Prokaryote species richness is positively correlated with eukaryote abundance in wastewater treatment biofilms.

Biological treatment represents a key step in nutrient removal from wastewater. Until now these process has mainly been considered prokaryotic, with the interactions between prokaryotes and eukaryotes not being properly explored. We therefore investigated the co-occurrence of eukaryotes and prokaryotes in biological nitrogen removal biofilms. We found that biofilms in the nitrifying reactor contained the highest diversity and abundance of both prokaryotes and eukaryotes, with nearly three times higher prokaryote species richness than for the denitrifying reactor. The positive associations between eukaryote abundance and prokaryote diversity could potentially be explained by mutualism - and/or predator/prey interactions. Further mechanistic insight, however, is needed to determine the main diversifying mechanisms. In summary, eukaryote and prokaryote interactions seem to play a fundamental yet underexplored role in biological wastewater treatment.

SIGNIFICANCE AND IMPACT OF THE STUDY: Eukaryote and prokaryote interactions may play an important role in wastewater treatment. This study found that prokaryote species richness was nearly three times higher in the aerobe nitrification than in an anaerobe denitrification reactor, coinciding with the highest level of eukaryotes. This knowledge can be important in process control, and potentially in the development of novel approaches based on nitrate accumulating denitrifying eukaryotes.