Anaerobic microbial communities and processes involved in the methane cycle in freshwater lakes-a focus on Lake Pavin

Abstract

The atmospheric concentration of methane (CH4), a major greenhouse gas, is mainly controlled by the activities of CH4-producing (methanogens) and CH4-consuming (methanotrophs) microorganisms. Freshwater lakes are identified as one of the main CH4 sources, as it is estimated that they contribute to 6-16 % of natural CH4 emissions. It is therefore critical to better understand the biogeochemical cycling of CH4 in these ecosystems. In this vein, the Lake Pavin provides a useful microbial ecosystem to investigate CH4 cycle in freshwater systems. Despite a significant production of CH4 in the deep anoxic water column and sediment, the amounts of CH4 emitted by Lake Pavin to the atmosphere are several orders of magnitude lower than those of temperate lakes suggesting intense consumption activities of this gas. This chapter focuses on CH4 cycle, but as methanogenesis and anaerobic methanotrophy build competitive and cooperative relationships with a number of bacterial metabolic groups, we also address bacterial processes that are tightly coupled with CH4 cycle (e.g., ferric iron reduction). Three main sections constitute this chapter: A presentation of CH4 cycle, including methanogenesis and methanotrophy, in freshwater systems and particularly in Lake Pavin, The relationships between CH4 cycle and some other biogeochemical processes in Lake Pavin (ferric iron reduction, sulfate reduction and fermentation), including a brief overview of anaerobic microbial metabolisms, Sections on methodologies enabling to access informations on the anaerobic metabolisms (e.g., biomarkers, isotopes, microcalorimetry, nucleic acid molecular markers, magnetoFISH).