Picky Eaters: Carbon Isotopic Evidence for the Uniform Bioavailability of Riverine Dissolved Organic Matter to a Model Marine Microorganism

Dissolved organic matter (DOM) is a key component of the global carbon cycle, with rivers delivering significant amounts of DOM to oceans. Urbanization and agricultural land‐use alter the age and chemical composition of riverine DOM, which likely impact the downstream bioavailability of riverine DOM. Here, we use bioreactor incubations of a marine bacterium ( Pseudoalteromonas sp. 3D05) to investigate DOM bioavailability from two distinct rivers: the Suwannee River (natural, non‐urbanized), and the Upper Mississippi River Basin (anthropogenically influenced). We measured rates of microbial CO 2 production and radiocarbon ages (as Δ 14 C) to assess DOM remineralization. We observed nearly identical cell densities and degradation patterns for both riverine DOM incubations. Respired DOM Δ 14 C values were also similar and decreased over time indicative of preferential utilization of recently synthesized “modern” substrates. These findings reveal unexpected similarities in riverine DOM bioavailability, indicating similar short term biological reactivity despite large DOM compositional differences.