Summary
Nitrogen, being one of the key abundant elements in biomass, is an important limiting nutrient in terrestrial and aquatic systems. Biological nitrogen fixation, the reduction of abundant atmospheric dinitrogen gas to biologically available ammonium is a key nitrogen input in the oceans. In oligotrophic open ocean waters, nitrogen fixation is an important source of “new” nitrogen that balances losses due to sinking of particulate matter to the deep ocean. For years, nitrogen fixation was believed to be largely due to the filamentous cyanobacterium Trichodesmium, an easily recognized microorganism using microscopy. The application of molecular biology and subsequently genomic technologies exploded this view with recognition of diverse bacteria and cyanobacteria, however, most of these species can only be detected by their DNA sequences and not visualized by microscopy. Nonetheless, it has been found that diverse species are widespread in the global ocean. One of these species, called UCYN-A, was found to be a very unusual cyanobacterium with a small genome, that is missing basic cyanobacterial metabolic pathways including photosynthesis, and ultimately was shown to be a symbiont of a specific coccolithophorid (Braarudosphaera bigelowii). Despite the fact that this organism was abundant and geographically widespread, little was known because of the inability to culture or to visualize in the sea. The recent successful culture and the application of advanced biology methods showed that in fact UCYN-A is not a simple symbiont, but is a nitrogen-fixing organelle, the first to be described, and making the host alga the first known nitrogen-fixing eukaryote. Technical challenges remain in the study of ocean nitrogen fixation, making it still difficult to assess the relative importance of Trichodesmium, B. bigelowii/UCYN-A and the other recently discovered nitrogen-fixing microbes in the balance of nitrogen in the sea.
Short biography
Dr. Zehr is a microbial oceanographer and is Distinguished Professor Emeritus of Ocean Sciences at the University of California, Santa Cruz. He received his B.S in Biology from Western Washington University in 1981 and his Ph.D in Ecology from the University of California, Davis, in 1985. Dr. Zehr did postdoctoral work in nitrogen cycling at the State University of New York, Stony Brook and Brookhaven National Laboratory, and in molecular biology at New England Biolabs, Inc. Dr. Zehr began his current position at UCSC in 1999. He is a Fellow of the Academy of Microbiology and the American Association for the Advancement of Sciences. His research has spanned nitrogen cycling by aquatic microorganisms and microbial diversity in freshwater, hypersaline and oceanic systems. His primary focus has been on nitrogen fixation in oligotrophic oceans, including the cyanobacterium Trichodesmium and the discovery of the globally important N2-fixing symbiont UCYN-A.