Sediment recycles the key nutrient phosphorus (P) to support marine productivity. Understanding sediment P cycles helps constrain the P budget and predict how ecosystems respond to environmental variability. Unlike coastal sediments where P recycling is largely controlled by iron cycles, effluxes of bioavailable P (i.e., soluble reactive phosphorus) from deep-sea sediments are mostly driven by the decomposition of organic matter due to its scarcity. However, by studying the sediments from Haima methane seeps in the South China Sea, we show that sediments at deep-sea methane seeps behave like organic-rich coastal sediments. Higher P recycling was found in high-methane sediments, where iron and sulfate reduction are actively coupled. Unlike coastal sediments where anaerobic reduction relies on the organic matter availability, at the methane seeps, Fe and sulfate reduction is driven by the anaerobic oxidation of methane (AOM), a process independent of the organic matter supply. Our findings highlight the role of chemoautotrophic communities in shaping the P recycling mechanisms and enhancing the fluxes of P and Fe, the two key nutrients supporting pelagic ocean productivity.

chemoautotrophy,methane seep,sediment,iron-sulfur,phosphorus