The physiology of dormancy and germination in cysts of the marine dinoflagellate Scrippsiella trochoidea

Abstract

Cysts of Scrippsiella trochoidea are representative of the thick-walled non-motile resting cells produced by many dinoflagellates. Although the influence of cysts on the biology and ecology of dinoflagellates may be significant, little is known of the factors controlling cyst dormancy and germination, or of the metabolic foundations of these processes. This study addresses these issues using cysts produced and manipulated under defined conditions in laboratory culture. Scrippsiella trochoidea cysts experienced a period of dormancy, lasting approximately 25 days, during which germination did not occur. The duration of this period was not affected by temperature. Cysts which had completed their dormancy period remained quiescent until permissive environmental conditions were established. Quiescent ~ trochoidea cysts remained viable at 3°C in the dark for at least 350 days. Germination in quiescent S. trochoidea cysts was photomorphogenically controlled: cysts deprived of light but otherwise provided with optimal environmental conditions failed to germinate. The light requirement was satisfied to a large extent by a single, brief, low intensity exposure (a photon fluence of 0.2 μmol m-2 "white" light elicited a 50% response). Yellow-green light (λ≈ 550nm) was found to be most effective. Temperature exerted significant control over germination as well. Germination rate was maximal above 14°C; it decreased rapidly as temperature decreased below this level. Ultimate achieved germination frequency, in contrast, was relatively insensitive to temperature. The temperature range optimal for cyst germination did not precisely coincide with that for vegetative growth. Thus germination was greatly retarded at low temperatures which supported good vegetative growth, but on the other hand proceded optimally at high temperatures which completely inhibited such growth. The most conspicuous compositional attribute of ~ trochoidea cysts was their increased carbohydrate content, which was 10 times that of exponentially growing cells. Cysts contained significantly less protein and chlorophyll-a than vegetative cells, while the lipid content of the two forms was comparable. The respiratory activity of quiescent cysts was estimated to be approximately 1.5% of that in vegetative cells. Although chlorophyll-a persisted in quiescent cysts, no photosynthetic activity could be detected therein. The germination of S. trochoidea cysts was accompanied by an immediate increase in respiratory activity, with carbohydrate serving as the principal substrate. Protein synthesis became apparent within 24 hr of activation, followed by a dramatic increase in both chlorophyll-a and photosynthetic activity just prior to excystment. Encystment and germination in S. trochoidea represent developmental patterns which are regulated by specific, albeit not yet well understood, environmental and biological parameters. The further elucidation of these processes and their regulation will lead to a better understanding of the dynamics of dinoflagellate populations in nature, as well as of the biology of dormancy and quiescence generally.