Co-existence of Methanogenesis and Sulfate Reduction with Common Substrates in Sulfate-Rich Estuarine Sediments

Abstract

The competition between sulfate reducing bacteria and methanogens over common substrates has been proposed as a critical control for methane production. In this study, we examined the co-existence of methanogenesis and sulfate reduction with shared substrates over a large range of sulfate concentrations and rates of sulfate reduction in estuarine systems, where these processes are the key terminal sink for organic carbon. Incubation experiments were carried out with sediment samples from the sulfate-methane transition zone of the Yarqon (Israel) estuary with different substrates and inhibitors along a sulfate concentrations gradient from 1 to 10 mM. The results show that methanogenesis and sulfate reduction can co-exist while the microbes share substrates over the tested range of sulfate concentrations and at sulfate reduction rates up to 680 μmol L^-1 day^-1. Rates of methanogenesis were two orders of magnitude lower than rates of sulfate reduction in incubations with acetate and lactate, suggesting a higher affinity of sulfate reducing bacteria for the available substrates. The co-existence of both processes was also confirmed by the isotopic signatures of δ³⁴S in the residual sulfate and that of δ¹³C of methane and dissolved inorganic carbon. Copy numbers of dsrA and mcrA genes supported the dominance of sulfate reduction over methanogenesis, while showing also the ability of methanogens to grow under high sulfate concentration and in the presence of active sulfate reduction.

Keywords: co-existence; estuaries; methanogenesis; substrates; sulfate reduction.

FIGURE 1

Yarqon estuary location map at the Israeli coast of the Eastern Mediterranean.

FIGURE 2

Experiment A – Evolution of methane concentrations in slurries with (A) 1 mM ${\mathrm{S}\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}$ ; (B) 2 mM ${\mathrm{S}\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}$ ; and (C) 9 mM ${\mathrm{S}\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}$.

FIGURE 3

Methanogenesis rates in Experiment A with and without sulfate reduction inhibitor (molybdate) under different sulfate concentrations.

FIGURE 4

Experiment B – Slurries with sediment amended with inhibitors of either sulfate reduction (molybdate) or methanogenesis (BES), control slurries (without inhibitor) and killed control (autoclaved). (A) Sulfate concentration; (B) δ³⁴S of residual ${\mathrm{S}\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}\mathrm{-}}$ ; (C) CH₄ concentrations; (D) δ¹³C-CH₄; (E) DIC concentrations and (F) δ¹³C-DIC. Legend in panel (A) refers to all panels.

FIGURE 5

Experiment C – (A) methane and (B) sulfate concentrations throughout the experiment with no substrate addition, acetate addition, lactate addition and killed control (autoclaved).

FIGURE 6

Rates of (A) methanogenesis and (B) sulfate reduction during Experiments B and C treated with inhibitors (BES, molybdate or without inhibitor) and with substrate (acetate, lactate or without substrate addition).

FIGURE 7

Changes in the relative abundance of (A) mcrA functional gene relative to the copy number of Archaea; and (B) dsrA functional gene relative to the copy number of Bacteria over the course of the experiment at two time points (average of duplicate bottles (each samples in triplicates).

FIGURE 8

Methanogenesis rates versus sulfate reduction rates in Experiments B and C. The rates of samples without addition of inhibitors are marked in blue symbols and rectangle and the rates of samples treated with molybdate are marked in green symbols and rectangle.