Elucidation of dominant energy metabolic pathways of methane, sulphur and nitrogen in respect to mangrove-degradation for climate change mitigation.

Mangroves play a key role in ecosystem balancing and climate change mitigation. It acts as a source and sink of methane (CH4), a major greenhouse gas responsible for climate change. Energy metabolic pathways of methane production (methanogenesis) and oxidation (methanotrophy) are directly driven by sulphur (S) and nitrogen (N) metabolism and salinity in coastal wetlands. To investigate, how mangrove-degradations, affect the source-sink behaviour of CH4; the pathways of CH4, S and N were studied through whole-genome metagenomic approach. Soil samples were collected from degraded and undisturbed mangrove systems in Sundarban, India. Structural and functional microbial diversities (KEGG pathways) of CH4, S and N metabolism were analysed and correlated with labile carbon pools and physico-chemical properties of soil. Overall, the acetoclastic pathway of methanogenesis was dominant. However, the relative proportion of conversion of CO2 to CH4 was more in degraded mangroves. Methane oxidation was higher in undisturbed mangroves and the serine pathway was dominant. After serine, the ribulose monophosphate pathway of CH4 oxidation was dominant in degraded mangrove, while the xylulose monophosphate pathway was dominant in undisturbed site as it is more tolerant to salinity and higher pH. The assimilatory pathway (AMP) of S-metabolism was dominant in both systems. But in AMP pathway, adenosine triphosphate sulfurylase enzyme reads were higher in degraded mangrove, while NADPH-sulfite reductase abundance was higher in undisturbed mangrove due to higher salinity, and pH. In N-metabolism, the denitrification pathway was predominant in degraded sites, whereas the dissimilatory nitrate reduction pathway was dominant in undisturbed mangroves. The relative ratios of sulphur reducing bacteria (SRB): methanogens were higher in degraded mangrove; however, methanotrophs:methanogens was higher in undisturbed mangrove indicated lower source and greater sink capacity of CH4 in the system. Microbial manipulation in mangrove-rhizosphere for regulating major energy metabolic pathways of methane could open-up a new window of climate change mitigation in coastal wetlands.