplankton in oligotrophic, tropical waters, with its latitudinal
distribution ranging from 45°N to 40°S, and it can account for a substantial fraction of
primary productivity (Olson et al. 1990). Like Synechococcus, Prochlorococcus is a
prokaryote without a membranous nuclear envelope (Fig. 2.1). These organisms have
a layered cell wall braced with murein, the wall polymer of bacteria. They have
several other distinguishing biochemical features, particularly dominance of the
photosynthetic pigments by a “divinyl chlorophyll pigment”, rather than the usual
form of chlorophyll-a dominant in all other plants (Wu & Rebeiz 1988). They share
with the Chlorophyta (green algae) and higher plants the inclusion of another