Summary of key findings Here we analysed the bacterial diversity of six oxygen reducing biocathodes formed from aerated sludges at different voltages. Pyrosequencing results show unexpectedly high bacterial diversity in cathodic biofilms. Some of the most electroactive bacterial communities are dominated by a new lineage belonging to Deinococci. Background and relevance Since the beginning of the 21st century, our knowledge on the diversity of bacteria able to catalyse electrochemical reactions has advanced fast, however the majority of the studies focused on anodic biofilms. On cathodes, the few analyses of the bacterial diversity have mostly relied on DGGE (Clauwaert, Van der Ha et al. 2007), clone library (Rabaey, Read et al. 2008, Sun, Wei et al. 2012) or PhyloChip (Wrighton, Virdis et al. 2010). In this study we used pyrosequencing to study the bacterial communities on oxygen reducing biocathodes. Results Four oxygen reducing biocathodes were formed on carbon-cloth at -0.2 V/CSE, and two at +0.4 V/CSE with a switch to -0.2V/SCE after 63 days without current. Chambers were inoculated with aerated sludge. Electrodes performances are summarized in Table 1. Bacterial communities of the cathodic biofilms were analysed after 18 days for electrode 1-4 and 66 days for electrodes 5-6 using sequences obtained from pyrosequencing. There were 179 genera and 41 classes identified in the dataset. The dominant classes (representing more than 5% of sequences in at least one sample) were Acidimicrobiia, Actinobacteria, Bacilli, Clostridia, Deinococci, Gemmatimonadetes, α, β and γ–Proteobacteria (cf. Figure 1). The most significant components of the community (relative abundance > 10%) are members of the genera Truepera, Alcaligenes, Thauera, Candidatus microthrix, Pseudomonas or of unclassified genera belonging to orders Xanthomonadales, Gemmatimonadales and Bacillales. Most of the genera identified here (Alcaligenes, Thauera, Candidatus microthrix and Pseudomonas) are common bacteria from activated sludge. However the most remarkable feature of the community is the high representation of Truepera (between 2% and 22% of the genera on all electrodes) belonging to the class Deinococci which includes extremely radiation resistant bacteria and several other lineages only represented by environmental 16S rRNA gene sequences. Discussion For electrodes made at -0.2 V, a high relative proportion of Deinococci is correlated with good electrochemical performances. Indeed, they constitute the major group for electrodes with high intensities Jlim and Jair (electrodes 2 to 4) whereas the major genus found on electrode 1 belongs to β-Proteobacteria. Interestingly, a deeper phylogenetic analysis shows that the Deinococci found here constitute a new lineage sister to Truepera (Albuquerque, Simoes et al. 2005). Deinococci are often present in environmental bacterial communities but almost always in small proportion. This new lineage might thus include electroactive species constituting potential new candidates for the formation of efficient oxygen reducing biocathodes. The bacterial richness of our oxygen reducing cathodes are surprisingly high ranging from 190 estimated total OTUs for electrode 5 to 413 OTUs for electrode 1. Our analysis by pyrosequencing reveals that cathodic biofilms are much more diverse than previously evaluated with DGGE (Clauwaert, Van der Ha et al. 2007), clone library (Rabaey, Read et al. 2008, Sun, Wei et al. 2012) or PhyloChip (Wrighton, Virdis et al. 2010). References Albuquerque, L., C. Simoes, M. F. Nobre, N. M. Pino, J. R. Battista, M. T. Silva, F. A. Rainey and M. S. da Costa (2005) Truepera radiovictrix gen. nov., sp nov., a new radiation resistant species and the proposal of Trueperaceae fam. nov., Fems Microbiology Letters 247(2): 161-169. Clauwaert, P., D. Van der Ha, N. Boon, K. Verbeken, M. Verhaege, K. Rabaey and W. Verstraete (2007) Open air biocathode enables effective electricity generation with microbial fuel cells, Environmental Science & Technology 41(21): 7564-7569. Rabaey, K., S. T. Read, P. Clauwaert, S. Freguia, P. L. Bond, L. L. Blackall and J. Keller (2008) Cathodic oxygen reduction catalyzed by bacteria in microbial fuel cells, Isme Journal 2(5): 519-527. Sun, Y., J. Wei, P. Liang and X. Huang (2012) Microbial community analysis in biocathode microbial fuel cells packed with different materials, AMB Express 2(1): 1-8. Wrighton, K. C., B. Virdis, P. Clauwaert, S. T. Read, R. A. Daly, N. Boon, Y. Piceno, G. L. Andersen, J. D. Coates and K. Rabaey (2010) Bacterial community structure corresponds to performance during cathodic nitrate reduction, Isme Journal 4(11): 1443-1455.