Anaerobic Oxidation of Short-Chain Alkanes in Hydrothermal Sediments: Potential Influences on Sulfur Cycling and Microbial Diversity

Abstract

Short-chain alkanes play a substantial role in carbon and sulfur cycling at hydrocarbon-rich environments globally, yet few studies have examined the metabolism of ethane ((C_2)), propane ((C_3)), and butane ((C_4)) in anoxic sediments in contrast to methane ((C_1)). In hydrothermal vent systems, short-chain alkanes are formed over relatively short geological time scales via thermogenic processes and often exist at high concentrations. The sediment-covered hydrothermal vent systems at Middle Valley (MV, Juan de Fuca Ridge) are an ideal site for investigating the anaerobic oxidation of (C_1–C_4) alkanes, given the elevated temperatures and dissolved hydrocarbon species characteristic of these metalliferous sediments. We examined whether MV microbial communities oxidized (C_1–C_4) alkanes under mesophilic to thermophilic sulfate-reducing conditions. Here we present data from discrete temperature (25, 55, and (75^{\circ}C)) anaerobic batch reactor incubations of MV sediments supplemented with individual alkanes. Co-registered alkane consumption and sulfate reduction (SR) measurements provide clear evidence for (C_1–C_4) alkane oxidation linked to SR over time and across temperatures. In these anaerobic batch reactor sediments, 16S ribosomal RNA pyrosequencing revealed that Deltaproteobacteria, particularly a novel sulfate-reducing lineage, were the likely phylotypes mediating the oxidation of (C_2–C_4) alkanes. Maximum (C_1–C_4) alkane oxidation rates occurred at (55^{\circ}C), which reflects the mid-core sediment temperature profile and corroborates previous studies of rate maxima for the anaerobic oxidation of methane (AOM). Of the alkanes investigated, (C_3) was oxidized at the highest rate over time, then (C_4), (C_2), and (C_1), respectively. The implications of these results are discussed with respect to the potential competition between the anaerobic oxidation of (C_2–C_4) alkanes with AOM for available oxidants and the influence on the fate of (C_1) derived from these hydrothermal systems.