Metagenomic analysis of biocathodic communities and its electron transfer proteins with bioinformatics tools

Abstract

Bioelectrochemical systems (BES) are environmental-friendly technologies that have huge potential. They employ organisms, redox reactions and electron transfer to produce either electricity or a chemical product. BES consist in a compartmentalized chamber with two electrodes in it, an anode and a cathode, bathed in a solution. The chamber is formed by an anodic region, where the oxidation reaction takes place, and a cathodic region, the site where the reduction reaction occurs. These two regions are (often) separated by a semi-permeable membrane and the electrodes are joined by an outside electron-conducting wire. Microbial electrosynthesis (MES), an instance of BES, is used to produce acetate (among other commodity chemicals). In a previous study, it was hypothesized that a MES system yielding acetate from CO2 and electricity, worked by the action of a consortium of two bacteria genera, Rhodobacter sp. at a biofilm surrounding the cathode and Clostridium sp. at the bulk of the liquid. The former would take both the electrons from the electrode and the H+ from the solution, to synthesize H2. While the latter, would employ that molecular hydrogen as an electron donor to produce acetate by reducing the supplied CO2. Whole metagenome shotgun (WMS) data generated from different types of samples from this system was preprocessed and inputted to DIAMOND, a Blast-like software, and aligned to a reverse translated database built from UniRef sequences of electron transfer proteins. The outputted data was screened, sorted and counted for the column categories ‘group of proteins’ and ‘genus’. Using that information, a relative protein frequency comparison between the samples and a taxonomy profile assessment of each sample, were performed. At the same time, two different methods for the taxonomical assignment of WMS data, one using 16S rRNA (Metaxa2) and another (MetaPhlan) that didn’t, were compared in order to assess the limitations associated with 16S rRNA profiling. The 16S rRNA taxonomical assignment method worked better than the alternative but the comparison was deemed inconclusive. Finally, the global results seem to support, at least partially, the hypothesis of the previously mentioned work, but further investigation needs to be performed on the biocathode community and especially in the mechanism of action of several electron external transfer proteins candidates