Microbially driven fate of terrigenous particulate organic matter in oceans

A long-standing enigma in oceanography is why terrestrial organic matter is missing in the global ocean, despite the considerable discharge into it every year. Although some explanations, such as mineralogical composition, hydrodynamic processes, and priming effect, have been proposed, we hypothesize that the essential mechanism behind the missing organic matter is microbial processing, for which the underlying coupled geochemical, molecular, and genetic evidence is unknown. An ultra-large-volume, long-term river-seawater stratified simulation system was constructed to unravel the microbially driven fate of terrigenous particulate organic matter (POM) in oceans. Analysis of combining the molecular with POM chemical composition data suggests that Bacteroidetes could act as pioneers in the processing of terrigenous POM in oceans, degrading high-molecular-weight, high-carbon compounds such as polysaccharides. Remaining low-molecular-weight nitrogenous organic matter is subsequently degraded by Planctomycetes and Proteobacteria. Isotopic signals show that this preferential degradation causes a distinct aging effect of POM, and along with nitrification enhanced by remineralization, causes a decrease in the POM C : N ratio. Degradation of terrigenous POM and bacterial biomass biosynthesis leads to positive deviations in delta N-15 and delta C-13. Relatively refractory hydrocarbons, aromatic compounds, and phenols are accumulated by microbial processes in this system. This study provides mechanistic insights into the missing chemical and isotopic signals and microbially driven fate of terrigenous POM in the ocean, with important implications for how riverine material input affects marine carbon and nitrogen cycling.

Automated summary

This study reveals the mechanism behind the missing terrestrial organic matter in the global ocean, attributing it to microbial processing. Through the degradation of high-carbon compounds by Bacteroidetes and subsequent degradation of nitrogenous organic matter by Planctomycetes and Proteobacteria, this study provides insights into the fate of terrigenous particulate organic matter in the ocean. The findings highlight the importance of riverine material input in marine carbon and nitrogen cycling.