Chemoautotrophy Enhances Iron and Phosphorus Recycling From Sediments at Deep-Sea Methane Seeps

Methane-rich cold seeps are oases of life in the deep sea, where microbial chemosynthesis of organic matter sustains thriving ecosystems independent of sunlight-derived energy. Here, we reveal a previously overlooked role of chemoautotrophy at seeps as powerful recyclers of scarce nutrients iron (Fe) and phosphorus (P). Investigations of sediments at Haima cold seeps (1,300–1,500 m deep) across varying methane seepage intensities showed that seep sediments released orders of magnitude more dissolved Fe and phosphate than background sediments, despite comparable organic matter remineralization rates. At Haima seeps with high methane, sediment phosphate effluxes reached 2.00–15.8 µmol m⁻² d⁻¹and dissolved Fe effluxes reached 2.24–47.4 µmol m⁻² d⁻¹, compared to background phosphate efflux of 1.21 µmol m⁻² d⁻¹ and dissolved Fe efflux of 0.412 µmol m⁻² d⁻¹. This enhancement in nutrient recycling stems from a cascade of coupled biogeochemical processes driven by the anaerobic oxidation of methane (AOM). Methane oxidation reduces Fe oxides, releasing both dissolved Fe and Fe-bound P. AOM also reduces sulfate to sulfide, precipitates dissolved Fe and suppresses the regeneration of P-binding Fe oxides, further promoting P release. These mechanisms maintained the disproportionately high benthic Fe and P recycling at seeps, which may significantly impact regional and global nutrient budgets, given the thousands of documented seeps and potentially orders of magnitude more undiscovered in the global ocean.