Behavior of Peridinium bipes (Dinophyceae) resting cysts in the Asahi Reservoir

The role of resting cysts includes short- and long-term survival under extreme conditions, bloom initiation, species dispersal, reproduction, and preservation of genetic variation. Accordingly, it is important to understand their behavior in a water environment, especially in lakes and reservoirs where dinoflagellate blooms are observed. In this study, we estimated the behavior of the Peridinium bipes cysts in the Asahi Reservoir using laboratory experiments and field surveys. It was observed that the amount of light strongly influenced excystment, and few cysts germinated under dark conditions in the laboratory experiment. The minimum temperature on excystment was inferred from the laboratory experiment and field surveys to be about 5°C. Although the frequency of excystment did not depend on the water temperature from 10° to 20°C, the average preparation period for excystment decreased with the increase of water temperature. In the Asahi Reservoir, the excystment was estimated to occur during the months of March and April. If excystment did not occur in spring, the dinoflagellate bloom was not be observed until about July, although the bloom often began to appear in about May in the Asahi Reservoir. Consequently, the blooming season in the Asahi Reservoir is affected by the biomass of the germinated cysts in spring. Received: August 31, 2000 / Accepted: March 1, 2001     

CONTROL OF GERMINATION OF ALEXANDRIUM TAMARENSE (DINOPHYCEAE) CYSTS FROM THE LOWER ST. LAWRENCE ESTUARY (CANADA)

Cysts of the toxic dinoflagellate Alexandrium tamarense (Lebour) Balech 1992 from the lower St. Lawrence estuary were used in a test of the following hypotheses: (1) cyst germination is triggered by a change in temperature, and (2) germination rate varies throughout the year and is controlled by a circannual internal biological clock. Results show that cyst germination was not affected significantly by temperature of incubation over the range 1°–16° C, and light showed no significant stimulation of germination. This is supported by the lack of effect of cyst incubation conditions during evaluation of the seasonal changes in germination rate (two temperatures: 4° and 15° C, and two light conditions: darkness and 150 μmol photons·m^?2·s^?1). Thus, direct environmental control through short-term increases in temperature and exposure to light has no effect on the germination of the cysts tested. The rate of germination, observed monthly over a 16-month period, showed low germination (<20%) over most of the period tested, except for a maximum reaching more than 50% germination in August to October of the second year of the experiment. This pattern was observed for cysts both from monthly field collections and from laboratory-stored cysts kept under constant environmental conditions (4° C, in the dark). The peak in germination observed under constant environmental conditions (in the laboratory), the almost coincidental increase in cyst germination observed for the field-collected cysts, and the absence of effects of temperature and light during incubation could be explained either by a temperature-controlled cyst maturation period (the time-temperature hypothesis of Huber and Nipkow 1923) or by the presence of an internal biological clock. However, the large decline in the rate of germination 2 months after the maximum provides strong support for the biological clock hypothesis. The ca. 12-month maturation (dormancy) period observed for the laboratory-stored cysts is the longest reported for this species to our knowledge; this might be related to the low storage temperature (4° C), which is close to bottom temperatures generally encountered in this environment (0° to 6° C). Similar field and laboratory storage temperatures could explain the coincidental increase in germination rate in the fall of the second year if cyst maturation is controlled by temperature. A fraction of the laboratory-stored cysts did not follow a rhythmic pattern: A rather constant germination rate of about 20% was observed throughout the year. This continuous germination of likely mature cysts may supplement the local blooms of this toxic dinoflagellate, as these often occur earlier than peak germination observed in late summer. It seems that two cyst germination strategies are present in the St. Lawrence: continuous germination after cyst maturation, with temperature controlling the length of the maturation period, and germination controlled by a circannual internal rhythm.     

Morphological Study of the Encystment and Excystment of Vermamoeba vermiformis Revealed Original Traits

