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.