Marine sediments cover almost ~75% of the Earth’s surface and are one of the largest global reservoirs of organic carbon. Microorganisms play a primary role in the decomposition of organic matter in marine sediments. Consequently, the activity of these microorganisms can have profound impacts on both local and global biogeochemical cycles. A major question in carbon biogeochemistry is determining what controls the accessibility or bioavailability of organic matter to microorganisms. It is not clear whether microorganisms themselves are ultimately controlling the degradation rate or whether it depends primarily on chemical and physical properties of the compounds and/or the depositional setting and sediment composition. In order to better understand microbial carbon cycling and the fate of recalcitrant organic matter, we constructed a novel bioreactor system to measure carbon isotopes during microbial degradation of complex organic matter. This system enables us to measure the natural isotopic signature (δ13C and Δ14C) of microbially-respired CO2, thereby allowing us to determine the age of the organic matter that is being respired. We investigated organic matter availability and transformation using bacterial isolates as well as natural sedimentary communities. Incubation of sediments from Falmouth, MA displayed a successive pattern of microbial respiration such that several peaks appear over the course of a 7-day incubation. This suggests that organic matter is degraded in a step-wise manner by a succession of microbial taxa. Isotopic signatures of CO2 fractions indicates that the microbial community is primarily are respiring labile organic matter from fast cycling pools. Separate incubations using two different marine bacterial isolates and sterilized marine sediment under oxic conditions showed that the rate and total quantity of organic matter metabolized by these two species differs. Continuous measurement of respiratory CO2 production and its carbon isotopic compositions (13C and 14C) shows species-specific differences in the rate, quantity, and type of organic matter remineralized. Taken together, this work sheds light on the degree to which organic matter cycling in marine environments may depend on the metabolic potential of the microbial populations that are present and active.

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

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.