The potential of Co-cultivation of plant growth promoting bacteria (PGPB) and Chlorella sp. LZ-29 to enhance the biomass and protein content of Chlorella sp. LZ-29

The co-cultivation of microalgae with bacteria that stimulate microalgae growth and metabolic changes is a promising strategy for improving microalgae yield and quality. In this study, two plant growth promoting bacteria (PGPB), Bacillus safensis BAI-6 (BAI-6) and Bacillus altitudinis FEN-9 (FEN-9), were isolated from a long-term open cultivation system of Chlorella sp. LZ-29. Both strains were confirmed to produce the growth-promoting phytohormone indole-3-acetic acid (IAA) and vitamin B12. The changes in biomass, protein, fat, polysaccharide, pigment and other active components of Chlorella sp. LZ-29 under co-cultivation with these strains were investigated. Compared with the axenic Chlorella culture, co-cultivation with BAI-6 or FEN-9 alone increased biomass by 46.57% and 58.75%, respectively, while the dual-strain co-culture achieved the highest biomass increase of 66.46%. Protein content increased by 7% with BAI-6, 2.56% with FEN-9, and 13.37% in the dual-strain co-culture. Conversely, fat content decreased by 3.05%, 3.67%, and 3.48%, respectively, while polysaccharide content also declined in all co-culture treatments ( p < 0.05). The synergistic effect of BAI-6 and FEN-9 in dual co-culture resulted in significantly enhanced biomass and protein production ( p < 0.01), likely due to increased secretion of IAA and vitamin B12, which promoted nitrogen assimilation and cell division, while inhibiting carbon flux towards lipid synthesis. These findings demonstrate that the dual-strain co-culture system has significant potential for enhancing both the yield and protein content of Chlorella sp. LZ-29.