Semi-continuously operated single strain microalgal-bacterial biofilm for nutrients removal: microbial succession and performance improvement over extended non-axenic operation.

Microalgal-bacterial biofilm could realize synergistic pollutants removal, CO sequestration, and resource transformation from wastewater. Pre-designed biofilm with clear microbial composition would benefit resource transformation, yet little is known about its nutrients removal performance under axenic conditions, not to mention the comparison with non-axenic conditions over extended operation. To fill in this knowledge gap, this study first investigated the growth characteristics and nutrients removal performances of a pre-designed microalgae dominant biofilm. The biofilm was composed of Chlorella sorokiniana and Staphylococcus sp. and was operated under hydraulic retention times (HRTs) of 1∼4 days. Optimum HRT of 2 d was selected considering the trade-off between stable biomass composition and efficient nutrients removal. Afterwards, the pre-designed single strain microalgal-bacterial biofilm was operated under axenic and non-axenic conditions for 52 days. Mixed-species community was developed under non-axenic condition, and the microbial succession was tracked. Due to enhanced "cross-feeding" of soluble extracellular polymeric substances (EPS) under increased biodiversity, residual nitrogen, phosphorus, and total organic carbon concentrations under non-axenic conditions were respectively 70.1 %, 50.3 %, and 32.3 % lower than under axenic conditions. Respectively 2.0∼7.2 and 2.5∼5.5 times of nitrogen and phosphorus turnover from loosely-bound EPS to tightly-bound EPS were observed under non-axenic operation. Staphylococcus sp. was soon substituted during non-axenic operation by cyanobacteria (54 %∼80 % relative abundance) and other biofilm-promoting and EPS-utilizing prokaryotes. Chlorella just maintained its dominance as the eukaryotes, and gradually formed a dual eukaryotic microalgae system with halotolerant Halochlorella. Prokaryotic metabolism and biofilm maintenance functions remained stable under non-axenic conditions, and the enrichment of genes associated with N/P/C metabolism favored pollutants removal over axenic operation. The pre-designed biofilm under non-axenic operation realized the trade-off between overall controllable microalgae dominance and easy maintenance, improving pollutants removal via gradually enhanced prokaryotic biodiversity.