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 HCl extractable iron concentrations were measured. Average TC was 52.6 ± 29.8 × 105 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 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 caused suppression in methane production. This is also indicative of the co-occurrence of iron reduction and methane oxidation in these sediments. •Potential prokaryotes influenced methane concentrations and iron activity.•Sediment geochemistry and microbial abundance correlated significantly.•Anaerobic prokaryotes positively influenced the different fractions of iron.•Presence of iron reduction and methane oxidation in microcosm experiments.