Insight into the formation mechanism of algal biofilm in soy sauce wastewater

Microalgal-biofilm-based wastewater treatment refers to a microalgae-dominated biofilm community in nonaxenic systems, in which co-existing bacteria and microbial extracellular polymeric substances (EPS) are crucial for the formation of algal biofilm. However, algae-bacteria consortium biofilm-forming mechanism has rarely been studied in wastewaters including soy sauce wastewater (SW). In this study, we conducted algal biofilm cultivation in raw SW (RSW) and sterilized SW (SSW), and determined the characteristics of algal biofilm and its EPS in the two SW treatments. The results demonstrated that the co-existing bacteria in the algal biofilm system significantly enhanced attached biomass and nutrient removal from RSW, compared to SSW without bacteria. The enhancement may be attributed to EPS excreted by dominated Acinetobacter and Comamonas (both Proteobacteria) in the co-existing bacteria community. The algae-bacteria consortium from algal biofilm in RSW excreted the contents of relatively high tightly bound EPS (TB-EPS) and its proteins (TB-PN), which were conducive to forming the algal biofilm. The high TB-EPS content was related to soluble microbial by-product-like substances (mainly proteins), and its high TB-PN content was mainly caused by the increases in hydrophobic glycine and serine contents for stable biofilm formation. Based on these results, we enhanced our understanding of the formation mechanism of algal biofilm in SW remediation, which provided the theoretical basis for biofilm formation by algae-bacteria synergism.

Automated summary

Microalgal-biofilm-based wastewater treatment involves the use of microalgae-dominated biofilm communities in nonaxenic systems, where co-existing bacteria and microbial extracellular polymeric substances (EPS) play a crucial role in biofilm formation. However, the mechanism of algae-bacteria consortium biofilm formation in wastewaters such as soy sauce wastewater (SW) has not been well studied. This study cultivated algal biofilms in raw SW (RSW) and sterilized SW (SSW), and analyzed the characteristics of the biofilms and their EPS in these two treatments. The results showed that co-existing bacteria significantly enhanced biomass attachment and nutrient removal in the algal biofilm system with RSW compared to SSW without bacteria. This enhancement was likely due to EPS excreted by dominant Proteobacteria species such as Acinetobacter and Comamonas in the bacterial community. The algal biofilm in RSW produced relatively high levels of tightly bound EPS (TB-EPS) and its proteins (TB-PN), which contributed to biofilm formation. The increased TB-EPS content was related to soluble microbial by-product-like substances, mainly proteins, and the high TB-PN content was mainly caused by an increase in hydrophobic glycine and serine content, promoting stable biofilm formation. Overall, this study enhances our understanding of algal biofilm formation mechanisms in SW treatment, providing a theoretical basis for algae-bacteria synergism in biofilm formation.