Here we report the discovery of cold-seep ecosystem and shallow methane hydrates (2-3 mbsf) associated with methane gas flares in the water column from the Indian EEZ for the first time. The seep-sites are located in the Krishna-Godavari (K-G) basin at water depths of 900-1800 m and are characterized by gas flares in the water-column images. The occurrence of methane gas hydrates at very shallow depths (2-3 mbsf) at some of the seep-sites is attributed to high methane flux and conducive P-T conditions, necessary for the stability of methane hydrate. Chemosymbiont bearing Bivalves (Vesicomidae, Mytilidae, Thyasiridae and Solemyidae families); Polychaetes (Siboglinidae family) and Gastropods (Provannidae family) are also identified from seep-sites.

Arabian Sea is one of the most productive regions of the world's ocean with seasonal upwelling and a characteristic oxygen minimum zone. It receives a continuous input of windborne iron-rich dust which possibly stimulates phytoplankton productivity. A sediment core from this area, which is on the western continental margin of India in the Kerala-Konkan basin was studied from the surface to 63 meters below seafloor, to establish the co-occurrence of microbial iron reducing activity and methane oxidation. Total bacterial abundance (TC), viable aerobic counts accounting for aerobic and facultative anaerobes (TVCa), viable anaerobic counts (TVCan), methane, sulfate, ferrous [Fe(II], hydroxylamine extractable iron and HCl extractable iron concentrations were measured. Average TC was 52.6±29.8 x 10 5 cells g -1 sediment while TVCa and TVCan were an order less. Methane and sulphate concentrations were 1.3 ppm and 23.2 mM, respectively. Average Fe(II) concentration, hydroxylamine and HCl extractable Fe was 0.32±0.26, 0.86±1.44 and 1.90±2.47 mmol g -1 sediment, respectively. Interestingly, Fe(II) significantly correlated with TVCan (r=0.66, p≤0.001). In the experimental tubes, the build-up of Fe (II) was accompanied by the disappearance of methane. Average methane concentration and Fe (II) concentration in the experimental sediment slurries was 0.56±0.76 ppb and 0.50±0.27 mmol g -1 sediment respectively. The microcosm experiment showed that Fe (II) determined was significantly higher (ANOVA, F=6.74, p≤0.014) after 5 days of incubation, thus implying microbial iron reducing activity. A strong negative linear correlation (p≤0.001) of methane concentration with Fe (II) suggests that increasing activity of microbial iron reduction M A N U S C R I P TA C C E P T E D ACCEPTED MANUSCRIPT 2 caused suppression in methane production. This is also indicative of the co-occurrence of iron reduction and methane oxidation in these sediments.

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