Oligo-cyanobacterial microalgae-bacteria granular sludge for mitigating cyanotoxin risk: Cultivation,...

Microalgae-bacteria granular sludge (MBGS) is a promising technology for wastewater treatment, due to its advantage of superior nutrient removal and potential for carbon-neutrality. However, cyanobacteria, which produce cyanotoxins and raise environmental concerns, was prevalent in previously cultivated MBGS. In this study, Oocystis borgei (O.borgei), which could inhibit cyanobacteria in microalgae-bacterial mixed culture (MBMC), was added in inoculum for the oligo-cyanobacterial MBGS cultivation. Flow cytometry and 16S/18S rRNA gene sequencing showed that the abundance of cyanobacteria was maintained at consistently low levels during the cultivation of MBGS. Moreover, the cyanotoxin concentration was only 0.15 μg/g-VSS, which was significantly lower than that in MBGS cultivated with other inoculums. Using autofluorescence-based cell sorting technology (AFCS), we found that the extracellular protein (PN) content of bacteria increased remarkably in the mixed culture with O.borgei, leading to the significant increase of extracellular polymeric substances (EPS) in MBMC. In addition, metagenomic analysis revealed that the abundance of lipoprotein (LPP) directed transport ATPase gene (lolD) and lipopolysaccharide (LPS) transport gene (lptB), key genes associated with EPS secretion, was increased from 493 to 343 fragments per kilobase of transcript per million mapped reads (FPKM) in inoculum to 775 and 548 FPKM in rapid granulation period. Genomic-annotation found that the Proteobacteria, encoded the EPS-producing genes, was dominated in oligo-cyanobacterial MBGS. These results suggested that microalgae could stimulate Proteobacteria to secrete PN, promoting MBGS formation. This work provides an effective approach for oligo-cyanobacterial MBGS cultivation and deep insight into the granulation mechanism, which provides theoretical support for the engineering application of MBGS technology.