Microbially driven fate of terrigenous particulate organic matter in oceans

Abstract

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 δ 15 N and δ 13 C. 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.Ongoing climate change is expected to lead to increased global river runoff (Labat et al. 2004;Andersson et al. 2015). Both ecological modeling and field research have indicated that the flux of terrestrial input to oceans increases with increased river runoff (Andersson et al. 2015). However, few molecular signals of abundant terrestrial organic matter are detected in marine environments (Hedges et al. 1997;Bianchi 2011;Kandasamy and Nath 2016), and only a trace of terrigenous organic matter (e.g., lignin) is present in the ocean (Opsahl and Benner 1997). A proposed explanation for this is a priming effect, in which the addition of labile organic carbon increases remineralization of the relatively refractory organic carbon (Bianchi 2011;Morling et al. 2017). Mineralogical composition and characteristics have also been proposed as important factors controlling the fate of continentally

Machine Learning Provides Opportunities to Recognize Greenhouse Gas Emissions from Water at a Large Scale

“…The fate of different carbon forms in water environments is determined by complex internal biogeochemical processes (e.g., mineralization) and external environmental drivers (e.g., geography and anthropogenic activities). ,, Carbon in inland waters may be deposited, converted into GHG emissions, or transported to the ocean . The ocean continuously exchanges carbon with the atmosphere through the air–sea interface, but in recent decades, problems such as ocean absorption of much CO 2 , leading to acidification, have reduced the ocean’s buffering capacity, increasing the ocean carbon cycle’s uncertainty .…”

Terrigenous particles regulate autotrophic and heterotrophic microbial assembly and induce humic-like FDOM accumulation in seawater

“…Time-series sample was collected from 1 m above the bottom for the following analysis of physicochemical parameters and characteristics of POM. Details on sample collection and measurements were described in previous studies (Zhang et al, 2021;Zhang et al, 2022).…”

“…All membranes were kept at -20°C until nucleic acid extraction. Please refer to the original work for detailed methods (Zhang et al, 2021;Zhang et al, 2022). Nucleotide sequence datasets used in this study are available in the NCBI Sequence Read Archive under the accession number PRJNA751271 (Zhang et al, 2022).…”

“…Further studies indicated that terrigenous organic matter influenced marine ecosystems, including increasing prokaryote abundance (Chen et al, 2022) and promoting organic matter transformation . Recent results based on this system clearly explicated how microbes modified particles in the seawater (Zhang et al, 2022). The composition and activity of microbial communities are the central issues in microbial ecology (Falkowski et al, 2008;Schimel and Schaeffer, 2012;Aronson et al, 2013).…”

Particulate Organic Carbon Released during Macroalgal Growth Has Significant Carbon Sequestration Potential in the Ocean

“…As an important fraction of marine organic carbon pool, POC plays a key role in ocean carbon cycle by regulating the sediment organic carbon (SOC), DOC, and dissolved inorganic carbon (DIC) pools. − When substantial amounts of POC are released during macroalgal growth, their labile components can be gradually mineralized by microorganisms into DOC, leaving behind the recalcitrant components of POC. , The labile fractions of the newly produced DOC from POC transformation can be further mineralized by microorganisms into DIC or carbon gas and be partially transformed by microorganisms into RDOC via the microbial carbon pump mechanism. − Those inert components of DOC and POC, whether buried in sediments, stabilized in seawater, or even transported to the deep sea, will contribute to the long-term carbon storage in the ocean. ,, In addition to microbial degradation, photodegradation may also substantially affect the fate and carbon sequestration effect of macroalgae-released POC. However, currently, the fate of POC released during macroalgal growth after bio- and photodegradation is still a mystery.…”