Time series analyses of Anaerobic Digesters at WWTPs reveal factors affecting community composition

Anaerobic digestion (AD) at wastewater treatment plants (WWTPs) is carried out by groups of microorganisms, active in complex metabolic networks. Recent studies of community structure have shown that there are two large fractions, growing(G) and non-growing (NG), the latter being up >40% of bacteria, all immigrating from influent and surplus activated sludge. Therefore, it is important to focus on G fraction to better understand ecology and ecophysiology of process-critical species.
The aim was to describe community composition and stability in mesophilic and thermophilic AD, and to investigate fate of specific G species during a change from meso- to thermophilic conditions, using MiDAS database.
AD samples were obtained every 2 weeks at Danish WWTPs from 2015-19. Bacteria and archaea were identified by 16S rRNA gene amplicon sequencing. MiDAS 3 taxonomy was used with species-level resolution.
Microbial communities at species-level showed clear differences between AD reactor types. NG taxa, such as Microthrix and Tetrasphaera, were only present because they were continuously added by food streams. Between G species, almost only uncharacterized genera and species were among most abundant in both mesophilic and thermophilic ADs. For all reactors, a high level of community stability was observed, with only minor changes over the 5 years period. However, this was not the case with NG fraction, which showed large fluctuations. A detailed study of an AD at Aalborg East WWTP, which was perturbated from mesophilic to thermophilic conditions, showed clear changes in community. This concludes importance of focus on G bacteria fraction when investigating ADs, and importance of temperature changes on top abundant bacteria.