Microbial modulation in the biomass and toxin production of a red-tide causing alga

The effect of S 10, a strain of marine bacteria isolated from sediment in the Western Xiamen Sea, on the growth and paralytic shellfish poison (PSP) production in the alga Alexandrium tamarense ( A. tamarense) was studied under controlled experimental conditions. The results of these experiments have shown that the growth of A. tamarense is obviously inhibited by S 10 at high concentrations, however no evident effect on its growth was observed at low concentrations. Its PSP production was also inhibited by S 10 at different concentrations, especially at low concentrations. The toxicity of this strain of A. tamarense is about (0.95–12.14) × 10 −6 MU/cell, a peak toxicity value of 12.14 × 10 −6 MU/cell appeared on the 14th day, after which levels decreased gradually. The alga grew well in conditions of pH 6–8 and salinities of 20–34‰. The toxicity of the alga varied markedly at different pH and salinity levels. Toxicity decreased as pH increased, while it increased with salinity and reached a peak value at a salinity of 30‰, after which it declined gradually. S 10 at a concentration of 1.02 × 10 9 cells/ml inhibited growth and the PSP production of A. tamarense at different pH and salinity levels. S 10 had the strongest inhibitory function on the growth of A. tamarense under conditions of pH 7 and a salinity of 34‰. The best inhibitory effect on PSP production by A. tamarense was at pH 7, this inhibitory effect on PSP production did not relate to salinity. Interactions between marine bacteria and A. tamarense were also investigated using the flow cytometer technique (FCM) as well as direct microscope counting. S 10 was identified as being a member of the genus Bacillus, the difference in 16S rDNA between S 10 and Bacillus halmapalus was only 2%. The mechanism involved in the inhibition of growth and PSP production of A. tamarense by this strain of marine bacteria, and the prospect of using it and other marine bacteria in the bio-control of red-tides was discussed.