Abstract: Synechococcus and its phycosphere bacteria can achieve mutualism through functional complementarity. In this study, the functional characteristics and ecological effects of stable symbiotic dominant bacteria were investigated by analyzing the differences of community structure and physiological indices of the algal-bacteria system before and after bacteria removal of in situ Synechococcus enrichment culture system. The results showed that Synechococcus mainly belonged to S5.1_VIII subgroup, and that the dominant classes of symbiotic bacteria were α Proteobacteria, γ Proteobacteria, and Bacteroides. After bacteria removal, community structure of recombinant stable symbiotic bacteria tended to be consistent, with Ruegeria and Marianobacter as the dominant populations. The results of function annotation showed that the relative abundance of bacterial groups involved in carbon and nitrogen cycling was 88.47% and 11.08%, respectively. Affected by symbiotic dominant bacterial community, phycobiliprotein contents and maximum photosynthetic capacity of Synechococcus significantly changed, and light conversion efficiency increased. Furthermore, Synechococcus released large amounts of silicon during cultivation, which may provide ballasting mineral for the aggregation and sinking of pico-phytoplankton and promote marine carbon cycling.

Key words: picocyanobacteria, phycosphere, bacteria-algae interaction, population diversity, ecosystem function