Nitrogen Fixation in Free Living Cyanobacteria

Among the free-living diazotrophs a prevailing interest is that addressed to cyanobacteria. This interest comes from the wide and abundant distribution of these microorganisms in all terrestrial and aquatic ecosystems as well as from their unique photosynthetic metabolism that makes the nitrogen fixation an apparently paradoxical event. Cyanobacteria, in fact, are the only prokaryotes that carry out oxygenic photosynthesis.

A very great number of these microorganisms is able to both fix N2 under aerobic conditions and produce O2 by photosynthesis. Filamentous cyanobacteria resolve this oxygenic photosynthesis-diazotrophy paradox by segregating the oxygen-sensitive machinery for N2 fixation in specialized nonphotosynthetic cells named heterocysts, and by maintaining the oxygen evolving photosynthesis in the neighboring vegetative cells. Thus, the simultaneous operation of the two basically incompatible processes is made possible through their spatial separation.

Nitrogen starvation leads to the appearance at regular intervals along the cyanobacterial filament of heterocysts which function as anaerobic factories for N2 fixation under external aerobic conditions. The ability to fix N2 ensues from changes that occur in vegetative cells that differentiate to heterocysts (Figure 6).

First they build up a very thick cell wall with an innermost laminated glycolipid layer, whose function is to provide an O2 permeability barrier to avoid the in-activation of nitrogenase inside the cell. The connections between heterocyst and neighbor cell occur through thin cytoplasmic channels (microplasmodesmata), which traverse the septum separating the two cells and the plag (polar body) filling the adjacent region. In addition, the photosynthetic apparatus undergoes a deep reorganization during the heterocyst differentiation: phycobilisomes disappear and the oxygen-evolving photosystem II is totally dismantled, while photosystem I, which produces ATP through cyclic photophosphorylation, persists in thylakoid membranes. The ATP necessary to fulfill the energy demand for nitrogenase activity, in fact, comes in heterocysts from cyclic photophosphorylation, while the reductant for the N2-fixing enzyme is furnished by neighboring photosynthetic cells as maltose. The sugar hydrolysis and glucose oxidation through the pentose

Thickened cell wall

Innermost laminated layer Polar body Microplasmodesmata

Glutamate

GS Glutamine

N2ase

ATP Fdx,

Metabolites f

Glutamate

Glutamate

GOGAT

Glutamine a-Ketoglutarate

N2ase

ATP Fdx,

GOGAT

Glutamine a-Ketoglutarate

Figure 6 Schematic drawing of a cyanobacterial heterocyst showing nitrogen fixation and metabolite exchange with the neighboring vegetative cell. Fdxred, reduced ferredoxin; gluc6P, glucose-6-phosphate; 6Pgluc, gluconate-6-phosphate; GOGAT, glutamate synthase; GS, glutamine synthetase; N2ase, nitrogenase; PSI, photosystem I; PSII, photosystem II; rib5P, ribulose-5-phosphate.

Heterocyst

Vegetative cell

Figure 6 Schematic drawing of a cyanobacterial heterocyst showing nitrogen fixation and metabolite exchange with the neighboring vegetative cell. Fdxred, reduced ferredoxin; gluc6P, glucose-6-phosphate; 6Pgluc, gluconate-6-phosphate; GOGAT, glutamate synthase; GS, glutamine synthetase; N2ase, nitrogenase; PSI, photosystem I; PSII, photosystem II; rib5P, ribulose-5-phosphate.

phosphate pathway produces NADPH used for ferre-doxin reduction.

The heterocysts of free-living cyanobacteria contain high levels of glutamine synthetase (GS), but are deficient in GOGAT that, on the contrary, is active in vegetative cells. Thus, after N2 reduction, the NH3 assimilation in glutamine is carried out in the heterocyst while the successive reaction leading to glutamate synthesis occurs in the near vegetative cell into which glutamine moves through microplasmodesmata.

A major role in protecting nitrogenase against O2 is played in heterocysts by the thick cell wall which prevents gas diffusion toward the cell. However, it is unlikely that this envelope provides a truly impermeable gas diffusion barrier, since this would also exclude nitrogen from the fixation site. Moreover, gases can reach the heterocyst cytoplasm through the junctions between them and the contiguous vegetative cells which produce O2 by photosynthesis. Thus, also cyanobacterial heterocysts, as the other aerobic diazotrophic organisms, need systems to remove oxygen that enters the cell. These include enhanced rate or respiration, presence of hemoproteins in the cytoplasm peripheral region, and activity of an uptake hydrogenase.

A great ecological interest arose from the unexpected finding that unicellular and nonheterocystous filamentous cyanobacteria are also able to both fix nitrogen and carry out oxygenic photosynthesis. These cyanobacteria, which are very abundant in the phytoplanktonic populations of marine environments, are responsible for most of the photosynthetic organic carbon provided to the ecosystem, and they may also account for a high percentage of the nitrogen fixed biologically worldwide.

The oxygenic photosynthesis-diazotrophy paradox is resolved by these nonheterocystous cyanobacteria with the temporal separation of the two physiological processes that should necessarily occur in the same cell. They carry out only the oxygenic photosynthesis during the day and fix N2 only during the night, when the photosynthetic O2 production does not occur. This timing of N2 fixation is also related to the fact that the nitrogen-ase is active exclusively during the dark period. Interestingly, the daily oscillation of nitrogenase activity occurs according to an endogenous circadian rhythm.

The finding that the nitrogenase of nonheterocystous N2-fixing cyanobacteria possesses this kind of rhythmic activity was also of great scientific importance since it was the first clearly recognized circadian rhythm in prokar-yotic organisms, which led to the backdrop of the former dogma that restricted the biological clocks to eukaryotes.

See also: Fermentation; Grazing.

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