Methanotrophy under Versatile Conditions in the Water Column of the Ferruginous Meromictic Lake La Cruz (Spain).

Kirsten Oswald,Carsten J Schubert,Corinne Jegge,Eduardo Vicente,Andreas Brand,Jakob Zopfi,Jana Tischer,Jasmine Berg,Moritz F Lehmann,María R Miracle,Xavier Soria

doi:10.3389/fmicb.2016.01762

Abstract

Lakes represent a considerable natural source of methane to the atmosphere compared to their small global surface area. Methanotrophs in sediments and in the water column largely control methane fluxes from these systems, yet the diversity, electron accepting capacity, and nutrient requirements of these microorganisms have only been partially identified. Here, we investigated the role of electron acceptors alternative to oxygen and sulfate in microbial methane oxidation at the oxycline and in anoxic waters of the ferruginous meromictic Lake La Cruz, Spain. Active methane turnover in a zone extending well below the oxycline was evidenced by stable carbon isotope-based rate measurements. We observed a strong methane oxidation potential throughout the anoxic water column, which did not vary substantially from that at the oxic/anoxic interface. Both in the redox-transition and anoxic zones, only aerobic methane-oxidizing bacteria (MOB) were detected by fluorescence in situ hybridization and sequencing techniques, suggesting a close coupling of cryptic photosynthetic oxygen production and aerobic methane turnover. Additions of nitrate, nitrite and to a lesser degree iron and manganese oxides also stimulated bacterial methane consumption. We could not confirm a direct link between the reduction of these compounds and methane oxidation and we cannot exclude the contribution of unknown anaerobic methanotrophs. Nevertheless, our findings from Lake La Cruz support recent laboratory evidence that aerobic methanotrophs may be able to utilize alternative terminal electron acceptors under oxygen limitation.

Highlights

Among all greenhouse gases, methane (CH4) has shown the highest atmospheric concentration increase since industrialization (Forster et al, 2007) with total emissions currently approximating ∼600 Tg CH4 a−1 (Ehhalt et al, 2001)
In permanently stratified and frequently in seasonally stratified lakes, an anoxic hypolimnion can be formed below the oxycline, where CH4 can potentially accumulate to high concentrations (Schubert et al, 2010; Blees et al, 2015; Lehmann et al, 2015)
Relative to the total irradiation at the water surface (∼1200 μE m−2 s−1) photosynthetically active radiation (PAR) decreased to 0.14% (1.9 μE m−2 s−1) at the oxycline, and could be detected at a maximum depth of 16.5 m (0.1 μE m−2 s−1; Figure 1B)

Summary

Introduction

Methane (CH4) has shown the highest atmospheric concentration increase (factor of 2.5) since industrialization (Forster et al, 2007) with total emissions currently approximating ∼600 Tg CH4 a−1 (Ehhalt et al, 2001). This only constitutes a small proportion compared to carbon dioxide (CO2) emissions, methane has a global warming potential which is 20 times higher over a 100 year period (Forster et al, 2007). Evidence for AOM proceeding concurrently with iron or manganese reduction exists for marine settings (Beal et al, 2009; Wankel et al, 2012; Slomp et al, 2013; Riedinger et al, 2014), but the involved microorganisms have not yet been identified

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Conclusion