A marine gammaproteobacterium, strain MS-02-063, was able to kill Chattonella marina, a noxious red tide phytoplankton. However, the algicidal activity of bacterial cells washed with the planktonic medium was significantly reduced. These results suggest that strain MS-02-063 produces an extracellular substance, the pigment, PG-L-1, that showed a potent algicidal effect on C. marina. The LD50 value of PG-L-1 was calculated to be approximately 8.5 μg ml-1. At the approximate LD50 concentration of 10 μg ml-1, a morphological change, which seemed to be due to the inhibition of cell division, was observed in C. marina. Almost all cells of C. marina were destroyed readily at 100 μg ml-1 of PG-L-1, and the cytostatic activity of PG-L-1 against this phytoplankton was observed at a concentration of 1 μg ml-1 during the 5 d of incubation. A sublethal concentration of PG-L-1 of 10 μg ml-1 significantly inhibited the reactive oxygen species (ROS) production by C. marina. ROS production has been previously reported to be essential for normal growth of C. marina (Oda et al. 1995; Biosci Biotechnol Biochem 59:2044-2048). Therefore, the inhibitory effect of PG-L-1 on ROS production may lead to growth inhibition of C. marina, at least in part. The pigment, PG-L-1, may be a useful compound not only as an applicable agent for the mitigation of harmful algal blooms, but also as an experimental tool to analyse the ROS production system in a red tide phytoplankton such as C. marina.
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