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Unicellular cyanobacteria and N2 fixation in the ocean

Nitrogen is often the limiting nutrient in the ocean, and so it is of vital interest to determine the degree to which biological nitrogen fixation takes place and what bacteria are responsible. For many years, the prime suspect has been the nonheterocystous, filamentous cyanobacterium Trichodesmium, but recent evidence indicates that the abundance of the cyanobacterium is not sufficient to account for observed levels of N2 fixation. Jonathan Zehr (U. California at Santa Cruz) and others now report that we need to look beyond Trichodesmium, demonstrating that highly abundant unicellular cyanobacterium also fix N2.

Zehr et al [Nature (2001) 412:635-638; comment 412:593-595] used RT-PCR with universal nifH primers to measure the amount of nif transcripts in ocean samples taken at various times and filtered to remove Trichodesmium. For samples taken at a depth of 25 m, nif expression was high during at night and low during the day, while with samples from 50 m and 100 m, the pattern of expression was reversed. Nitrogenase expression in Trichodesmium is controlled by circadian rhythm, so that levels are high during the day [Chen et al J Bacteriol (1998) 180:3598-3605]. Sequences of most of the RT-PCR products proved to be similar to those from cultivated unicellular cyanobacteria, while the remainder showed similarity to nif genes from proteobacteria.

It is difficult to assess the amount of N2 fixed by unicellular cyanobacteria as compared to Trichodesmium, and it remains to be shown whether N2-fixing unicellular strains have a range beyond the subtropical North Pacific Ocean station where the samples were taken. Nonetheless, the now proven existence of a new source of biological N2 fixation in the open ocean should be enough to send modelers scurrying back to their computers.