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We investigated how environmental factors initiate Heterosigma akashiwo and Skeletonema costatum blooms from resting stages in bottom sediments in a shallow port over 2 yr. Using field-collected sediments, we also conducted laboratory experiments on how light intensity affects germination of resting stages and growth of the germinated cells. Both phytoplankton species bloomed only in summer, when water temperature and solar radiation were high enough for growth. All three blooms of H. akashiwo and the earliest bloom of S. costatum in a year occurred right after transmission of strong light (.200 mmol quanta m 22 s 21 ) to the bottom layer and a peak occurred in dissolved inorganic phosphorus (DIP). In the laboratory, resting stages of H. akashiwo and S. costatum germinated even in dim light (20 and 65 mmol quanta m 22 s 21 , respectively), but germinated cells required stronger light of .130 and 280 mmol quanta m 22 s 21 , respectively, for rapid growth. This value is much higher than the threshold for survival, and is higher than the half-saturating light intensity for growth of vegetative cells. Abundance of the resting stages of both species in the sediments rapidly increased during blooms and logarithmically decreased during nonbloom periods, suggesting that resting stages are continuously consumed. For both species, our results suggest that blooms initiate when transmission of sufficient light permits: first, germination of cells from the sediment; second, rapid growth of these germinated cells. Temperature and DIP must also exceed a facilitating threshold.

We investigated growth interactions between Skeletonema costatum (Greville) Cleve and Heterosigma akashiwo (Hada) Hada ex Hara et Chihara using bi-algal cultures under axenic conditions. When inoculated at high cell densities, growth of both species was coincidentally suppressed. In other combinations of inoculation density, the species first reaching stationary phase substantially reduced maximum cell densities of the other species. When cultured together under conditions without cell contact, growth of S. costatum and H. akashiwo were both suppressed. Furthermore, despite re-enrichment with nutrients, filtrates from dense cultures of S. costatum and H. akashiwo reciprocally reduced their maximum cell densities. In additional experiments, growth of Chaetoceros muelleri was also suppressed with filtrates from the above cultures, but growth of Prorocentrum minimum was not. Therefore, growth interactions between these species strongly suggest the involvement of allelopathic substances secreted by both species. Finally, growth and interaction of S. costatum and H. akashiwo in bi-algal cultures were simulated using a mathematical model. This model indicated that S. costatum and H. akashiwo steadily approach a stable equilibrium point of about 3.4 × 10 5 cells ml -1 and 4.8 × 10 5 cells ml -1 , respectively, when the 2 species coexist.KEY WORDS: Allelopathy · Bacillariophyceae · Skeletonema costatum · Raphidophyceae · Heterosigma akashiwo · Bi-algal culture · Growth inhibition Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 339: [83][84][85][86][87][88][89][90][91][92] 2007 Honjo (1994) and Smayda (1998) indicated that most pelagic blooms attributed to O. luteus were almost certainly those of H. akashiwo. Therefore, we will treat O. luteus described by Pratt (1966) and Honjo & Tabata (1985) as H. akashiwo.In in situ and in vitro experiments, high concentrations of Olisthodiscus luteus inhibit the growth of Skeletonema costatum, while lower concentrations stimulate the growth of S. costatum (Pratt 1966). Similarly, Honjo et al. (1978) reported that Heterosigma akashiwo and S. costatum alternated in forming red tide blooms in the fishing port of Hakozaki. They found that filtrate from dense cultures of H. akashiwo, reenriched with nutrients, suppressed the growth of S. costatum. Furthermore, Honjo & Tabata (1985) reported an apparent and reciprocal codominance between O. luteus and diatoms in a 70 m 3 outdoor tank with flowing coastal water.Unfortunately, none of the in vitro experiments mentioned above used axenic strains. In this study, we used axenic strains of Heterosigma akashiwo and Skeletonema costatum. First, we conducted bi-algal culture experiments with several combinations of initial cell densities of the 2 species. Second, we examined allelopathic interactions between H. akashiwo and S. costatum by way of both growth experiments using culture filtrates and of bi-algal culture experiments under noncontact conditions. Finally, we simulated the...

The goal of this study was to examine the significance of allelopathy by the raphidophyte Heterosigma akashiwo in a multispecies phytoplankton community in the field. Towards this aim, we sought allelochemicals of H. akashiwo, which had allelopathic effect both in laboratory experiments and in the field. As an initial step, we showed that the allelopathic effects of H. akashiwo filtrate were both species-specific and dependent upon the cell density of the target species. Secondly, we found for the first time that extracellular, high-molecular weight allelochemicals [that is, polysaccharideprotein complexes (APPCs)] were produced by a marine phytoplankton species, H. akashiwo. Thirdly, we indicated that the purified APPCs selectively inhibited the growth of the diatom Skeletonema costatum that is a major competitor of H. akashiwo, and thereby tended to promote the formation of monospecific H. akashiwo blooms. Furthermore, we demonstrated that the inhibitory effect of APPCs on the growth of the diatoms was determined by binding to the cell surface of the target species. Finally, we succeeded in the detection of APPCs in the field samples at concentrations exceeding their experimentally determined action threshold during the H. akashiwo bloom. Strategies for ecosystem control, including mitigation of harmful algal blooms (HABs), should take into account that red-tide organisms like H. akashiwo are already part of complex webs involving inter-specific allelopathic inhibition and ecosystem control during their dense blooms.