Single Granules as highly-replicated whole-ecosystems. Single Methanogenic Granules

Anaerobic granules are characteristic of wastewater treatment bioreactors such as upflow anaerobic sludge blanket (UASB) reactors. Granules are naturally-occurring spherical biofilms in bioreactors composed of many microorganisms involved in the anaerobic digestion process: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Optimal bioreactor performance depends on the thermodynamics and the crucial metabolic interactions of fermenters, syntrophs and methanogens. Many of these processes remain poorly understood and the specific species involved in granular growth and development are rarely studied at the level of a single granule.Previous data supported the idea that granules could be considered as distinct, replicated, whole ecosystems. On this basis, and by using novel, ‘micro-sequencing batch reactors’ (μSBRs), a series of high-throughput experiments was set up to investigate the single, whole-ecosystem microbiology of granules under a number of distinct environmental conditions.Individual granules in μSBRs were fed every 48 hours over 42 days. Granules were subjected to various, distinct conditions of pH (4, 7 and 10), carbon source (a VFA mixture, acetate, cellulose and glucose), trace elements availability (e.g. cobalt- deprivation) and temperature (37°C and 23°C). For each condition, assays were set up with and without the methanogenic inhibitor, 2-bromoethanesulfonate (BES), to also investigate non-methanogenic pathways. RNA was extracted from individual granules at the conclusion of μSBR trials and cDNA was sequenced.An analysis of the volatile fatty acids (VFA) present in BES-treated granules indicated that there was homoacetogenic activity. The core findings were that community structure succession shifted in response to environmental conditions and that species patterns shifted in the same way within triplicate granules.