Analysis suggests that feedbacks between carbon (C), nitrogen (N), and oxygen (O) cycles helped prevent the oxidation of Earth in the Paleoproterozic. This stabilizing feedback, which was ultimately overridden, led to the contemporary nitrogen cycle where nitrate, rather than ammonium, was the stable form of fixed inorganic nitrogen in the oceans. Barring some minor changes in the trace element composition in nitrogenases, the core proteins remained essentially unchanged following the transition to an oxidized atmosphere. In the contemporary ocean, approximately 20%-30% of nitrogenase activity is inhibited at any moment in time by O2. This inhibition results in a negative feedback which constrains the upper level of O2 on Earth. Three central aspects of cyanobacterial nitrogen fixation remain curious. First, although some trace elements have been altered in the evolution of nitrogenases, the core proteins have remained virtually unchanged. Second, while there is abundant evidence of lateral transfer of nitrogenase genes between prokaryotes, in the endosymbiotic appropriation of cyanobacteria into heterotrophic hosts to photosynthetic eukaryotes, nitrogenases were lost. Third, although free-living heterocystous cyanobacteria are abundant in lakes and brackish water ecosystems, they appear to be rare in the open ocean. © 2008
|Title of host publication||Nitrogen in the Marine Environment|
|Number of pages||27|
|State||Published - 2008|
Bibliographical noteFunding Information:
We thank two anonymous reviewers for their constructive comments. Y.G.was supported by the following programs: The US NASA Ocean Biology and Biogeochemistry Program (Award # NNG04G091G).