Chemosynthetic activity prevails in deep-sea sediments of the Central Indian Basin.

It is hypothesized that in the deep-sea, under psychrophilic, barophilic and oligotrophic conditions, microbial community of Central Indian Basin (CIB) sediments could be chemosynthetic. In the dark, at near ambient temperature, 4 ± 2°C, 500 atm pressure, pelagic red clay could fix carbon at rates ranging from 100 to 500 nmol C g(-1) dry wt day(-1). These clays accumulate in the deepest and the most remote areas of the ocean and contain <30% biogenic material. These clays with volcanic signatures fixed 230-9,401 nmol C g(-1) dry wt day(-1) while siliceous radiolarian oozes of the basin fixed only 5-45 nmol C g(-1) dry wt day(-1). These rates are comparable to those of white smoker waters and are 1-4 orders of magnitude less than those of bacterial mats and active vents recorded at other localities worldwide. The experimental ratios of carbon fixation to metal oxidation in the sediments were 0-1 order of magnitude higher than the corresponding average theoretical ratio of 0.0215 (0.0218, 0.0222, 0.0207 and 0.0211 for Fe, Mn, Co and Ni, respectively) in the siliceous ooze. In case of pelagic red clay it was 0-2 orders higher than theoretical ratio. Thus, chemosynthetic activity could be more widespread, albeit at low rates, than previously considered for abyssal basins. These environments may be dependent partially or even wholly on in situ microbial primary production for their carbon requirements rather than on photosynthetically derived detritus from surface waters.