Studies On The Relationships Between Microalgae And Phycosphere Bacteria And Molecular Identification Of Culturable Bacteria Isolated From Typical Red Tid Microalgae

Community composition and abundance of bacteria which isolated from the phycosphere ofGyrodinium instriatum, Skeletonema costatum, and Chattonella marina were studied in order toknow the bacterial community composition in different algal growth stages. Meanwhile theinhibitory effects of bacteria on the growth of algal cells were studied. and revealed role ofbacteria in the red tide generating and vanishing process. The purpose of the study is tounderstand the role of bacteria in in the initiation and collapse of algal blooms, and to providesome basis information in mechanisms of bloom occurrence and means of bloom controlling.Culturable bacteria were isolated from the different growth stages of G. instriatum, S.costatum and C. marina using gradient dilution method, and were identified based on sequencesof16S rDNA V3region. Phylogenetic trees were constructed using sequences in this study andthe closest related sequences from GenBank. The genetic distances were calculated. Meanwhile,bacteria in different growth stages of these algae were analyzed quantitatively. Results showedthat12different strains of bacteria were isolated from G. instriatum,7strains from S. costatumand C. marina, respectively, which belonged to Alphaproteobacteria, Gammaproteobacteria,Bacteroidetes, and Actinobacteria. Alphaproteobacteria was the most diversified and abundantgroup, followed by Bacteroidetes and Actinobacteria. The bacterial community structures weresimilar within different algal species, but varied among growth stages. The species number ofculturable bacteria was highest in the stationary stage, and the abundance peaked in early declinestage. Results suggested that bacterial community might play important roles in the growth,population competition, and bloom dynamics of algal cells.Results of the screen experiments showed that the bacteria strain Gi-1isolated from G.instriatum had significantly inhibitory effects on growth of the tested alga Chaetoceroscurvisetus, and had no significantly inhibitory effects on growth of G. instriatum. The bacteriastrain Gi-1was identified as Acinetobacter baumannii, which belongs to γ-proteobacteria. Gi-1had significant inhibitory effects on the growth of S. costatum, P. tricornutum, C. curvisetus, S.trochoidea and K. mikimotoi. Gi-1had inhibitory effects on growth of S. costatum, S. trochoidea,K. mikimotoi and P. tricornutum at all concentrations in this experiment (7.5×104-7.5×107CFU/mL), however inhibited the growth of C. curvisetus only at concentration of7.5×107CFU/mL. The abundance of Gi-1increased as well in co-cultures with microalgae, were higherin cultures with significantly inhibitory effects. C. curvisetus was less sensitive to Gi-1, then followed by P. tricornutum. The results suggested that Gi-1had strong inhibitory effects on thegrowth of most microalgal species tested in this study, and the bacterium might play animportant role in phytoplankton interspecific competition and bloom occurrence of G.instriatum..