Experimental studies on the stable carbon isotope biogeochemistry of acetate in lake sediments

Acetate is an important intermediate in the anaerobic degradation of organic matter and highly relevant for the cycling of carbon in nature. The water soluble C2 compound is produced by fermentation of organic matter as well as by reduction of CO2 with H2 via the acetyl-CoA pathway (acetogenesis) and serves as an important substrate for a variety of microorganisms including sulfate reducing bacteria and methanogenic archaea. The relative importance of the different metabolic processes in acetate production and consumption is thought to be reflected in the stable carbon isotopic composition of pore water acetate. δ13C values of acetate have been shown to be highly variable in marine sediments, ranging from –85‰ to –3‰, and distinct isotopic fractionations have previously been reported to be associated with fermentation, acetogenesis and acetoclastic methanogenesis in pure microbial cultures. Aiming to better understand the stable carbon isotope biogeochemistry of sedimentary acetate, this study investigates if process related carbon isotopic signals of acetate are also expressed in the pore water pool of complex freshwater environments. We report findings from incubation experiments with lake sediments, in which we manipulated the relative importance of single processes by addition of specific substrates and inhibitors. In particular, we found: (a) δ13C values of acetate closely resembled δ13C values of total organic carbon in the sediment’s solid phase (TOC) where fermentation was the dominant process; (b) a distinct 13C enrichment of the acetate pool relative to δ13CTOC where acetate was consumed by acetoclastic methanogenesis; and (c) a distinct 13C depletion of acetate relative to both δ13CTOC and δ13C values of dissolved inorganic carbon (DIC) where high levels of H2 stimulated acetogenesis. Our study provides further evidence for the diagnostic value of the stable carbon isotope chemistry of acetate for the detection of biogeochemical processes in natural systems.