Shellfish Toxicity and Dormant Cysts in Toxic Dinoflagellate Blooms

Abstract

Several of the dinoflagellates responsible for toxic blooms include a dormant cyst stage in their life cycles. These resistant cells have a variety of potential functions in the overall ecology of the toxic dinoflagellates. As Wall (1) discussed, cysts can theoretically act as "seed" populations to initiate blooms, as a survival mechanism through environmental extremes, as agents for species dispersal, and as means for genetic recombination through sexuality. Two additional functions can be added to this list: cysts can be direct sources of toxicity, and their formation can be a major factor in bloom termination. The purpose of this paper is to examine our present knowledge of the cysts of the toxic Gonyaulax species to see whether these hypothetical considerations are valid representations of the actual roles cysts play in toxic dinoflagellate blooms and shellfish toxicity.Nearly 100 years ago, dinoflagellate resting cysts were first observed in plankton samples, but nearly 50 years passed before their affinity to motile dinoflagellates was demonstrated (2.3). Early researchers investigating paralytic shellfish poisoning episodes (PSP) speculated that a benthic cyst population germinating at specific times could explain the seasonal nature of toxic dinoflagellate blooms (4-7), but it was only in the last few years that true resting cysts were finally identified and described for toxic dinoflagellates (8-10).It is now generally accepted (10, 30, 39, 40) that the toxic Gonyaulax species reproduce asexually through vegetative division (Figure 1), but that this process can switch to sexual reproduction through the formation of gametes which fuse together (Figure 2) to yield a large, swimming zygote (planozygote; Figure 3). This presumably diploid cell swims for up to a week before it is transformed into the thick-walled resting cyst or hypnozygote ( Figure 4). Upon germination, the cyst releases one cell (Figure 6) which

Contribution of benthic cysts to the population dynamics of Scrippsiella spp. (Dinophyceae) in Onagawa Bay, northeast Japan

“…Considering that the minimum temperature in winter was above 6°C (Fig. 2), a part of the vegetative population must survive from the previous blooms a n d may grow slowly during the winter season, contributing to the winter population It has been suggested that cysts play an important role In bloom initiation (Wall 1971, Anderson & Wall 1978, Dale 1983, Anderson 1984. The evidence obtained in this study, however, demonstrates that high germination rates or synchronized mass germination of the cysts from the surface sediment is not necessary for Sci-ippsiella spp.…”

“…Among these factors, nutrient depletion, especially nitrogen and/or phosphorus deficiency, has been indicated to be the primary trigger for encystment (e.g. von Stosch 1973, Pfiester 1977, Anderson et al 1984, 1985, Chapman & Pfiester 1995. Watanabe et al (1982) observed that the encystment of S. trochoidea was enhanced by nutrient limitation, either of nitrogen or phosphate, but found that it also occurred in nutrient-rich control cultures.…”

“…The significant function or role of such cysts has been emphasized by many studies (e.g. Wall 1971, Anderson & Wall 1978, Dale 1983, Anderson 1984. The concept that the cysts act as 'seed populations' or 'seed beds' in their population dynamics (Prakash 1967, Wall 1971, Steidinger 1975) is now widely accepted.…”

“…Germination is thought to be responsible for the initiation of blooms (e.g. Dale 1983) and bloom termination a direct result of cyst deposition (Anderson 1984). Therefore, direct measurements in situ of these 2 processes are indispens- …”

Coupling planktonic and benthic shifts during a bloom of Alexandrium catenella in southern Chile: Implications for bloom dynamics and recurrence

“…A role for benthic cysts has been cited in species dispersion, resistance to unfavorable conditions, population resilience through sexual recombination, and bloom onset and termination (Anderson et al, 1982;Anderson, 1984;Garcé s et al, 1999;McGillicuddy et al, 2003). Moreover, given that the toxicity of resting cysts can be higher than that of vegetative cells, resting cyst formation must be taken into account when analyzing the possible effects of a bloom on human health (Dale et al, 1978;Lirdwitayaprasit et al, 1990;Oshima et al, 1992).…”