Identification of the bacteria associated to the phycosphere of the Chlorella-like strain SEC_LI_ChL_1

The associations and the co-evolution of bacteria and eukaryotic microalgae are raising great interest in the last years, especially in the "phycosphere", that is the area around the algal cells, where extracellular products of the algae are used by bacteria; here, different interactions between microalgae and bacteria might occur. The Chlorella-like strain SEC_LI_ChL_1 was previously characterized with a multidisciplinary integrated approach based on phylogenetic reconstructions, morphological-ultrastructural analysis and physiological characterization in presence of different trophic conditions. In this work, the isolation and characterization of fifteen cultivable and one uncultivable bacterial strains strictly associated to the phycosphere of strain SEC_LI_ChL_1 was carried out. The molecular identification followed by the taxonomic reconstruction allowed to highlight the presence of a "primary" culturable associated bacterium taxonomically related to Stenotrophomonas rhizophila, and other culturable bacterial strains taxonomically related to Actinobacteria (Microbacterium), Gammaproteobacteria (Pseudomonas, Stenotrophomonas and Pseudoxanthomonas), Alphaproteobacteria (Bosea, and Brevundimonas), Bacteroidetes and Firmicutes (Priestia and Bacillus). Since the microalgal strain was tolerant to different metal concentrations, the cultivable bacterial strains were exposed to the same metals at the same concentrations, highlighting similar tolerance patterns to the microalgae. The IAA production ability was tested as well in order to highlight this PGP trait in the isolated strains. An uncultivable bacterium taxonomically related to Shinella sp. was also molecularly characterized from the DNA of washed algal cells thus suggesting, in this case, an even more strict connection with the algae. A possible role in the microalgal growth promotion and defence against environmental pollutants is here discussed for all the bacterial strains associated to the SEC_LI_ChL_1 phycosphere.