Abstract
Methanogens are restricted to a few genera of Archaea, however they have great importance in the carbon cycle, impacting climactic considerations, and also find a role in renewable energy in the form of biogas. Here, we examine the microbial contribution to the production of methane in a sargassum fed anaerobic saltwater bioreactor, which are poorly characterized compared to fresh water bioreactors, using a comprehensive functional metagenomics approach. Despite abundant production of methane, we detected a low proportion of Archaea in the system using 16S rRNA community profile analyses. We address the low representation using an additional 16S rRNA analysis of shotgun data and a consideration of CO2:CH4production. Using a novel network alignment and tree building approach, we measured similarity between the meta-metabolic capabilities of different anaerobic microbial communities. The saltwater bioreactor samples clustered together, validating the approach and providing a method of determining meta-metabolic similarity between microbial communities, with a range of potential applications. We also introduce a number of additional approaches for examining and interpreting meta-metabolic network topology. The low abundance of methanogens appears as a common property of such anaerobic systems and likely reflects the relatively poor energetics of methanogens, while examination of key enzymes confirms that hydrogen producing bacteria are the major fermentative guild. Our results indicate that the use of readily available seawater and marine macroalgae is a promising approach to the production of biogas as a source of renewable energy.
Highlights
Anaerobic bioreactors produce biogas from a variety of organic feedstocks through the process of methanogenesis
The two 2L bioreactors were operated during a six month campaign that began in April 2013
The true values of the organic loading rate (OLR), based on actual experimental measures of volatile solids in 7% suspensions of ground sargassum suggest that the initial OLR was 1.40 g VS∗L−1∗d−1, while the final value at the end of the campaign was approximately 5.7 g VS∗L−1∗d−1
Summary
Anaerobic bioreactors (or anaerobic digester, AD, systems) produce biogas from a variety of organic feedstocks through the process of methanogenesis. The resulting biogas possesses a high proportion of methane, an important greenhouse gas, being several times more potent than carbon dioxide Such is the climactic importance of methane; the evolution and subsequent population expansion of methanogens have been causally linked to the Permian extinction [2]. Capture of this gas via anaerobic digestion of organic waste material and subsequent use as biofuel represents a potentially important manner of ameliorating greenhouse gas emission as it represents the conversion of methane to carbon dioxide. The minor proportion of Archaea in the majority of these bioreactor systems has not been adequately addressed and is explored in detail here using a functional metagenomics approach and a range of novel bioinformatics approaches, tailored for this study but with potential applications elsewhere
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