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The open ocean environment is typified by low concentrations of inorganic nutrients and trace elements, and most turnover rates are high. Nitrogen fixation has only recently been recognized as a quantitatively important process in the open ocean (12,16). Since concentrations of ammonium and nitrate are typically in the low nanomolar range, N2-fixing species should have an ecological advantage in the open ocean. For most of the last four decades, it has been assumed that the major, and perhaps the only, nitrogen fixers were the free-living filamentous, colony-forming, nonheterocystous cyanobacteria of the genera, Trichodesmium (6), and symbiotic heterocystous cyanobacteria (Richelia) (20), which live in association with certain genera of diatoms. A little more than half of the global biological nitrogen fixation occurs in the ocean (141 Tg N yr -1 in the pelagic zone alone; 10,11), of which 10% (16) to over 50% (10) has been ascribed to Trichodesmium (85 Tg N yr -1 ). Several years ago, we detected nitrogenase (specifically nifH) genes in the Pacific Ocean subtropical gyre as well as in the Atlantic Ocean, which were not derived from either Trichodesmium or Richelia. The phylogenetic analysis of these nifH genes suggested that some of the organisms were unicellular cyanobacteria because the sequences clustered with those of Cyanothece, Gloeothece and Synechococcus (renamed Cyanothece) (Figure 1). Microscopic observations confirmed their presence in the plankton (22). Some isolates of unicellular cyanobacteria had previously been recovered from the marine environment, but were not believed to be abundant in the oceans. The unicellular cyanobacteria are much more difficult to observe than the colony-forming Trichodesmium or the symbionts in diatoms. These include Cyanothece sp. ATCC 51142, Synechococcus sp. BG 043511, and Crocosphaera (marine Synechocystis) sp. WH501. The strain of Cyanothece, which has been the most studied (strain ATCC 51142), was isolated from an inter-tidal area in the Gulf of Mexico. The “Synechococcus” appears to be in the Cyanothece group and is reported to have been isolated from shallow waters in the Bahamas (21). True marine Synechococcus are |
