Microbially driven Carbon Transformation in marine Sediments (MCT-S): a framework for understanding the role of sediment microbes in climate change

Marine sediment is Earth’s largest reservoir of organic carbon and harbors >50% of the microbial cells in the ocean. Whether the organic carbon deposited to seabed is mineralized back to CO₂ or preserved over geological timescale, is largely controlled by the microbial processes occurring at sediment-water interface and those key geochemical transition zones below the seafloor. Despite the intimate connection of sediment microbes to Earth’s carbon cycle, the mechanisms, fluxes, and ecological effects of the microbially driven carbon transformation processes in marine sediments, as well as their potential contributions to climate change and mitigation, remain largely unsolved. To this end, we propose the conceptual framework MCT-S (Microbially driven Carbon Transformation in marine Sediments) to guide the understanding and research on sediment carbon cycles. Under the guidance of MCT-S, we integrated interdisciplinary techniques of geochemistry, microbiology, molecular ecology, mathematical modeling, environment simulation as well as designing and fabricating in situ process measuring instruments, to resolve the mechanisms and quantify the contributions of microbial processes in benthic carbon cycling. Our recent studies revealed novel microbial lineages, metabolisms, and/or interplays that are involved in the degradation of refractory organic macromolecules (e.g. lignin, long-chain alkanes), the production and consumption of methane, and the cycling of iron-bound organic carbon at seafloor. These advances produce insights that are necessary to understand the inner workings of carbon cycle at and below the sediment-water interface, underpinning the foundation for elucidating the potential responses and contributions of seafloor microbes to climate change and the related oceanic mitigation actions.