Direct processing of alginate-immobilized microalgae into polyhydroxybutyrate using marine bacterium of Saccharophagus degradans

[Display omitted] •Microalgae harvested from wastewater treatment was rich in carbohydrate and protein.•S. degradans effectively degraded alginate-immobilized microalgae as carbon source.•When pH was lower than 6 S. degradans growth was significantly inhibited.•When glucose was available S. degradans degradation of alginate was repressed.•PHB produced by S. degradans was degraded when available carbon source is limited. Alginate immobilized microalgae (AIM) was found efficient in algal cells separation and pollutants removal, however, its processing required alginate removal. In present study, polysaccharide-degrading bacterium of Saccharophagus degradans was used to biodegrade alginate and microalgae in AIM and produce polyhydroxybutyrate (PHB). Results showed that AIM cultivated in wastewater contained 34.0% carbohydrate and 45.7% protein. S. degradans effectively degraded and utilized polysaccharide of AIM to maintain five-day continuous growth at 7.1–8.8 log CFU/mL. Compared with glucose, S. degradans metabolism of mixed polysaccharide in AIM maintained the medium pH at 7.1–7.8. Increasing the inoculum concentration did not enhance AIM utilization by S. degradans due to the carbon catabolite repression of glucose which likely inactivated hydrolysis enzymes. PHB production in S. degradans peaked at 64.9 mg/L after 72 h cultivation but was later degraded to provide energy. Conclusively, S. degradans was effective in direct processing of AIM while showing potential in PHB production.