Co-utilization of microalgae and heterotrophic microorganisms improves wastewater treatment efficiency

Wastewater treatment using co-culture systems of microalgae and heterotrophic microorganisms is expected to be useful under atmospheric dilute carbon dioxide conditions. In this study, we investigated the combination of microalgae and heterotrophic microorganisms to improve the efficiency of wastewater treatment. Furthermore, to elucidate the cause of the changes in wastewater treatment efficiency in the co-culture system, changes in gene expression were revealed through transcriptome analysis. Three types of microalgae and five heterotrophic microorganisms were used in combination for wastewater treatment. The combination of Chlamydomonas reinhardtii NIES-2238 and Saccharomyces cerevisiae SH-4 showed the highest wastewater treatment efficiency. Using this combination for artificial wastewater treatment, the removal rates of TOC (Total organic carbon), PO4 3- , and NH 4 + reached 80%, 93%, and 63%, respectively, after 18 h of treatment. Transcriptome analysis revealed that the combined wastewater treatment altered the expression of 1371 and 692 genes in C. reinhardtii and S. cerevisiae , respectively. The genes upregulated in C. reinhardtii included those related to molecular and ion transport. Genes upregulated in S. cerevisiae included those related to cell protection from various types of damage and stress. To the best of our knowledge, this is the first study to show that a combination of green algae and yeast improves the efficiency of wastewater treatment. As both the green alga C. reinhardtii and the yeast S. cerevisiae are highly safe microorganisms, the establishment of their effective combination for wastewater treatment is highly significant.