Utilization of Indole-3-acetic acid–Secreting Bacteria in Algal Environment to Increase Biomass Accumulation of Ochromonas and Chlorella

Application of plant hormones is an effective strategy to further improve the accumulation of microalgae biomass. In this study, the IAA (indole-3-acetic acid) production capacity of symbiotic bacteria isolated from the culture system of Ochromonas and Chlorella was detected and compared, with the bacterial community structure revealed. The results demonstrated that Algoriphagus, Porphyrobacter, Roseococcus, and Rhizobium were the dominant bacterial genera both in Ochromonas and Chlorella system, distributed in Proteobacteria and Bacteroidetes. Sixteen of 40 strains classified into 15 genera isolated from the two genera of microalgae could produce IAA, with Agrobacterium and Rhizobium as the main representatives with highest IAA production level (22.53–34.18 mg/L in 30 h). And through determination of changes of chlorophyll content in the co-culture system of microalgae and the supernatant of IAA high-yielding bacteria suspension from medium with or without tryptophan (synthetic precursor of IAA), Rhizobium (C-3) and Agrobacterium (O-16, C-2) were proved to promote the growth of Ochromonas and Chlorella through secreting IAA. The biomass yield of Ochromonas and Chlorella could be respectively increased by 78.98% and 72.73% through the co-culture with Agrobacterium (C-2), by 68.15% and 69.93% for Rhizobium (C-3) and by 80.89% and 65.73% for Agrobacterium (O-16). Moreover, compared with the application of exogenous IAA to improve the microalgae biomass, it is more efficient and economical to construct algae bacteria symbiosis system using IAA secretory bacteria commonly existed in microalgae culture system. In addition, the growth promoting effect of IAA secretory bacteria on microalgae was different among the species of algae.