IRON STIMULATION OF PHOTOSYNTHESIS AND NITROGEN FIXATION IN ANABAENA 7120 AND TRICHODESMIUM (CYANOPHYCEAE)

Abstract

Iron availability may limit carbon and nitrogen fixation in the oceans. The freshwater cyanobacterium, Anabaena, was used as a laboratory model for the biochemical and physiological effects of iron. Increased iron nutrition, in the range of 10−8 M to 10−6 M resulted in increases of approximately four fold in carbon and nitrogen fixation rates. Chlorophyll concentration increased, and the relative amount of in vivo fluorescence was reduced with more iron. Natural samples of Trichodesmium, collected off Barbados and incubated with increased iron for two days, showed similar effects. Trichodesmium responded to iron additions indicating that it may be Fe limited in its natural environment. These responses to iron are consistent with the biochemical roles of iron in photosynthesis and nitrogen fixation. The results are discussed in the geochemical context of the sporadic total iron input to tropical oceans and possible implications to spatial and temporal patterns of productivity.