| Summary: | The present study reviewed the carbon-biogeochemistry-related observations concerning CO<sub>2</sub> and CH<sub>4</sub> dynamics in the estuaries adjoining the Indian Sundarbans mangrove ecosystem. The review focused on the partial pressure of CO<sub>2</sub> and CH<sub>4</sub> [<i>p</i>CO<sub>2(water)</sub> and <i>p</i>CH<sub>4(water)</sub>] and air–water CO<sub>2</sub> and CH<sub>4</sub> fluxes and their physical, biogeochemical, and hydrological drivers. The riverine-freshwater-rich Hooghly estuary has always exhibited higher CO<sub>2</sub> emissions than the marine-water-dominated Sundarbans estuaries. The mangrove sediment porewater and recirculated groundwater were rich in <i>p</i>CO<sub>2(water)</sub> and <i>p</i>CH<sub>4(water)</sub>, enhancing their load in the adjacent estuaries. Freshwater-seawater admixing, photosynthetically active radiation, primary productivity, and porewater/groundwater input were the principal factors that regulated <i>p</i>CO<sub>2(water)</sub> and <i>p</i>CH<sub>4(water)</sub> and their fluxes. Higher chlorophyll-<i>a</i> concentrations, indicating higher primary production, led to the furnishing of more organic substrates that underwent anaerobic degradation to produce CH<sub>4</sub> in the water column. The northern Bay of Bengal seawater had a high carbonate buffering capacity that reduced the <i>p</i>CO<sub>2(water)</sub> and water-to-air CO<sub>2</sub> fluxes in the Sundarbans estuaries. Several authors traced the degradation of organic matter to DIC, mainly following the denitrification pathway (and pathways between aerobic respiration and carbonate dissolution). Overall, this review collated the significant findings on the carbon biogeochemistry of Sundarbans estuaries and discussed the areas that require attention in the future.
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