Microbial iron reduction and methane oxidation in subsurface sediments of the Arabian Sea

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 m 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 HCI 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 sulfate concentrations were 1.3 ppm and 23.2 mM, respectively. Average Fe(II) concentration, hydroxylamine and HCI 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 caused suppression in methane production. This is also indicative of the co-occurrence of iron reduction and methane oxidation in these sediments. (C) 2015 Elsevier Ltd. All rights reserved.