Free‐living amoebae are ubiquitous protozoa commonly found in water. Among them, Acanthamoeba and Vermamoeba (formerly Hartmannella) are the most represented genera. In case of stress, such as nutrient deprivation or osmotic stress, these amoebae initiate a differentiation process, named encystment. It leads to the cyst form, which is a resistant form enabling amoebae to survive in harsh conditions and resist disinfection treatments. Encystment has been thoroughly described in Acanthamoeba but poorly in Vermamoeba. Our study was aimed to follow the encystment/excystment processes by microscopic observations. We show that encystment is quite rapid, as mature cysts were obtained in 9 h, and that cyst wall is composed of two layers. A video shows that a locomotive form is likely involved in clustering cysts together during encystment. As for Acanthamoeba, autophagy is likely active during this process. Specific vesicles, possibly involved in ribophagy, were observed within the cytoplasm. Remarkably, mitochondria rearranged around the nucleus within the cyst, suggesting high needs in energy. Unlike Acanthamoeba and Naegleria, no ostioles were observed in the cyst wall suggesting that excystment is original. During excystment, large vesicles, likely filled with hydrolases, were found in close proximity to cyst wall and digest it. Trophozoite moves inside its cyst wall before exiting during excystment. In conclusion, Vermamoeba encystment/excystment displays original trends as compare to Acanthamoeba.     

SEXUALITY AND CYST FORMATION IN THE DINOFLAGELLATE GONYAULAX TAMARENSIS: CYST YIELD IN BATCH CULTURES1

Encystment of the toxic dinoflagellate Gonyaulax tamarensis Lebour (var. excavata) was monitored in batch cultures exposed to a variety of nutritional and environmental treatments. Limitation by nitrogen (as ammonium or nitrate) or phosphorus (as phosphate) resulted in cyst formation. When the initial concentration of limiting nutrient was varied, total cyst yield (mL^?1) was directly proportional to the cell yield at all but the highest nutrient concentrations (where encystment was minimal). Encystment efficiency was relatively constant (0.1–0.2 cysts · cell^?1) over a 5-fold range of cell densities, indicating that 20 to 40% of the vegetative populations successfully encysted. Cyst formation was negligible in nutrient-replete medium, even with a significant reduction in growth rate due to non-optimal light, temperature, or to high batch culture cell densities. Low light levels did decrease cyst yield once encystment was initiated by nutrient limitation, but this was probably linked to smaller motile cell yield and not to a specific inhibition of encystment. In contrast, encystment was more sensitive to temperature than was growth rate: optimal cyst production occurred over a relatively narrow temperature range and no cysts were formed at [Page missing]     

ENVIRONMENTAL AND ENDOGENOUS REGULATION OF CYST GERMINATION IN TWO FRESHWATER DINOFLAGELLATES

The role of excystment in relation to seasonal succession was investigated in two freshwater dinoflagellates, Ceratium hirundinella (O.F. Müller) Dujardin and Peridinium aciculiferum (Lemmermann). Field studies and laboratory experiments were performed to determine which factors regulate the timing of cyst germination. Environmental factors (temperature, light, nutrients, and anoxia) and endogenous factors (maturation period and biological clock) were investigated. Our main results indicate that temperature and internal maturation period determine when germination can occur. C. hirundinella had a maturation period of 4.5 months and germinated in the laboratory and in the field at temperatures above 6° C. P. aciculiferum had a maturation period of 2.5 months and germinated in the laboratory and in the field at temperatures below 7° C. In addition, our results indicated that both species were regulated by a biological clock. Furthermore, anoxia prevented the germination of C. hirundinella, contrary to results in earlier studies. To conclude, we could explain the appearance in plankton of the two dinoflagellate species through two main factors regulating excystment, that is, temperature and maturation period.     

HIGH ENCYSTMENT SUCCESS OF THE DINOFLAGELLATE SCRIPPSIELLA CF. LACHRYMOSA IN CULTURE EXPERIMENTS 1

Close to 100% encystment efficiency and a yield above 10⁵ cysts·mL ^{? 1} were routinely achieved in full strength f/2 medium‐based batch cultures (883 μM NO₃ ^? and 36 μM PO₄ ^{? 3}) of the marine dinoflagellate Scrippsiella cf. lachrymosa Lewis. Increases in cell density led to nutrient depletion in this enriched medium, which was the most likely cause for initiation of cyst formation. Lowering the concentration of either nutrient to 1/10 the initial levels decreased the encystment efficiency, whereas use of ammonium as the N source resulted in both low cell yield and low encystment efficiency. The mandatory dormancy period was ca. 60 days and was not affected by cold dark storage of the cysts. Cysts produced in the initial phase of sexual reproduction were relatively large (length 47 μm, width 31 μm) with a heavy calcareous cover. Cysts produced thereafter lacked apparent calcareous cover and were smaller (length 29 μm, width 19 μm). The decrease of cyst volume (by a factor of 0.24–0.4) suggested strong resource limitation during the course of encystment. However, after the mandatory dormancy period, germination success of the smaller cysts was higher (80%), compared with the larger cysts that had been produced initially (50%). Germling survival (74%) was independent of cyst type but was enhanced by higher nutrient concentration during incubation. The ratio of initial nutrient concentration in the medium to the cyst yield was used as a proxy to estimate the cellular nutrient quota. The conservative estimates of 9 pmol N·cyst ^{? 1} and 0.4 pmol P·cyst ^{? 1} obtained in this manner are at the low end of the range of previous published estimates for other dinoflagellate cysts. Given the high encystment observed in laboratory experiments, we have no reason to assume an inherently lower encystment success in dinoflagellate field populations. Our results do not challenge the low nutrient paradigm for dinoflagellate sexuality. We believe that the high encystment success and cyst yield of this particular species is at least partly due to its ability to achieve very high cell densities in cultures, which evidently leads to nutrient depletion even in f/2 medium.     

