Getting a Better Picture of the Ocean's Nitrogen Budget

摘要

The most common nitrogen-fixing cyanobacterium in the oceans is more abundant than previously estimated and can therefore account for at least a portion of the apparent oceanic nitrogen deficit. The deficit in the biologically available nitrogen supply required to support new production (photosynthetic activity that results in permanent carbon fixation) in tropical and subtropical oceans represents one of the most intriguing conundrums in oceanography Upwelling of the deep, nutrient-rich waters through mechanisms of winter mixing due to storm activity, eddy pumping, and diffusion can supply only a fraction (40 to 60%) of the necessary nitrate (I). The remainder must be provided by reduction of atmospheric dinitrogen (N_2), which requires breaking the ultrastable triple nitrogen-nitrogen bond. In the ocean, most of this process is performed by a nonheterocystous photosynthetic prokaryote, Trichodesmium sp The estimates of nitrogen fixation by this dia-zotroph average ~90 Tg N year1 (2), which represents less than 50% of the required new nitrogen (5) (1 Tg = 10~12 g) During the past 10 years, a substantial effort has been expended to account for the missing part in the ocean's nitrogen budget Researchers have sought to reassess the diazotrophic activity of Trichodesmium or to identify other organisms that may fix the "missrng" nitrogen. Although new, single-cell dra-zotrophic bacteria have been identified (4), their potential contribution to nitrogen fixation has been difficult to quantify On page1517 of this issue, Davis and McGillicuddy (.5) decided to have another "look" at this problem, literally Using a towed undulating video camera, they captured images of Trichodesmium colonies (in puff and tuft morphologies; see the figure) over large areas of the Sargasso Sea. On the basis of the unexpectedly high abundance of these colonies, they postulate that the apparent nitrogen deficit in the world's oceans is simply due to an accounting error, .