Salinity-coupled metal pollution alters microbial suppression-driven carbon sequestration in surface sediments at the land-water interface of the Hooghly River Estuary, India

Tropical estuaries, despite being hotspots of carbon sequestration and emission, are understudied in respect of stressors-microbial processes coupling. The objective of this study was to quantify microbial extracellular enzyme (EE) activities and associated changes in CO 2 emission at land-water interface (LWI) along a 170 km stretch of the Hooghly River Estuary, a tidal distributary of the Ganga River (India). Samples were collected during summer low flow for three consecutive years (2016–2018). The CO 2 emission and EE activities were significantly influenced by the concentration of Cd, Cr, Cu, Ni, Pb, and Zn. This relationship was found to become stronger as the bioavailable fractions of metal pollutants increased. We found significant negative correlations of the bioavailable fraction of metals with enzyme activities ( R 2 = 0.56–0.77; p < 0.001) and CO 2 emission ( R 2 = 0.79; p < 0.001). The CO 2 emission and EE activities increased with salinity up to 2.0 (practical salinity, PSS-78) but decreased thereafter as the salinity increased further. Even under relatively high bioavailability, low salinity conditions supported elevated enzyme activity and CO 2 emission, suggesting enhanced organic matter decomposition. However, the fixed factor regression ANOVA model and structural equation modelling revealed that the bioavailability of metal pollutants was the major factor suppressing EE activities and CO 2 emission. We show that metal pollution along human-impacted river-estuary gradient reduces organic matter degradation, enhancing C storage relative to flux. Understanding these patterns enable informed management decisions, highlighting how pollutant–salinity interactions modulate estuarine carbon sequestration and blue carbon sustainability in tropical estuaries.