Refractory humic-like dissolved organic matter fuels microbial communities in deep energy-limiting marine sediments

Humic-like dissolved organic matter (DOM), usually regarded as refractory, is a major component of DOM in marine sediment pore waters. However, its bio-reactivity remains poorly explored in natural environments, which makes its roles in supporting subsurface microbial communities and regulating long-term carbon cycling elusive. Here, the bio-reactivity of humic-like DOM was evaluated by modeled reaction rates together with its interactions with microbial communities in five sediment cores collected from the eutrophic Pearl River Estuary to the oligotrophic deep-sea basin in the northern South China Sea. We revealed contrasting relationships between humic-like DOM and microbes in the coastal and deep-sea sediments. In eutrophic coastal sediments, specific microbial groups enriched in the deep layers co-varied with humic-like DOM, while most microbial groups were significantly correlated with protein-like DOM, microbial transformation of which likely resulted in the production of humic-like DOM. On the contrary, in energy-limiting deep-sea sediments, over 70% of the microbial groups were found closely correlated with humic-like DOM, a net consumption of which was demonstrated in deep layers. The consumption of humic-like DOM in deep-sea sediments reduced its total production flux in the uppermost ~5-meter layer to about one-tenth of that in coastal sediments, which could consequently decrease the refractory DOM flux to the overlying seawater and influence long-term oceanic carbon cycling.