Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land‐to‐ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.
The microbial carbon pump (MCP) hypothesis suggests that successive transformation of labile dissolved organic carbon (DOC) by prokaryotes produces refractory DOC (RDOC) and contributes to the long-term stability of the deep ocean DOC reservoir. We tested the MCP by exposing surface water from a deep convective region of the ocean to epipelagic, mesopelagic, and bathypelagic prokaryotic communities and tracked changes in dissolved organic matter concentration, composition, and prokaryotic taxa over time. Prokaryotic taxa from the deep ocean were more efficient at consuming DOC and producing RDOC as evidenced by greater abundance of highly oxygenated molecules and fluorescent components associated with recalcitrant molecules. This first empirical evidence of the MCP in natural waters shows that carbon sequestration is more efficient in deeper waters and suggests that the higher diversity of prokaryotes from the rare biosphere holds a greater metabolic potential in creating these stable dissolved organic compounds.
Marine and freshwater prokaryotes feed primarily on bioavailable labile dissolved organic carbon (BDOCL), as well as the bioavailable fraction of the semilabile DOC (BDOCSL) pool. These fractions are operationally defined here as the DOC consumed within a month and greater than a month to a year and a half, respectively. Organic matter from these different pools comes from various autochthonous and allochthonous sources, but their relative bioavailability is unknown across aquatic ecosystems. To fill this gap, we compiled literature information that included 653 batch culture DOC biodegradation experiments across eight aquatic ecosystem types over the past 20 years. We show that the proportion of BDOCL across all aquatic ecosystems was surprisingly consistent (6.1%) despite a 2 order of magnitude variation in initial DOC concentrations, suggesting an overall tight balance between carbon supply and consumption. A higher proportion of BDOCL, 16.3% on average, was observed in high productivity ecosystems. BDOCSL, on the other hand, gradually decreased from 16.0% in lakes to 7.2% in estuaries to undetectable in the open ocean, suggesting that terrestrial connectivity regulates BDOCSL across the continuum. Our results support that recent primary production fuels short‐term prokaryotic DOC needs with an increasing reliance on the abundant BDOCSL pool as ecosystems approach the land‐water interface. Batch culture experiments show that BDOCSL is metabolizable in freshwater and coastal environments but not in the open ocean. We estimate that BDOCSL can sustain 62% of total prokaryotic biomass in inland waters and coasts and an estimated total of 16.7% across aquatic biomes.
Diverse prokaryotic communities consume and transform a broad suite of molecules in the dissolved organic matter (DOM) pool, which controls major biogeochemical cycles. Despite methodological advancements that provide increasingly more detailed information on the diversity of both prokaryotic communities and DOM components, understanding how these two component parts are structured to influence ecosystem functioning remains a major challenge in microbial ecology. Using empirical data collected along a gradient of productivity in the Labrador Sea, we characterized relationships among DOM compounds, metabolic processing, and prokaryotic diversity by structuring prokaryotic communities using spatial abundance distribution (SpAD) modeling. We identified strong associations of different SpAD taxonomic groups with specific organic substrates as well as with metabolic rates. Amplicon sequence variants (ASVs) with more cosmopolitan distributions (i.e. normal-like) such as Bacteroidia were related to fresher DOM substrates such as free and combined amino acids whereas rare ASVs (i.e. logistic) like δ-proteobacteria were associated with complex forms of organic matter. In terms of ecosystem function, rates of respiration and production were most strongly predicted by the abundance of certain SpAD taxonomic groups. Given the importance and complexity of linking environmental conditions, prokaryotic community structure, and ecosystem function, we propose a framework to bridge the gap between prokaryotic diversity, microbial ecology, and biogeochemistry among methods and across scales. Our work suggests that SpAD modeling can be used as an intermediate step to link prokaryotic community structure to both finer DOM details and larger ecosystem scale processes.
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