Drs. Capone and Carpenter propose to continue their investigations of the biochemistry, physiology and ecology of N2 fixation in planktonic microorganisms, and particularly the non-heterocystous cyanobacterium Trichodesmium. These studies are important as Trichodesmium is a substantial component of the phytoplankton populations of tropical and subtropical seas and a major contributor of new nitrogen to these systems. Furthermore, the nitrogenase enzyme system of Trichodesmium appears distinct in several respects from the more well studied non-heterocystous analogues. An understanding of the biochemical and molecular mechanisms used by Trichodesmium to maintain active nitrogen fixation would be of broad relevance. They will determine the factors (biochemical and ecological) which regulate N2 fixation and growth in Trichodesmium, as well as its importance in the marine food web. Specifically, they will pursue studies of: 1) the mechanisms for protecting nitrogenase from oxygen deactivation (oxygen protective enzyme systems such as superoxide dismutase, uptake hydrogenases etc.); 2) factors controlling the apparent endogenous rhythms of nitrogenase synthesis and activity, and of regulation of nitrogenase synthesis and activity, ATP/ADP ratios); 3) how fixed nitrogen is transferred between N2 fixing and non-fixing cells within a colony; 4) the causes and significance of diel changes in buoyancy; and 5) the fate of nitrogen and carbon recently fixed by Trichodesmium in near surface waters. This research program is unique in that it brings together a diverse group of investigators to work on a broad array of topics concerning the biology and ecology of Trichodesmium and other open ocean nitrogen fixers.
