The nitrogen physiology of the marine N-2-fixing cyanobacteria Trichodesmium spp.

Trichodesmium spp, have proved to be enigmatic organisms, and their ecology and physiology are unusual among diazotrophs, Recent research shows that t hey can simultaneously fix N-2 and take up combined nitrogen. The co-occurr ence of these two processes is thought to be incompatible, but they could b e obligatory in Trichodesmium spp, if only a small fraction of cells within a colony or along a filament are capable of N-2 fixation, Combined nitroge n is released from cells during periods of active growth and N-2, fixation, and concomitantly taken up by Trichodesmium spp, or cells living in associ ation with colonies, Although the nitrogenase of Trichodesmium spp, is affe cted by high concentrations of combined nitrogen, it might be relatively le ss sensitive to low concentrations of combined nitrogen typical of the olig otrophic ocean and culture conditions. Nitrogenase activity and synthesis e xhibits an endogenous rhythm in Trichodesmium spp, cultures, which is affec ted by the addition of nitrogen.