Priliminary Study On The Prevention To The Red-tide Of Karenia Mikimotoi With The Competition Of Marine Economic Microalgae

Karenia mikimotoi, a hemolytic toxin and cytolytic toxin producer, causes a high mortality of fish and invertebrate. It is one of the most notorious red-tide forming dinoflagellates in the world. In order to controll or mitigate harmful algal blooms with biological strategies, five marine economic microalgae were compared for their competition with Karenia mikimotoi in growth. The mechanism underlining such competition at diverse nutrient levels and allelopathy were studied preliminarily.The main findings were outlined as the following:1. Since their wide adaptation range of temperature and salinity, easiness in high density cultivation and rapid propagation, five typical marine microalgae (Dunaliella salina, Latymonas subcordiformis, Pavloca viridis, Isochrysis galbana, and Phaeodactylum tricornutum) were studied for their competitions with Karenia mikimotoi. It was found that such competition was species and initial inoculation amount dependent. The microalgae used can be categorized into 3 groups: strong inhibitor, moderate inhibitor and victim. As a strong inhibitor, Platymonas subcordiformis caused the death of Karenia mikimotoi to almost completion in 9 days of co-cultivation when the ratio of cell number was 1:1 and in 7 days when the ratio of either cell surface or cell volume was 1:1. Pavloca viridis, Isochrysis galbana and Phaeodactylum tricornutum showed moderate inhibition to Karenia mikimotoi in a manner similar to that of Platymonas subcordiformis. However, Karenia mikimotoi survived the whole cultivation period. Dunaliella salina was not able to inhibit the growth of Karenia mikimotoi. Reversely, it was killed by Karenia mikimotoi during co-cultivation in 9 days of co-cultivation when the ratio of cell number was 1:1 and in 11 days when the ratio of cell surface was 1:1. While it could only keep a low density which was 2 times as the initial cell density when the ratio of cell volume was 1:1. Platymonas subcordiformis was selected as the strongest growth competitor of Karenia mikimotoi.2. Under inorganic nitrate and phosphate(added with NaNO3 and NaH2PO4, respectively) limitation, four test groups of both N and P limitation (N/P=24.4,N: 9350-74800μg/L, P: 550-4400μg/L) and six test groups of only N or P limitation(N: 74800μg/L, P: 550-2200μg/L and P: 4400μg/L, N: 9350-37400μg/L)were set under laboratory conditions so as to study the competition between Platymonas subcordiformis and Karenia mikimotoi. The results figured:①under the condition of monoculture, higher concentrations of nitrate and phosphate were beneficial to the growth of two microalgae. Moreover, it had more important effects in the group of P limitation than N limitation to the growth of the two microalgae. However, the differences occurred when the two microalge cultivated under the lowest nutrient concentrations. Karenia mikimotoi had the stronger endurance capabilities under the environment with lower concentrations of nitrate and phosphate.②u nder the mixed culture, Platymonas subcordiformis became the predominant species in all of experimental groups, and the inhibitory effects were more and more obvious as the nutrient concentrations of the environment increased gradually. When the nutrition of the environment was more enriched, all of Karenia mikimotoi would die almostly. While the nutrition of the environment was more spare, Karenia mikimotoi could exist with a low density which was 0.8 to 3 times as the initial cell density.3. Effects of the used medium and cell inclusion of Platymonas subcordiformis at different growth phases on the growth of Karenia mikimotoi were determined. The used medium and cell inclusion of Platymonas subcordiformis inhibited the growth of Karenia mikimotoi in a density dependent manner at the beginning. Such effect attenuated (decayed??) with the increase of time, and finally, Karenia mikimotoi recovered its growth. In contrast, cell inclusion more intensively inhibited the growth of Karenia mikimotoi than the used medium. Accordingly, both the secrete and the cell inclusion of Platymonas subcordiformis were believed to contain allelopathy components inhibiting the growth of Karenia mikimotoi. The allelochemicles may be synthesized and accumulated at the exponential growth phase, reaching the highest concentration at the end of exponential growth phase or plateau phase and being released with the death of Platymonas subcordiformis.