Early microalgal-bacterial flocs
DNA extraction from each time point was conducted in triplicate. In addition, the starting inoculum was also extracted in triplicate. Specifically, 15 mL of harvested sludge was centrifuged at 5500 rpm and the supernatant discarded. The cell pellets were immediately frozen at -80 C until further processing. DNA was extracted by using the DNeasy PowerSoil Isolation Kit (Qiagen GmbH, Hilden, Germany). The quantity and quality of DNA were spectrophotometrically determined with a NanoDrop (ThermoFisher Scientific, USA). The 16S rRNA gene V3/V4 variable region was amplified using primer pair 341F (CCTACGGGNGGCWGCAG) and 805R (GACTACHVGGGTATCTAATCC) (Herlemann et al., 2011).