Microbial Gladiators: Unraveling the dynamics of carbon substrate competition among heterotrophic microbes

Growing evidence suggests that interactions among heterotrophic microbes influence the efficiency and rate of organic matter turnover. These interactions are dynamic and shaped by the composition and availability of resources in their surrounding environment. Heterotrophic microbes inhabiting marine environments often encounter fluctuations in the quality and quantity of carbon inputs, ranging from simple sugars to large, complex compounds. Here, we experimentally tested how the chemical complexity of carbon substrates affects competition and growth dynamics between two heterotrophic marine isolates. We tracked cell density using species-specific PCR assays and measured rates of microbial CO 2 production along with associated isotopic signatures ( 13 C and 14 C) to quantify the impact of these interactions on organic matter remineralization. The observed cell densities revealed substrate-driven interactions: one species exhibited a competitive advantage and quickly outgrew the other when incubated with a labile compound while both species seemed to coexist harmoniously in the presence of more complex organic matter. Rates of CO 2 respiration revealed that co-incubation of these isolates enhanced organic matter turnover, sometimes by nearly twofold, compared to their incubation as mono-cultures. Isotopic signatures of respired CO 2 indicated that co-incubation resulted in a greater remineralization of macromolecular organic matter. These results demonstrate that simple substrates promote competition while high substrate complexity reduces competitiveness and promotes the partitioning of degradative activities into distinct niches, facilitating coordinated utilization of the carbon pool. Taken together, this study yields new insight into how the quality of organic matter plays a pivotal role in determining microbial interactions within marine environments.