MICROBIAL WEATHERING OF SEDIMENTARY ORGANIC MATTER

Carbon cycling relates to the transfer of carbon among reservoirs of the lithosphere, hydrosphere, atmosphere and biosphere. Two processes within the geochemical carbon cycle hold particular importance for their role in controlling the O₂ concentration in earth’s atmosphere. These are the burial of organic matter in sediments, and the weathering of ancient organic carbon-rich sedimentary rocks. The small fraction of primary production that escapes respiration to become buried in sediments can be interpreted as photosynthesis without respiration: a net addition of O₂ to the atmosphere. When ancient sedimentary rocks are uplifted and eroded on the earth’s continents, organic matter becomes exposed to an oxidizing surficial environment. This organic matter is oxidized to inorganic carbon, consuming O₂. It is believed that the rates of organic matter burial and weathering are approximately balanced on geologic timescales, both to maintain equable atmospheric O₂ concentrations and to preserve carbon isotope mass balance.

Greater than 90% of organic matter in ancient sedimentary rocks occurs as the macromolecule kerogen. Kerogen is the insoluble, randomly polymerized and cross-linked remains of degraded algal and bacterial biomass. Oxidation of kerogen releases appreciable free energy, and thus may provide a carbon source for organisms able to access this material. However, the composition of kerogen and its setting in impermeable rocks with highly acidic porewaters presents a barrier to microbial heterotrophy. Microorganisms able to metabolize kerogen in rock weathering environments must be tolerant of extremes and fluctuations in pH and O₂ availability, and have adopted novel carbon utilization pathways. Detailed organic geochemical analyses reveal the stepwise degradation of kerogen during weathering, compound-specific radiocarbon analyses of cell membrane lipids confirm microbial assimilation of ancient organic matter, and phylogenetic analyses help place these kerogen-degrading communities within the tree of life.