THE EFFECTS OF TEMPERATURE,IRRADIANCE, AND NITROGEN ON THE ENCYSTMENT AND GROWTH OF THE FRESHWATER DINOFLAGELLATES PERIDINIUM CINCTUM AND P. WILLEI IN CULTURE (DINOPHYCEAE)1

Effects of temperature, irradiance, and nitrogen availability on the encystment and growth of the freshwater dinoflagellates Peridinium cinctum Ehrenberg and Peridinium willei Huitfeld-Kaas were studied in culture. Lack of nitrogen was the main trigger of encystment in both species. Irradiance had a secondary effect on the percentage of the population of each species that encysted. Temperature did not significantly affect encystment in either species. In both species, only a small percentage of the population underwent encystment. Low light had an inhibitory effect on the growth of P. willei growing in nitrogen-sufficient medium.     

Generalist Life Cycle Aids Persistence of Alexandrium ostenfeldii (Dinophyceae) in Seasonal Coastal Habitats of the Baltic Sea

In seasonal environments, strong gradients of environmental parameters can shape life cycles of phytoplankton. Depending on the rate of environmental fluctuation, specialist or generalist strategies may be favored, potentially affecting life cycle transitions. The present study examined life cycle transitions of the toxin producing Baltic dinoflagellate Alexandrium ostenfeldii and their regulation by environmental factors (temperature and nutrients). This investigation aimed to determine whether genetic recombination of different strains is required for resting cyst formation and whether newly formed cysts are dormant. Field data (temperature and salinity) and sediment surface samples were collected from a site with recurrent blooms and germination and encystment experiments were conducted under controlled laboratory conditions. Results indicate a lack of seasonal germination pattern, set by an endogenous rhythm, as commonly found with other dinoflagellates from the Baltic Sea. Germination of quiescent cysts was triggered by temperatures exceeding 10°C and combined nutrient limitation of nitrogen and phosphorus or a drop in temperature from 16 to 10°C triggered encystment most efficiently. Genetic recombination was not mandatory for the formation of resting cysts, but supported higher numbers of resistant cysts and enhanced germination capacity after a resting period. Findings from this study confirm that A. ostenfeldii follows a generalist germination and cyst formation strategy, driven by strong seasonality, which may support its persistence and possibly expansion in marginal environments in the future, if higher temperatures facilitate a longer growth season.     

Cysts of Danish Gymnodinium nolleri Ellegaard et Moestrup sp. ined. (Dinophyceae): studies on encystment, excystment and toxicity

Life cycle dynamics of Gymnodinium nolleri Ellegaard et Moestrupsp. ined. were studied under different temperature and nutrientconditions. Five culture strains originating from cysts foundin Danish marine sediments were used for the experiments. Bothencystment and excystment were found to vary with temperature.Maximal encystment occurred at 22–28°C, with no cystsformed below 13°C or above 33°C. Cyst production wasslightly higher under phosphorus limitation than under combinednitrogen and phosphorus limitation. Maximal excystment occurredat 26.5°C with negligible excystment under 11°C andover 35°C. The resting period for cyst maturation was typically3–4 weeks. Cysts produced under both phosphorus and nitrogenlimitation were tested for paralytic shellfish poisoning (PSP)toxins by HPLC, and none were detected. It remains unclear whyvegetative cells of this species have not yet been recordedin plankton samples from Scandinavia, despite the widespreaddistribution of the cysts.     

