Marine Plug Flow Bioreactors Provide a High‐Throughput Assay of Potential DOC Bioavailability

Dissolved organic carbon (DOC) forms the largest reservoir of reduced carbon in the ocean, yet its bioavailability and the microbial processes governing its transformation remain poorly constrained. Traditional seawater batch cultures have been indispensable for measuring bioavailable DOC (BDOC), but their long incubation times and limited throughput constrain comparisons across environmental gradients and treatments. Here, we adapted marine plug flow bioreactors, glass columns packed with inert glass beads colonized by natural microbial assemblages, to provide a high‐throughput operational assay of potential DOC bioavailability. After approximately 6 months of conditioning, single‐column bioreactor assays consistently removed DOC from coastal surface waters, deep seawater, and surface seawater amended with diatom‐derived organic matter within 4–7 hr. Linking bioreactors in series increased cumulative DOC removal although the number of columns contributing additional removal varied among source waters. In paired experiments, terminal bioreactor DOC removal equaled or exceeded that measured at the final sampling point of seawater batch cultures incubated for weeks to months. Bioreactor‐associated microbial communities were enriched in Planctomycetota and Thermoproteota relative to the free‐living communities in batch cultures, reflecting the distinct microbial assemblages supported by the two approaches. Because bioreactors operate under high‐biomass, attached community conditions that differ from the open ocean, their measurements represent internally comparable estimates of potential DOC bioavailability rather than realized in situ utilization. Used alongside seawater batch cultures, marine bioreactors provide a complementary framework for distinguishing potential DOC bioavailability from realized DOC utilization.