Exploration of applying growth-promotion bacteria of Chlorella sorokiniana to open cultivation systems

Nowadays, artificial construction of bacteria-algae consortia to enhance microalgal biomass is prevalent in enclosed systems, while few are built in an open culture. In this study, Achromobacter sp. and Rhizobium sp., isolated from an open pond of Chlorella sorokiniana , were the microalgal growth-promotion bacteria and selected to build the bacteria–algae consortia with axenic C. sorokiniana in open cultivation systems. To examine the performance of these two artificial bacteria–algae consortia in open culture under stable cultivation conditions, the co-cultivation experiments were conducted under constant temperature and light intensity indoors. It was found that Achromobacter sp. gradually lost the dominance of the population in the co-culture and failed to promote the growth of C. sorokiniana during open cultivation. However, the Rhizobium sp. maintained its dominant population of bacterial community in open culture and could promote the growth of C. sorokiniana , with an enhancement of 13.76%. To further evaluate the effects of Rhizobium sp. on microalgae under variations of temperature and sunlight intensity conditions, the open co-cultivation experiments were built outdoors. Results showed that the growth of C. sorokiniana could rise 13.29% only when Rhizobium sp. was added to the culture continuously, and addition of bacterial solution in log-phase of microalgae could help Rhizobium sp. dominate in the bacterial community. In this way, addition of Rhizobium sp. in the log-phase of C. sorokiniana should be an effective process to be applied to open ponds cultivation. Our findings are a step toward applying growth-promotion bacteria for C. sorokiniana for applications in open cultivation systems.