Different excystment patterns in two calcareous cyst-producing species of the dinoflagellate genus Scrippsiella

Scrippsiella rotunda and Scrippsiella trochoidea var. aciculifera (order Peridiniales, subfamily Calciodinelloideae) are autotrophic orthoperidinioid dinoflagellates producing calcareous resting cysts which are at times abundant in coastal marine sediments. We have carried out laboratory experiments to investigate features of cyst germination in the two species, including dormancy length, germination pattern and germination success, over an annual cycle and under different light and temperature conditions. The maturation period for S. rotunda cysts was between 17 and 24 weeks, while that of S. trochoidea var. aciculifera was much shorter, ranging between 2 and 5 weeks. Both species required exposure to light for germination, while temperature shifts (from 14 to 20C) in the dark did not induce excystment of mature cysts. In both species, germination was not synchronous, but distributed over a variable time interval, suggesting a high physiological diversity within the cyst pool. Moreover, exposure to light of S. rotunda cysts that had not completed maturation impaired the germination of a great percentage of the cysts. Differences in dormancy length may partially explain the distinct cyst production patterns observed for the two species in the Gulf of Naples.      

Factors regulating excystment of Alexandrium in Puget Sound,WA, USA

Factors regulating excystment of a toxic dinoflagellate in the genus Alexandrium were investigated in cysts from Puget Sound, Washington State, USA. Experiments were carried out in the laboratory using cysts collected from benthic seedbeds to determine if excystment is controlled by internal or environmental factors. The results suggest that the timing of germination is not tightly controlled by an endogenous clock, though there is a suggestion of a cyclical pattern. This was explored using cysts that had been stored under cold (4 °C), anoxic conditions in the dark and then incubated for 6 weeks at constant favorable environmental conditions. Excystment occurred during all months of the year, with variable excystment success ranging from 31–90%. When cysts were isolated directly from freshly collected sediments every month and incubated at the in situ bottom water temperature, a seasonal pattern in excystment was observed that was independent of temperature. This pattern may be consistent with secondary dormancy, an externally modulated pattern that prevents excystment during periods that are not favorable for sustained vegetative growth. However, observation over more annual cycles is required and the duration of the mandatory dormancy period of these cysts must be determined before the seasonality of germination can be fully characterized in Alexandrium from Puget Sound. Both temperature and light were found to be important environmental factors regulating excystment, with the highest rates of excystment observed for the warmest temperature treatment (20 °C) and in the light.     

LIFE CYLE AND SEXUALITY OF THE FRESHWATER RAPHIDOPHYTE GONYOSTOMUM SEMEN (RAPHIDOPHYCEAE)1

Previously unknown aspects in the life cycle of the freshwater flagellate Gonyostomum semen (Ehrenb.) (Raphidophyceae) are described here. This species forms intense blooms in many northern temperate lakes, and has increased in abundance and frequency in northern Europe during the past decades. The proposed life cycle is based on observations of life cycle stages and transitions in cultures. Viable stages of the life cycle were individually isolated and monitored by time‐lapse photography. The most common processes undertaken by the isolated cells were: division, fusion followed by division, asexual cyst formation, and sexual cyst formation. Motile cells divided by two different processes. One lasted between 6 and 24 h and formed two cells with vegetative cell size and with or without the same shape. The second division process lasted between 10 and 20 min and formed two identical cells, half the size of the mother cell. Planozygotes formed by the fusion of hologametes subsequently underwent division into two cells. Asexual cyst‐like stages were spherical, devoid of a thick wall and red spot, and germinated in 24–48 h. Heterogamete pairs were isogamous, and formed an angle of 0–90° between each other. Planozygote and sexual cyst formation were identified within strains established from one vegetative cell. The identity of these strains, which was studied by an amplified fragment length polymorphism analysis, was correlated with the viability of the planozygote. Resting cyst germination was described using cysts collected in the field. The size and morphology of these cysts were comparable with those formed sexually in culture. The excystment rate was higher at 24°C than at 19 or 16°C, although the cell liberated during germination (germling) was only viable at 16°C. The placement of G. semen within the Raphidophyceae family was confirmed by sequence analysis of a segment of the 18S ribosomal DNA.     

