Isolation and characterization of bacteria from natural microbiota regrown with Chlamydomonas reinhardtii in synthetic co-cultures

Microalgae coexist with bacteria in natural aquatic environments and mass-cultivation systems, and these associated bacteria can have a potential impact on host microalgal growth. However, the characteristics of microalgae-associated bacteria and their effects on host microalgae have not yet been fully studied. In particular, no reports on the isolation of bacteria associated with Chlamydomonas reinhardtii, a green microalga, have been made. This study aimed to isolate the bacteria associated with C. reinhardtii from synthetic co-cultures of C. reinhardtii and various freshwater bacteria and to characterize them, focusing on their taxonomic characteristics, growth-promoting factors, and growth-promoting or -inhibiting effects on the host, C. reinhardtii. In this study, 147 bacterial strains were isolated from C. reinhardtii cultures. Of the isolated bacteria, 72.1 % and 19.7 % belonged to the phyla Pseudomonadota and Bacteroidota, respectively. Of the isolated bacteria, 54.4 %, 74.1 %, and 66.0 % produced indole-3-acetic acid (IAA), siderophores, and vitamin B₇, respectively, which are possible growth-promoting factors for microalgae. More than half of the bacteria exhibited two or three of these factors. Furthermore, 65.3 % of the isolated bacteria promoted C. reinhardtii growth, whereas 17.7 % inhibited it. Thus, promotive and inhibitory bacteria co-constructed the C. reinhardtii-associated bacterial community. In particular, Emticicia sp. JSL2 and JSL3, Caulobacter sp. SCP3, and Sediminibacterium sp. SCP4 exerted the strongest growth-promoting effect on C. reinhardtii. Among the isolated bacteria, IAA production was a key factor that promoted C. reinhardtii growth. These positive bacteria could be useful for improving C. reinhardtii biomass production via C. reinhardtii-bacteria co-cultures.