Abstract

Trichodesmium and other diazotrophs are common in tropical and subtropical oligotrophic waters where they fix dinitrogen (N2), thereby introducing new N and potentially alleviating system-wide N-limitation. While we know much more about N2 fixation by Trichodesmium, unicellular diazotrophs may have a more cosmopolitan global distribution and therefore may have a greater contribution to total marine N2 fixation. Although new production by marine N2 fixers has been quantified in recent years, growth rates, N2 fixation rates and nitrogen and carbon based doubling times all vary widely within and among studies of Trichodesmium. Recent research using cultured and natural populations suggest that one reason for these widely varying rates includes rapid recycling of elements, particularly N and P. Studies suggest that up to 90% of the recently fixed N2 can be rapidly regenerated as ammonium and dissolved organic nitrogen. These N compounds are readily available to co-occurring organisms including both autotrophs and heterotrophs. Similarly, P regeneration rates and uptake of regenerated P is also elevated in and around Trichodesmium colonies. In the Gulf of Mexico, blooms of Karenia brevis (red tides) commonly occur simultaneously or subsequent to Trichodesmium blooms. Evidence also suggests that regeneration of organic compounds fuels heterotrophic microbial activity and regenerated production in the euphotic zone. This may contribute to respiratory loss of recently fixed C from the ocean rather than substantial export as sinking particles, however the fate of new production from N2 fixation has not been broadly examined and the stoichiometric relationships between C, N and P uptake.

Light refreshments are served in the Interaction Area (4th floor of the Oceanography/Physics Building) at 4:00 p.m.

All seminars begin at 3:00 p.m. and are held in room 200 of the Oceanography/Physics building.