Algae support populations of heterotrophic, nitrifying, and phosphate-accumulating bacteria in the treatment of poultry litter anaerobic digestate

The objective of this study was to investigate the impact that algae have on bacterial treatment of poultry litter slurry anaerobic digestate. We hypothesized that algae could provide benefits to heterotrophic and nitrifying organisms, due in part to provision of dissolved oxygen. No other studies, to our knowledge, have shown that algae provide benefits to nitrifying bacteria in the treatment of anaerobic digestates. Batch cultures of the native digestate community were grown alone or with the addition of activated sludge microorganisms and/or the green algae, Chlorella sorokiniana. Water quality measurements, 16S rRNA sequencing, and quantitative PCR were utilized to elucidate the impacts of algae on bacterial populations involved in organics removal and nutrient cycling. The presence of activated sludge promoted algal growth by 29%, nitrogen uptake by 53%, and phosphate removal by 21%. In turn, the presence of algae promoted a 57% increase in organics removal by heterotrophic bacteria. Algae also promoted full oxidation of ammonium to nitrate by nitrifying bacteria whereas incomplete nitrification was observed in the absence of algae. This result was likely driven by algal support of the commamox bacteria, Nitrospira, which otherwise died off in the anaerobic digestate. We also show here, for the first time, that C. sorokiniana sustained the relative abundance of the phosphate-accumulating bacteria, Candidatus Accumulibacter, whereas this organism declined in relative abundance by 94% when algae were absent (p < 0.001). These results suggest that C. sorokiniana supports particular heterotrophs, nitrifiers, and phosphate-accumulating bacteria, thereby improving treatment of poultry litter anaerobic digestate.