The Impact of Selenium on an Archaea-dominated, Methanogenic Granular Sludge Consortium

Selenium (Se) is one of the latest chemical elements identified as a terminal electron acceptor in anaerobic respiration. Before this important discovery, Se was mainly known for its toxicity and its limited use as an essential element for microbial metabolism (Staicu et al., 2019). This study screened the presence of selenium respiratory genes/operons and the impact of selenium on the microbial community from a methanogenic anaerobic granular sludge. The inoculum was collected from a full-scale bioreactor treating dairy wastewater (Ireland). To identify the presence of selenium respiration, the following primers were designed: for selenate, SeO42- (Thauera selenatis, Bacillus selenatarsenatis SF-1) and for selenite, SeO32- (Bacillus selenitireducens MLS10). The primers were designed based on the available sequences and refined using the UniPriVal algorithm developed by our group at University of Warsaw. Apart from Se respiration, the study also investigated the change in bacterial diversity using 16S rRNA genes as a function of incubation time and exposure to selenate (with acetate as electron donor) and selenite (with lactate as e-donor). An additional focus of the study attempted to shed light into the biomineralization of elemental Se, Se0, since this process is still not fully understood (e.g. intracellular vs extracellular nucleation, growth of the particles by an Ostwald-like ripening process) (Staicu and Barton, 2017). The results indicate the high contribution of Archaea (22% - 37%) to the community of the granular sludge. The addition of 10 mM selenate/selenite did not have an impact on the Archaean community, nor did it trigger the reduction of selenate/selenite to Se0 via a known respiratory pathway. On the other hand, selenium addition did not particularly affect the community structure. There was a shift towards Bacteroidetes during the 7-day incubation period, with a high and consistent contribution of Firmicutes over Bacilli, which are putative host of selenate and selenite reductases. Furthermore, no significant Betaproteobacteria (Thauera selenatis) were identified, other putative host of selenate reductase. The biogenic Se0 particles were localized both outside and at the intracellular level, displaying polydisperse size distribution (<100-500 nm), indicative of different biomineralization mechanisms involved. Overall, this dataset indicates the reduction of selenium might be a purely detoxification process in methanogenic Archaea-dominated microbial communities, which brings into question the use of high energy-dense selenium oxyanions for respiratory purposes in certain biogeochemical conditions. Alternatively, this might emphasize the uncharted respiratory potential of selenium, since the process was reported to have diverse ecosystem distribution (Steinberg and Oremland, 1990). Reference Staicu LC, Barton LL (2017) Microbial metabolism of selenium – for survival or profit. In: Bioremediation of selenium contaminated wastewaters, van Hullebusch ED (Ed.), Springer, 1-31. Staicu LC, Simon S, Guibaud G et al. (2019) Biogeochemistry of trace elements in anaerobic digesters. In: Trace elements in anaerobic biotechnologies, Fermoso FF et al. (Eds.), IWA, 23-50. Steinberg NA, Oremland RS (1990) Dissimilatory selenate reduction potentials in a diversity of sediment types. Appl Environ Microbiol 56, 3550-57.