Systems Biogeochemistry of Major Marine Biomes. Группа авторов

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СКАЧАТЬ Name of OMZ O₂ (μM) In situ microbial processes detected so far (nitrogen metabolism) Microbiology or molecular technique used for decrypting biogeochemical process Taxonomic groups responsible for in situ metabolic process References 1 Eastern Tropical SouthPacific (ETSP) <detection limit Dissimilatory reduction of nitrate to ammonia (DNRA) Seafloor imaging Thioploca Sommer et al., 2016; Bohlen et al., 2011 Sulfur metabolism 2 Arabian Sea OMZ <detection limit sulfate reduction 16S rRNA gene and partial genes sequencing Desulfurococcus, Desulfurolobus, Fernandes et al., 2018 3 Arabian Sea OMZ <detection limit Sulfur oxidation Metagenome, Functional gene analysis, whole genome, pure culture isolation Cereibacter, Guyparkeria, Halomonas, Methylophaga, Pseudomonas, Sulfitobacter Thioploca Bhattacharya et al., 2020; Schmaljohann et al., 2001 Methane metabolism 4 Arabian Sea OMZ <detection limit Anaerobic methane oxidation 16S rRNA gene and partial genes sequencing – Fernandes et al., 2018, Bhattacharya et al., 2021

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.

REFERENCES

      1 Abdulla, H.A., Burdige, D.J. and Komada, T. (2020). Abiotic formation of dissolved organic sulfur in anoxic sediments of Santa Barbara Basin. Organic Geochemistry, 139, 103879. https://doi.org/10.1016/j.orggeochem.2019.05.009

      2 Acharya, S.S. and Panigrahi, M.K. (2016). Eastward shift and maintenance of Arabian Sea oxygen minimum zone: Understanding the paradox. Deep Sea Research Part I: Oceanographic Research Papers 115: 240–252. https://doi.org/10.1016/j.dsr.2016.07.004

      3 Agnihotri, R., Bhattacharya, S.K., Sarin, M.M. et al. (2003). Changes in surface productivity and subsurface denitrification during the Holocene: a multiproxy study from the eastern Arabian Sea. The Holocene 13 (5): 701–71. https://doi.org/10.1191/0959683603hl656rp

      4 Al Azhar, M., Lachkar, Z., Lévy, M. et al. (2017). Oxygen minimum zone contrasts between the Arabian Sea and the Bay of Bengal implied СКАЧАТЬ