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Takeshi YOSHIKAWA, Associate Professor
Department of Aquatic Environment Science
Education and Research Center for Marine Resources and Environment
"Prevention of the Fisheries Damage Caused by Red Tides"
Red tides are known as a phenomenon with discoloration of the sea surface caused by massively grown unicellular microalgae. In some cases, their density reaches thousands or ten thousands of cells per milliliter.
Red tide-causing microalgae are known to be ichthyotoxic and to have often caused mass mortality of aquacultured fish. In Kagoshima Prefecture, aquaculture fishery industries, especially yellowtail aquaculture, have suffered serious economical damage, amounting to a few billion yen (a few million US dollars). In addition, new bloom-forming algal species are invading Japanese coastal area, perhaps due to global warming: in Yamagawa Bay, located in the south of Satsuma Peninsula, Kagoshima Prefecture, a dictyochophyte Pseudochattonella verruculosa bloomed in February 2012, which was the first appearance in the prefecture. Because of its ability to grow under a low temperature, blow 10˚C, outbreak of the bloom was observed in winter, the season when few red tide outbreaks have been reported.
We are engaged in studies to prevent fishery damages caused by red tides. Phytoplankton dominancy, including causative agents of red tides, as well as physical and chemical factors such as water temperature, salinity and inorganic nutrients, is monitored in Yamagawa Bay and Yatsushiro Sea. Molecular biological techniques, including quantitative PCR and denaturing gradient gel electrophoresis (DGGE), are applied to detect and quantify the bloom-forming microalgae in the natural environment with ease and high sensitivity. Moreover, development of the red tide alga-controlling methods is attempted: some chemical agents, magnesium hydroxide and sodium percarbonate, have been found to be potential candidates to manage microalgal biological activity and growth.
Yamagawa Bay is an inlet which was a volcanic crater in ancient times. The basin is characterized by the very shallow entrance (5–8 meter depth) and very deep central area (40–50 meter depth). Low water exchange and resultant high residence time of the seawater make the sea bottom highly polluted by sedimented organic matter, resulting in oxygen deficiency and high sulfide concentration. Red tide algae are known to produce cysts in association with the collapse of the bloom: the cysts with no swimming ability sediment to the sea floor and “hibernate” by resisting the unsuitable condition, such as low temperature and irradiance. However, by using a quantitative PCR technique, we observed rapid decrease of the cyst numbers in the sediment of Yamagawa Bay after the bloom termination. The result also demonstrates the deteriorating environment of the sea floor.
Although some marine microalgae cause negative impacts especially on fishery industries, most of the benign phytoplankton plays an important role as a primary producer in the marine ecosystem. In addition, they are quite attractive organisms: some of the dinoflagellates, whose cells look armored by being covered by thecal plates, are luminescent by the luciferin-luciferase system. Some dinophytes are also known to be heterotrophic or mixotorophic: they feed on microalgae or marine bacteria. Interestingly, some of the heterotrophs keep chloroplasts of the ingested microalgae undigested and make them photosynthesized intracellularly to obtain produced organic nutrients. These are called as “kleptochloroplasts” (= stolen chloroplasts). These characteristics show notable trophic versatility of dinoflagellates.
When you observe seawater microscopically, phytoplankton shows very attractive appearance and behavior. I would appreciate it if you would have an interest in the world of tiny life.
round shape |
formation of projections |
fusion of the cells |
resultant cell aggregates |
| Cell morphology of a red tide-causing phytoplankton Pseudochattonella verruculosa. The cell morphology is very diverse |
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| Training and research vessel, Nansei-Maru, in Yamagawa Bay, Kagoshima Prefecture. |
Sediment sampled from the sea floor of Yamagawa Bay. The sediment was muddy and black, because of the high content of sulfides. |
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Micrograph of a dinoflagellate, Cochlodinium polykrikoides, forming chains with 2, 4, 8 or 16 cells. |
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