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 humiclike 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.