Networked Speaker Series #19

September 20, 2018

Dr. Nagissa Mahmoudi, McGill University

Uncovering microbial species-specific effects on organic matter transformation in marine sediments


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 (Δ14C and δ13C) signature of microbially-respired CO2 to constrain the type and age of organic matter that is bioavailable to each species. Separate incubations using bacterial isolates (Vibrio spp. and Pseudoalteromonas spp.) and sterilized sediment under oxic conditions showed that the rate and total quantity of organic matter remineralized by these two species differs. Approximately twice as much respired CO2 was collected during the Pseudoaltoermonas spp. incubations compared to the Vibrio spp. incubation. Isotopic (Δ14C and δ13C) signatures of respired CO2 show selective utilization of different carbon pools by each species. These differences in OM utilization may be due to the physiological characteristics of each species and the gene copy number and substrate specificity of degradative enzymes. 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.

Speaker Biography

Dr. Nagissa Mahmoudi is an Assistant Professor in the Department of Earth and Planetary Sciences at McGill University. She received her undergraduate degree from the University of Toronto and completed her Ph.D at McMaster University. Dr. Mahmoudi’s research employs a variety of field and laboratory based tools, ranging from molecular ‘omics to isotope geochemistry, to gain insight into microbial pathways and interactions that underpin carbon cycling in marine environments. As a C-DEBI postdoctoral fellow at Harvard University, she applied these techniques to investigate the microbial controls on organic carbon reactivity and transformation in marine sediments.



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