Exploring the Bacterial Preference for Terrestrial or Marine Organic Matter in Estuarine Sediments Using Compound Specific Stable Carbon Isotope Ratios: A Degradation Kinetics Study

Preserving the health of estuarine ecosystems has been an increasing challenge in the recent past with the spreading of areas affected by deep-water hypoxic conditions. Hence, it is of critical importance to identify the causes of such perturbation, triggered by changing ocean circulation and increasing inputs of organic matter (OM), which results in serious threats to living species. Estuaries are large deposition centers for organic matter (OM) where stable carbon isotope ratios of either bulk OM or specific organic compounds provide detailed information about carbon cycling and the tracing of OM sources and transformations along the terrestrial-marine continuum. In particular, the ∂¹³C values of biomarkers that are specific to heterotrophic bacteria (branched iso- and anteiso-C15:0 fatty acids) can be used to assess the type of OM that they preferentially degrade as the ∂¹³C values of marine organic carbon (OC) are more enriched in ¹³C than those of terrestrial OC. However, very little is known on the dynamics between the seasonally varying relative inputs of terrestrial vs. marine OM and the ∂¹³C values of these bacteria-specific fatty acids. In this study, we will use a kinetic batch incubation approach in which natural sediments from the St. Lawrence Estuary and Gulf, amended with fresh terrestrial or marine OM characterized by a very different ¹³C/¹²C ratio (difference of between 10 and 14 ‰ depending on the sampling station), will be incubated for varying amounts of time. Quenching of the incubations followed by the extraction, quantification and isotopic characterization of the bacterial fatty acids will allow determining the rate and temporal extent of change of their compound-specific ∂¹³C values. Bulk elemental (OC and total nitrogen) and isotopic (∂¹³C and ∂¹⁵N) mass balances will be precisely monitored throughout the experiment. Acquisition of this knowledge, combined with results from other studies carried out in our lab, will provide a better understanding of the relative importance of terrestrial and marine OM processing in the onset of hypoxia and will be exploited as a guide for remedial efforts aiming to improve the health of such an important ecosystem.