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

Mixed microbial communities (e.g., granular sludge) are an invaluable research topic owing to their high and uncharted biodiversity, as well as to the numerous biotechnological applications they can sustain. This chapter explores the impact of selenium (Se) oxyanions (selenite and selenate) on an anaerobic granular sludge consortium. We used a bioinformatics approach (taxonomic assignment of 24 genes related to Se metabolism using the NCBI NT database), coupled with experimental data (PCR screening of 13 genes from three operons involved in Se respiration). In addition, the study investigates the change in bacterial diversity using 16S rRNA genes, as well as the impact of Se on the methanogens present in the granular sludge and the biomineralization of Se0. The microbial consortium had a high methanogenic archaeal abundance (Euryarchaeota and Halobacterota) ranging from 19.4% to 37.7%, which was unaffected by the exposure to Se oxyanions. In contrast, the bacterial phyla displayed much higher diversity, evident at the phylum and genus levels. Three diversity indices (Shannon’s index, Faith’s phylogenetic diversity, and richness) showed a decrease in microbial diversity in the presence of both selenite and selenate, with a higher impact induced by selenite. Of the two oxyanions, only selenite was reduced/biomineralized to red Se0. No known gene/operon capable of respiring Se was detected; the reduction of selenite was likely mediated by a detoxification strategy or by an unknown respiratory gene/operon. Se0 biomineralization was characterized by red, globular particles, present intra- and extracellularly. Overall, the results indicate the high resilience of methanogenic archaeal communities against toxic Se oxyanions and the concomitant biomineralization of Se0, opening the prospects for the joint biotechnological methane and Se0 recovery.