Biogeochemical cycling in the low oxygen marine environments
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Date
2020Author
Shenoy, D.M.
Naik, H.
Kurian, S.
Gauns, M.
Ward, B.
Burkill, P.
Smetacek, V.
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Biogeochemistry is the interdisciplinary science of the composition and cycling of chemicalelements. Nitrogen and oxygen are two elements that are essential for living organisms and the sustenance of life on Earth. While oxygen is needed for respiration, nitrogen is required for the genetic material, proteins, peptides and enzymes in all living organisms. Oxygen minimum zones (OMZs) are regions in the world’s oceans where the dissolved oxygen (DO) concentration in the
sub-surface waters falls below the normal levels (<90 µM; hypoxic). These regions are situated in the eastern tropical north and south Pacific, eastern tropical north and south Atlantic and the northern
Indian Ocean. When the DO concentrations fall below 5 µM (suboxic), the facultative bacteria switch to utilizing nitrate as a terminal electron acceptor in their respiration. This results in the loss of nitrogen through denitrification and anammox processes in the OMZs (please see marine nitrogen cycling in Fig. 1; Naqvi, 1991; Ward et al., 2009; Lam et al., 2011). Where the oxygen concentration is low, a suite of organisms (from microbes to mammals) can adapt and survive in these extreme conditions. In addition to these specialised pelagic regions, ~ 0.3% of the seafloor is permanently exposed to low oxygen conditions (Helly and Levin, 2004). In these regions, benthic organisms are adapted to low oxygen conditions and are involved in carbon preservation.
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