More than meets the eye: microalgal-bacterial association in polyhydroxybutyrate (PHB) accumulating cultures of Desmodesmus communis

The focus on microalgae within a circular economy framework has been growing due to their ability to utilize inexpensive substrates and produce high-value biomolecules. Among these, the biopolymer polyhydroxybutyrate (PHB) can be accumulated as a storage compound in several microalgal biomasses under nutrient- and light-limiting conditions. Herein, the Chlorophyta Desmodesmus communis was grown under mixotrophic conditions (low light, phosphorus-free medium, 1 g L⁻¹ sodium acetate) in a 1 L semicontinuous system over 25 days. PHB accumulation progressively increased, reaching 57 % w/w on day 25. Since mixotrophic conditions also favor bacterial growth, the associated bacterial community was analyzed through 16S rRNA gene metabarcoding at different time points (day 0, 12, 21, 25) to investigate its potential contribution to PHB accumulation. A selection in bacterial genera was observed after prolonged cultivation. Moreover, antibiotic treatments markedly reduced bacterial diversity and PHB content, indicating a central bacterial role in PHB production and highlighting key genera likely involved. Environmental Scanning Electron Microscopy (ESEM) and Transmission Electron Microscopy (TEM) revealed close interactions between algal and bacterial cells, suggesting a potential endophytic presence of PHB-accumulating bacteria within D. communis cells. These findings propose a new perspective on microalgae-associated bacterial communities, viewing them not as contaminants but as essential contributors to microalgal growth and valuable metabolites production. The results emphasize the importance of considering the bacteria-microalgae consortium in PHB production research, rather than focusing solely on photoautotrophic organisms.