MSI Seminar

What’s on the menu? Investigating the selective diet of microbes using novel isotopic tools

Nagissa Mahmoudi

McGill University

The biogeochemical transformation and cycling of carbon in marine sediments is driven by the activity of complex and diverse microbial communities. Reactions initiated by microbial enzymes at the molecular scale drive the rate and extent of organic matter (OM) remineralization to CO₂ and CH₄. A major unknown in carbon biogeochemistry is determining what controls the reactivity (or bioavailability) of OM to microorganisms. Recent studies have found that the ability to use different carbon sources varies among microorganisms, suggesting that the reactivity of certain pools of carbon can be specific to the taxa that utilize the pool. Here, we ask the question: to what extent is the reactivity and transformation of OM dependent on the species that are present and active in the environment. Using a novel bioreactor system (IsoCaRB), we carried out time-series incubations using model marine bacterial isolates and sterilized organic-rich sediment collected from Guaymas Basin (Gulf of California). The IsoCaRB system allows us to measure the production rate and natural isotopic (¹⁴C and ¹³C) signature of microbially-respired CO₂ to constrain the type and age of organic matter that is bioavailable to each species. Our results suggest that organic matter transformation in marine environments may depend on the metabolic capabilities of the microbial populations that are present and active.