Department of Marine Science and Resources, College of Bioresource Sciences Nihon University, Fujisawa, Kanagawa, JAPAN

jhiromi@brs.nihon-u.ac.jp

　

The authors have formerly found that several marine diatom species such as Cylindrotheca closterium produced different kinds of polyunsaturated fatty acid (PUFA) and stearic acid-like substances both of which kill harmful algal species, e.g., a raphidophycean Heterosigma akashiwo, at the concentration of about 50-80 ng / ml, and that a few of these algaecides acted as allelopathy to H. akashiwo.

　

In the present study the authors examined in vitro the utility of the algaecides produced by the marine diatom species for the collapse of red tide of H. akashiwo. The number of cells of H. akashiwo inoculated (initially at 1,200 cells / ml) into the medium of C. closterium being in the stationary period (2,500 cells / ml) decreased rapidly, and died out by six days. In the course of bialgal culture, the concentration of algaecides in the medium declined but increased again after H. akashiwo dying out. This result suggested that H. akashiwo presumably accumulated algaecides in the cell. This suggestion was evidenced by measuring the concentration of arachidonic acid in dead cells of H. akashiwo cultured in the medium containing arachidonic acid which was an algaecide produced by C. closterium, and the lethal dose was calculated to be about 0.03 ng / cell of H. akashiwo.

　

These experimental results suggest that the growth of H. akashiwo can be inhibited by sustaining the population of C. closterium in seawater. Furthermore it is expected that the diatom prevalence including C. closterium will lead to the collapse of harmful algal blooms (HABs) in natural waters, taking into consideration that many species of diatoms are known to produce allelochemicals such as PUFA.

　

　

¹Environmental Studies Programme, School of Science & Technology The Open University of Hong Kong, Hong Kong SAR, CHINA

kcho@ounk.edu.nk

　

²Department of Ecology & Biodiversity, The University of Hong Kong Hong Kong SAR, CHINA

　

Since the 1980s, waters in Hong Kong and Southern China have suffered from occasional attacks of harmful algal blooms (HABs). An episode in 1998 resulted in a total loss of 350 million Hong Kong dollars in the aquacultural industry. While most HABs were triggered by special oceanographic and climatic conditions, microalgal biomass was closely related to nutrient supplies and dinoflagellate-caused HABs were found to be associated with changes in N:P (atomic) ratio when the minimum concentration of 0.1 mg-N/L of TIN and 0.02 mg-P/L of DIP was reached. Recent findings indicate that interspecific competition between diatoms and dinoflagellates should not be overlooked. Silicate (Si), the limiting factor for diatom growth, has played a significant role in determining the dominant species and the magnitude of red tides. The Redfleld Ratio should be critically reviewed in terms of its application to HABs dominated by dinoflagellates and the Si:N ratio should be considered in parallel with N:P ratios. From data Hong Kong and Southern China show that dinoflagellate blooms are favored by N:P (atomic) ratio of 10-22 and Si:N (atomic) ratio < 1; diatom blooms are favored by larger N:P and Si:N ratios; whereas micro-flagellate blooms generally occur after collapse of diatom and dinoflagellate blooms. Therefore, besides discharges of domestic sewage and agricultural wastes, which contain a huge quantity of TIN and DIP, variation of Si input due to urbanization, soil erosion and river diversion should be considered.

　

　

School of Fisheries Sciences, Kitasato University Sanriku, Iwate, JAPAN

kodama@kitasato-u.ac.jp

　

Toxins responsible for paralytic shellfish poisoning (PSP) are accumulated in shellfish by ingestion of toxic dinoflagellates such as Alexandrium tamarense occurred in the environment. The accumulated toxins are gradually released to the seawater. Thus the amount of toxins in shellfish is considered to be the difference between supply and release of the toxins in the shellfish. However, it has been pointed out in the field survey that the change of shellfish toxicity is not always parallel to the abundance of toxic dinoflagellates. Shellfish toxicity often increases after toxic dinoflagellates disappeared from the environment. The similar phenomena are also observed in the feeding experiments in which cultured A. tamarense cells were fed to scallop in the aquarium. These facts suggest that toxins or toxin-related substances are metabolized in shellfish, though little is known about the metabolism of toxins in shellfish or dinoflagellates. Recently, we found that some toxin components are decomposed in shellfish. At the same time, some components are formed in shellfish. Peptidase and protease are involved in these reactions. These facts indicate that toxins or toxin-related substances are metabolized in shellfish and that the mechanism for shellfish to accumulate PSP toxins is more complex than have been considered.