Shedding light on biogas: a transparent reactor triggers the development of a biofilm dominated by Rhodopseudomonas faecalis that holds potential for improved biogas production

Abstract

Conventional anaerobic digesters intended for the production of biogas usually operate in complete darkness. Therefore, little is known about the effect of light on microbial communities operating in anaerobic digesters. In the present work, we have studied through 16S rRNA gene amplicon Nanopore sequencing and shotgun metagenomic sequencing the taxonomic and functional structure of the microbial community forming a biofilm on the inner wall of a lab-scale transparent anaerobic biodigester illuminated with natural sunlight. The biofilm was composed of microorganisms involved in the four metabolic processes needed for biogas production. The biofilm proved surprisingly rich in Rhodopseudomonas faecalis, a 2 4 6 8 10 12 14 16 18 20 22 24 26 28versatile bacterium able to carry out a photoautotroph metabolism when grown under anaerobic conditions. Our results suggest that this bacterium, able to fix carbon dioxide, could be considered for its use in transparent biogas fermenters in order to contribute to the production of optimized biogas with a higher CH4:CO2 ratio than the biogas produced in regular, opaque digesters. To the best of our knowledge, this is the first study supporting illuminated bioreactors as a new bioprocess for the obtention of biogas enriched in methane. Keywords: Anaerobic digestion, Rhodopseudomonas faecalis, optimized biogas, shotgun metagenomic sequencing, waste water treatment, phototrophism, 16S rRNA gene amplicon Nanopore sequencing.30 32 34 36 38 40The microbial communities operating in the digester are the final key players responsible for the quality of the produced biogas. The role of different microorganisms in the four metabolic steps carried out during the AD of organic matter (hydrolysis, fermentation, acetogenesis, and methanogenesis) has been widely studied (Zinder, 1984). A diverse number of Bacteria are known to be involved in the hydrolysis and further fermentation of complex polymers, whereas the oxidation of intermediate fermentation products to acetate is performed by either hydrogen-or formate-producing acetogens (Stams & Plugge, 2009). Lastly, methane synthesis is mainly derived from acetate and H2/CO2 by acetoclastic and hydrogenoclastic methanogenic Archaea. Therefore, an improved understanding of the microbial communities 42 44 46 48 50 52 54 56 58 60 62 64 influence of light in the process (Sawayama, 2000; Tada et al., 2006; Wei et al., 2016;Reverso, 2017), mainly because the obvious fact that conventional AD systems operate in complete darkness. Interestingly, a previous study reported an increment of the relative concentration of methane when an anaerobic digester was operated under the influence of light (Tada et al., 2006). However, a holistic study of the effect of light on the entire microbiome of anaerobic digesters had yet to be addressed.The aim of the present work was to analyse the effect of natural sunlight on the microbial community of a lab-scale anaerobic co-digester, in order to explore the potential of this strategy to produce high quality biogas. In o...