Lytic Effects Of Bacteria In The Phycosphere Of Red-tide Algae

Bacteria have a unique ecological relationship with algae in the phycosphere, due to the utilization of the algal extracellular products as growth nutrient. Some bacteria in the phycosphere could control the growth of algal cells and even lyse them. As a result, it shows significant importance to study the bacteria in the phycosphere and apply them to inhibit the growth of red-tide algae.In the present study, the lytic effects of bacteria on cultured microalgae Skeletonema costatum and Scrippsiella trochoidea were studied by adding 1%(v/v) and 10%(v/v) of the bacteria culture medium 2216E to the cultures of two red tide microalgae S. costatum and S. trochoidea, respectively. The microalgal cultures without additions of 2216E medium were used as the first negative control, while the axenic microalgal cultures (obtained by antibiotics treatment) with additions of 10% (v/v) of 2216E medium were used as the second negative control. The results showed that the growth of microalgae, S. costatum and S. trochoidea, in the treatment of 10% bacteria medium addition was dramatically inhibited (P< 0.001) in the algicidal experiment. After 96 hours of co-culture, most of the algal cells were lysed and exhibited very low levels of both algal cell density, chlorophyll-a concentration and variable chlorophyll fluorescence (Fv/Fm) (P< 0.001). Meanwhile, the bacterial abundance in the two microalgal cultures increased about 250-300 times following the algal lysis, which suggests that the cause-effect relationship between the increase of bacterial abundance and the dissolution of algal cells.By exploring the sole-carbon-source utilization profiles using Biolog ECO microplates, average well color development (AWCD) and diversity indexes were not significantly changed at different times of the algicidal process, but the utilization of carbon source type was evidently different.Eleven bacterial colonies were isolated from the phycosphere, of which five were from the phycosphere of S. costatum and six from S. trochoidea. The 16S rDNA sequence analysis showed that these bacteria belong to Proteobacteria taxa and Bacteroides groups. Bacteria in the phycosphere of algae had different growth rate, and the faster-growing bacteria showed relatively better algicidal effect. For example, the bacterium SK-5 in the phycosphere of S. costatum and the bacterium ST-6 in the phycosphere of S. trochoidea both grew the best and showed distinct lysis ability (P< 0.001). The specificity and threshold concentration of algae-lysing bacteria have the practical significance in the control of red tide. The effects of the selected algae-lysing bacteria SK-5 and ST-6 on supplied algae were related to inoculated concentration, and the higher concentration showed more significant effect. SK-5 and ST-6 had non-species-specific algicidal effect.