Symbiotic association among marine microalgae and bacterial flora: a study with special reference to commercially important Isochrysis galbana culture

Bacterial associations play a substantial role in large-scale culture and production systems of microalgae which are used in finfish and shellfish hatcheries as live feed and also as products such as algal paste, spray dried and freeze dried algal powders. The present study explores interaction between the marine microalga Isochrysis galbana MBTDCMFRI S002 and its associated bacteria. The algal-bacterial interaction was investigated by co-culturing axenic algal culture with two bacterial symbionts-Alteromonas sp. (MBTDCMFRI Mab 25) and Labrenzia sp. (MBTDCMFRI Mab 26). There was an increase in algal biomass accumulation and growth rate in the presence of added bacterial symbionts which indicates their algal growth promoting role. In addition, our study showed the potential of these bacterial strains to release various growth stimulatory compounds such as antioxidants, siderophores and indole-3-acetic acid which could have a significant positive impact on algal growth. The heterotrophic growth of these bacterial strains on extracellular carbon produced by I. galbana is evident in the present work. Thus, our findings showed that a mutually beneficial associations exists between I. galbana and coexisting bacterial flora which can be further explored to improve productivity and sustainability of aquaculture algal rearing systems.