Download Nitrogen fixation in the Ocean from cyanobacteria

EMBC
Biogeochemical Processes and Climate change
2009
Nitrogen fixation in the Ocean from cyanobacteria
Recent studies in fields of biological oceanography, molecular ecology, geochemistry and
paleoecology all tend toward a same conclusion: the role of cyanobacteria in Nitrogen
cycling in the oceans has been underestimated so far. A low concentration in 15N is the main
indicator of the occurrence of biological fixation. Such situation has been observed in tropical
and subtropical ecosystems, which are usually highly stable.
Nitrogen fixation by cyanobacteria happens in the surface mixed layer of oceans. A specific
index N* has been developed, which is “a linear expression of the relative regeneration of
nitrate and phosphate” in ocean cycles. A positive value of N* is a signature of biological
fixation of nitrogen before it reaches the sediments. Organisms which can fixate N2, making
it available for other organisms, are called diazotrophs. N2-fixation is highly correlated to
atmospheric carbon absorption. Paleology studies even draw a link between global evolution
of the earth climate and the activity of these organisms.
Trichodesmium, a non-heterocytous cyanobacteria, is the most studied cyanobacteria genus
due to its ease of detection by satellite (light emission). Trichodesmium is found both as
microscopic free filaments called trichomes (about 100 cells) and as macroscopic aggregates
composed of several hundred trichomes. However, the accuracy of satellite evaluation is
controversial. In order to better account for all organisms (other cyanobacteria) able of
nitrogen fixation new molecular techniques have been developed, which track a gene
associated with the activity of the enzyme Nitrogenase. Nitrogenase is a highly conserved
enzymcomplex, which is irreversibly inhibited by molecular oxygen and reactive oxygen
species. Diazotrophs have developed mechanisms and morphological traits to avoid the
contact of those two molecules; some of these are explained here after:
(i) fixing nitrogen under microaerobic conditions (eg. Plectonema spp., Phormidium, and
some species of Pseudoanabaena,
(ii) temporal separation of photosynthesis and N2-fixation (at night-time); in nonheterocystous filamentous Symploca and Lyngbya majuscula, and the unicellular Gloeothece,
Synechoccus and Cyanothece.
(iii) temporal and spatial separation of photosynthesis and N2-fixation (at day-time) in
Trichodesmium. N2-fixation happens for about 6 hours in the middle of the photoperiod,
while photosynthesis is inhibited. Cells can turn photosynthetic activity on or off within 10–
15 min. Individual cells modulate oxygen production and consumption during the
photoperiod.
(iv) spatial separation. N2-fixation in heterocysts (eg. Anabaena variabilis). Heterocysts are
micro-anaerobic cells, characterized by a thick membrane that slows the diffusion of O2.
Some species of picoplankton, heterotophic bacteria and diatoms (from endosymbiosis with
cyanobacteria) also display such characteristcs. Synechoccus species have drawn the specific
attention of scientists due to their high abundance in marine waters. Nitogen fixation by such
organisms is thus to take into account for N-cycling.
Nitrogen fixation by diazotrophs is limited by phosphorus and iron availability in the water, as
these are both involved in nitrogenase enzyme complex. Understanding processes that control
nitrogen fixation can allow us to evaluate the capacity of oceans to absorb carbon and, in the
long run, to make predictions on consequences of global climate change. However, some
aspects of nitrogen cycling are still to study. Specifically, the fate of fixed nitrogen is badly
understood as cyanobacteria only undergo grazing by specialized copepods. Other
hypotheses focus on the production of DON by extracellular release, viral lysis of
cyanobacteria, and remineralization by bacterial exoenzyme activity.
Camille Vogel, Maria Klein