Photosynthetic activity assessment in mixotrophically cultured Chlorella vulgaris biofilms at various developmental stages

Abstract

Biofilm reactors have been used to grow the microalga Chlorella vulgaris, which under mixotrophic conditions can effectively grow as biofilm with reduced liquid volumes and low energy consumption requirements. Under mixotrophic conditions, CO2 is internally generated and can be reduced to carbohydrates or lipids, with changes in gene expression. However, our knowledge of the photosynthetic activity and gene expression profiles of C. vulgaris cultured in a biofilm reactor using glycerol and urea in the absence of external CO2 is limited. Our goal was to assess the 1) chlorophyll-a fluorescence, 2) oxygen evolution, and 3) gene expression levels of C. vulgaris biofilms in the exponential growth (0–96 h) and stationary phases at the onset of lipid biosynthesis (168 h). The maximum quantum efficiency of PSII (Fv/Fm) varied between 0.64 and 0.69, indicating no stress during growth for the mixotrophic C. vulgaris biofilms. In photosynthetic oxygen evolution measurements C. vulgaris mixotrophic cells exhibited dark respiration (higher than 25 mg O2 mg Chl-a−1 s−1); at the same time, they were photosynthetically active with electron transport rate (ETR) values up to 50 μmol electrons m−2 s−1. The change in gene expression of light-regulated enzymes linked to carbon fixation, such as rbcl, was slightly higher in the stationary phase, while the gene expression of other enzymes, such as G3PD and PRK, was reduced in 168-hour-old biofilms. On the other hand, the gene expression of MDH for carbon metabolism and of enzymes ACC and DGAT for lipid biosynthesis was upregulated in 168-hour-old biofilms. These results show that in algal biofilms under mixotrophic growth conditions, autotrophy is not limited by glycerol and is fully complemented by the heterotrophic metabolism through a cycle in which photosynthesis utilizes the CO2 released by the respiration of C. vulgaris in the presence of glycerol and urea.