Systems Biogeochemistry of Major Marine Biomes. Группа авторов
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1.11. OXYGEN MINIMUM ZONE EXPANSION
Several studies have shown that oceanic OMZs are currently expanding as a result of global warming (e.g. Stramma et al., 2008; Schmidtko et al., 2017; Breitburg et al., 2018). Climate change caused by anthropogenic emissions of CO2 leads to an increase in surface seawater temperature, thereby decreasing the solubility of O2 in surface waters, leading to enhanced stratification (Bopp et al., 2002; Stramma et al., 2009) that could further prevent ventilation of the interior of the ocean.
Deutsch et al. (2011) predicted that an increase in surface ocean temperatures by 1°C could result in a threefold increase in seawater volume containing <5 μM O2. Expansion of OMZs could have a significant impact on the biogeochemical cycles of major elements such as Fe, P, and N in the ocean (Deutsch et al., 2011), ultimately influencing the burial of organic carbon (Ruvalcaba Baroni et al., 2020)
1.12. CONCLUSION
Marine oxygen minimum zones (OMZs) are typical biogeochemical milieus that play crucial roles in regulating the global ocean’s productivity and organic matter burial. The OMZ sediments are key sites for C–S–N cycling characterized by high labile organic matter preservation and diminished bioturbation activity, thereby exerting a strong influence on the benthic abundance, diversity, and composition biota, and gas and metal fluxes across the sediment–water interface. The present review has emphasized the significance of biogeochemical cycling in the OMZ sediments with respect to the pore fluid and solid phase geochemistry and microbial ecology, along with a brief account of the mechanism of formation of OMZ in the northern Indian Ocean. The OMZ in the northern Indian Ocean (AS and BoB) arises from high monsoon‐driven primary productivity, high respiratory O2 demand, and poor intermediate water ventilation owing to its landlocked geography. The AS in the northwestern Indian Ocean contains the thickest and most intense perennially DO depleted water mass in the world associated with SNM attributed to denitrification. In contrast, the OMZ in the BoB is less intense and thinner than the AS. The low DO concentration sufficient to support nitrite oxidation in the BoB inhibits denitrification to some extent.
Oxygen minimum zones host unique microbial communities that depend on different electron acceptors for metabolism, leading to fixed N loss and production of greenhouse gases. The high abundance and diversity of small benthic organisms (meiofauna and microbes) over macrofauna are observed in OMZs as a result of decreased predation and food competition. The prevalence of anammox over denitrification as the predominant N loss pathway is reported from the ETSP while the opposite is the case in the AS. Other than microbes capable of N compound metabolism, recent metagenomic and geochemical studies also identified sulfur oxidizing and reducing bacterial communities from the OMZ water column, suggesting its importance as a biochemical pathway in oxygen‐depleted conditions. Microbial sulfate reduction and methanogenesis are important biogeochemical processes in anoxic marine sediments. The prevalence and activity of sulfate reducing/sulfide oxidizing bacterial communities and methanogens/methanotrophs are also reported from the OMZ sediments.
The DO depleted water mass globally is increasing due to global warming and anthropogenic input of nutrients leading to eutrophication. The expansion of OMZs would also affect the intensity of biogeochemical processes in the underlying sediments, influencing the benthic fluxes across the sediment–water interface. High sulfate reduction rates and shallowing of SMTZ can lead to seepage of H2S into the water column, which has a deleterious effect on the benthic community. This effect has been reported from OMZs of Peru and Chile.
Perturbations in the homeostasis of the OMZs would have a momentous impact on the biogeochemical cycling of nutrients (particularly the C–N–S cycles) and the maintenance of ecological balance in the global ocean. Nonetheless, it is worthy of mention that in the future, new discoveries and research focusing on a deeper understanding of OMZ microbiology and geochemistry will unravel novel metabolic strategies or pathways in this critical ecosystem than have not been anticipated to date.
ACKNOWLEDGMENT
We acknowledge Director‐NIO and funding from CSIR for the research program. We thank the CSIR‐NIO publication committee for carrying out the internal review.
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