Factors affecting N fixation by the cyanobacterium Trichodesmium sp. GBRTRLI101.

Abstract Various factors affecting N(2) fixation of a cultured strain of Trichodesmium sp. (GBRTRLI101) from the Great Barrier Reef Lagoon were investigated. The diurnal pattern of N(2) fixation demonstrated that it was primarily light-induced although fixation continued to occur for at least 1 h in the dark in samples that had been actively fixing N(2). N(2) fixation was dependent on the light intensity and stimulated more by white light when compared with blue, green, yellow and red light whereas rates of N(2) fixation decreased most under red light. Inorganic phosphorous concentrations in the lower range of treatments up to 1.2 muM significantly stimulated N(2) fixation and further additions promoted little or no increase in N(2) fixation. Organic phosphorous (Na-glycerophosphate) also stimulated N(2) fixation rates. Added combined nitrogen (NH(4) (+), NO(3) (-), urea) of 10 muM did not inhibit N(2) fixation in short-term studies (first generation), however it was depressed in the long-term studies (fifth generation).