Nitrogen-fixation strategies and Fe requirements in cyanobacteria

Diazotrophic (nitrogen-fixing) cyanobacteria are important contributors of new nitrogen to oligotrophic environments and greatly influence oceanic productivity. We investigated how iron availability influences the physiology of cyanobacterial diazotrophs with different strategies for segregating nitrogen fixation and photosynthesis. We examined growth, photosynthesis, nitrogen fixation, and Fe requirements of the filamentous nonheterocystous Trichodesmium, the filamentous heterocystous Anabaena, and the unicellular Cyanothece under a range of Fe concentrations. Under similar Fe concentrations the three species differed in N₂-fixation rates, photosynthetic activity, the relative abundance of the photosynthetic units PSI:PSII, elemental stoichiometry, and Fe use efficiency. Complex colonial forms such as Trichodesmium and Anabaena are more likely to be Fe limited in their natural environments and are more efficient at utilizing Fe than unicellular diazotrophs such as Cyanothece. The varied physiological responses to Fe availability of the three cyanobacteria reflect their nitrogen-fixation strategies, cell size, unicellular or colonial organization, and may explain, at least in part, the ecological distribution of these photosynthetic bacteria.