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 B 7 , 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. • Total of 147 bacteria were isolated from Chlamydomonas reinhardtii cultures. • Of the isolated bacteria, 66 % promoted C. reinhardtii growth. • Most isolated bacteria (95 %) had possible microalgal growth-promoting factors. • IAA from isolated bacteria was the key factor that promoted C. reinhardtii growth. • Emticicia sp., Caulobacter sp., Sediminibacterium sp. promoted growth more strongly.