Pelagic Oxygen Minimum Zone Microbial Communities
Marine oxygen minimum zones (OMZs) are oxygen-starved regions of the ocean harboring diverse microbial communities that drive biogeochemical processes of global significance, particularly those associated with the nitrogen cycle, with resulting impacts on
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tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Phylogenetic Diversity of OMZ Prokaryotes . . . . . . . . . . . . . . . 113 Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Archaea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 OMZ Prokaryotes as Biogeochemical Players . . . . . . . . . . . . . 119 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Introduction Oxygen minimum zones (OMZs) are regions of the global ocean in which dissolved oxygen in the water column is reduced or totally absent due to poor ventilation, sluggish circulation, and a high demand of oxygen by microbial aerobic respiration. Open-ocean OMZs are prominent in the eastern tropical and subarctic Pacific and the northern Indian Oceans (> Fig. 7.1). The actual concentration of dissolved oxygen varies among recognized OMZs, and determining whether or not they reach total anoxia based exclusively on oxygen measurements has until recently been a problem due to technical limitations (Revsbech et al. 2009; Thamdrup et al. 2012). This is highly relevant because microbial-encoded enzymes mediating aerobic and anaerobic transformations of elements (e.g., nitrogen, sulfur, and carbon) manifest different oxygen sensitivities. It has long been recognized that many OMZs are hotspots for oxygen-sensitive nitrogen transformations, where nitrate serves as the main terminal electron acceptor for the oxidation of organic matter (Lam and Kuypers 2011). In such cases, denitrification and anaerobic ammonium oxidation (anammox) contribute to the removal of fixed nitrogen as N2, with resulting impacts on global nutrient cycles and the climate system (Codispoti et al. 2001). Geochemical signs of the functioning of these anaerobic processes include the presence of an inorganic fixed nitrogen deficit relative to phosphorus in addition to the accumulation of nitrite and excess N2 in the oxygen-deficient regions of the water column. Processes occurring in the boundary regions of OMZs
also contribute to the production of the potent greenhouse gas nitrous oxide (N2O), due primarily to the activity of nitrifiers at low oxygen levels. Thus, continued OMZ expansion is an emerging environmental concern, as it will likely exacerbate the loss of fixed nitrogen from the ocean in addition to increasing N2O production (Keeling et al. 2010; Codispoti 2010). Certain coastal regions also develop periods of oxygen starvation during part of the year, either naturally or induced by anthropogenic eutrophication, affecting marine ecosystems and coastal economies (Diaz and Rosenberg 2009). Some examples of where this phenomenon occurs include the continental shelves off Namibia, western India, and in the Gulf of Mexico (> Fig. 7.1). Moreover, in certain enclosed or semienclosed basins, such as inland seas (e.g., the Baltic Sea, the Black Sea), fjords (e.g., S
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