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Abstract
Nitrogen is the limiting nutrient in most of the surface ocean, and understanding the fate and supply of nitrogen in the marine system has been the subject of extensive research over the past few decades. Recent discoveries in nitrogen cycling (e.g., the anammox bacteria and the ammonia-oxidizing archaea) have raised questions about the pathways structuring marine N cycling. In the open ocean, new nitrogen for supporting primary production is supplied by nitrogen fixation and upwelling of nitrate from the deep ocean, although there is potential for a significant role for nitrification in supplying recycled nitrate as well. I measured rates of the first step of nitrification, ammonia oxidation, at three stations in the Arabian Sea, not only in the surface ocean but under low oxygen conditions, and in the mesopelagic zone to explore the controls on nitrification and the ammonia-oxidizing community. Ammonia oxidation rates were highest within the primary nitrite maximum, but detectable everywhere they were measured.
Nitrogen loss occurs through linked nitrification-denitrification, which together consume bio-available forms of dissolved inorganic nitrogen and convert them into nitrogen gas. One key side product of these pathways is N2 O, which is a potent greenhouse gas. The major sites of N-loss from the marine system are high-nutrient, coastal sediments and in oxygen-depleted areas of the water column (oxygen minimum zones, OMZs). In estuaries, like Chesapeake Bay, these N-loss processes are critical to ameliorate eutrophication. Using a functional gene microarray, I showed that nirS diversity decreased along an environmental gradient ranging from eutrophic headwaters to oligotrophic marine water, and the most abundant groups of denitrifiers were also the most active.
The final goal of this research was to investigate the interactions between N-loss pathways and the controls on nitrogen cycling in and around the Arabian Sea OMZ. The dynamics between the two major nitrogen loss pathways in the OMZs, anammox and denitrification, have been vigorously debated in the last few years. Although it has been proposed that the nitrogen cycle within the OMZs be re-written to exclude denitrification, my findings showed denitrification to be the dominant N loss pathway in the Arabian Sea OMZ.
