Growth promotion of Chlorella by symbiotic bacteria under adverse environments

This study is aimed at searching for microalgae growth-promoting bacteria (MGPB) that can promote Chlorella growth in adverse environments. Fifty strains of symbiotic bacteria were isolated from four Chlorella species and identified by 16S rRNA gene sequences. Function screening and binary culture experiments classified forty-three of these strains as MGPB. In adversities (iron deficiency, high salinity, or unsuitable seawater environment), a significant increase in growth parameters was observed for MGPB-treated microalgae compared to the uninoculated control, and Dinoroseobacter shibae was the most effective strain. Using inorganic nutrient-rich and microalgae-free seawater (S-seawater and Q-seawater) as media, aseptic Chlorella thrived in Q-seawater but could not grow in S-seawater, which was relatively rich in Alteromonas and Vibrio. However, Chlorella could thrive when 250 μL /L D. shibae (OD600 = 0.1) was added into S-seawater. Chlorella growth rates in S- and Q-seawater inoculated with D. shibae were not significantly different (P > 0.05), but were all significantly (P < 0.05) lower than in Q-seawater without D. shibae addition. 16S rRNA gene profiling revealed that similar phycosphere bacterial communities emerged in microalgae grown in Q- and S-seawater supplemented with D. shibae, manifested by recruiting more Rhodobacteriaceae (such as Ruggieella sp.) and fewer Pseudomonadaceae (such as Pseudomonas sp.). These results indicate that phycosphere bacterial communities could be modified by symbiotic MGPB addition. MGPB inoculation showed a positive effect on the growth of Chlorella under stress conditions but might be invalid under favorable environments. These also suggest that these isolated MGPBs, specifically D. shibae, can be exploited as a bioinoculant for the initial establishment of microalgal communities in adverse environments.