Molecular ecology of nitrogen fixation
 in the sea: From gene expression
 to global carbon cycling

Jonathan P. Zehr, Ph.D.
University of California, Santa Cruz

Wednesday, September 5, 2001
3:00 p.m.–Pacific Forum

The availability of nitrogen is one of the crucial factors constraining oceanic primary productivity and, ultimately, controlling global atmosphere-ocean carbon balances. Nitrogen-fixing organisms should be keystone microorganisms in nitrogen-limited waters, and yet only a few nitrogen-fixing microorganisms have been previously reported from oceanic waters.

Nitrogenase genes and nitrogenase gene transcripts (mRNA), indicative of active transcription of the nitrogenase genes, were detected from surface waters at station ALOHA in the North Pacific Subtropical Gyre. Nitrogenase genes were expressed in
a distinct daily pattern throughout the upper 150 meters of the water column. Analysis of the gene sequence led to identification and cultivation of a novel unicellular cyanobacterium, which appears to be an important, previously unrecognized nitrogen fixer particularly in the North Pacific Subtropical Gyre. These results have important implications for the global controls on primary productivity by the availability of nitrogen.