Encystment of Peridinium gatunense: occurrence, favourable environmental conditions and its role in the dinoflagellate life cycle in a subtropical lake

1. The abundance of cysts of the bloom‐forming dinoflagellate Peridinium gatunense in the sediments of Lake Kinneret and the effects of environmental conditions on encystment were studied in relation to bloom dynamics. Peak cyst formation coincided with the highest growth rate of the population, prior to bloom peak. 2. Peridinium cysts were counted in water and sediment corer samples from 2000 to 2003 and in archived sediment trap samples collected during 1993–94. The cyst data were examined in relation to ambient temperature and nutrient records, and revealed no direct correlation. 3. In laboratory encystment experiments with Peridinium cells collected from the lake, 0.2–3% of the vegetative cells encysted. Temperature, light and cell density had no significant effect on the percentage of encystment. 4. Cysts were always present in the lake sediments but their abundance in ‘non Peridinium’ years was much lower than after a massive bloom. Vegetative cells were always present in the water column after the collapse of the annual dinoflagellate bloom, potentially serving as the inoculum for the next bloom. We propose that the hardy cysts serve as an emergency ‘gene bank’ to initiate population build up following catastrophic die outs.     

The effects of temperature, growth medium and darkness on excystment and growth of the toxic dinoflagellate Gymnodinium catenatum from northwest Spain

The chain-forming dinoflagellate Gynmodinium catenatum Grahamcauses recurrent outbreaks of paralytic shellfish poisoning(PSP) in the Galician Rias Bajas (northwest Spain). A sedimentsurvey in Ria de Vigo in April 1986 indicated that the highestconcentrations of cysts of this species were located in themiddle sections of the ria, with maximum abundance of 310 cystscm^–3. The effects of temperature, growth medium compositionand irradiance on the germination of laboratory-produced restingcysts were investigated. Newly formed cysts required very littletime for maturation, as excystment was possible within 2 weeksof encystment. Growth media did not affect germination success.In contrast, the excystment rate was retarded signifiantly indarkness. Germination was also strongly affected by temperature,with {small tilde}75% excystment success at 22–28°Cand little or no germination below 11°C after 1 month ofincubation. In culture, the optimum growth rate of vegetativecells was between 22 and 28°C, the highest rate being 0.53divisions day^–1 at 24°C. Growth did not occur at temperatures< 11°C or >30°C. These results are important withrespect to the different hypotheses proposed to explain theinitiation of G.catenatum blooms in the Galician Rias Bajasand Northern Portugal. The pattern of G.catenatum bloom developmentalong this coast has been related to seasonal upwelling in thearea, with major blooms occurring during the autumn as warmeroffshore surface water is transported towards the coast whenupwelling relaxes. The landward transport of established offshorepopulations of G.catenatum with the warm surface layer remainsa viable explanation for the observed blooms within the rias,but alternatively, our data suggest that cysts within the riascan provide the inoculum population at times conducive to growthand bloom formation. Even though newly formed G.catenatum cystshave a very short maturation time and can germinate in darknessacross a wide temperature range, bloom development will be significantonly during the late summer and early autumn, since in othermonths light levels at the sediment surface and temperaturesthroughout the water column are too low for significant germinationor growth.     

Strict requirement of fluctuating temperatures as a reliable gap signal in <Emphasis Type="Italic">Picris hieracioides</Emphasis> var. <Emphasis Type="Italic">japonica</Emphasis> seed germination

Picris hieracioides var. japonica (Asteraceae), which grows in occasionally disturbed habitats such as riverbanks, is rarely observed under dense vegetation. We examined the effect of the experience and timing of receiving leaf-transmitted light in gap-detecting seed germination in this plant. Seeds under unfiltered light, which simulated the light conditions of seeds on the soil surface in a canopy gap, germinated at a constant temperature of 20°C. However, most seeds in darkness, which simulated the light conditions of seeds buried in the soil without receiving leaf-transmitted light, germinated under temperature fluctuations of over 4°C. Seeds in darkness after receiving leaf-transmitted light for 1 week, which simulated the light conditions of seeds buried in the soil after receiving leaf-transmitted light, germinated under temperature fluctuations of over 8°C. Finally, seeds under continuous leaf-transmitted light, which simulated the light conditions of seeds on the soil surface below preexisting vegetation, germinated under temperature fluctuations of over 12°C. Seeds that experience unfiltered light, which suggests that they are in a gap, should not delay germination. In contrast, seeds that have received leaf-transmitted light should delay germination until the vegetation above is removed. Seeds exposed to leaf-transmitted light required larger temperature fluctuations in darkness than did untreated seeds, and seeds under continuous leaf-transmitted light required the largest temperature fluctuations. The various germination reactions to each gap signal in P. hieracioides var. japonica seeds allow the more reliable detection of gaps for subsequent seedling establishment. The requirement for gap signals that created high precision of timing in the germination process results in the germination of this species only in gaps. Therefore, P. hieracioides var. japonica is rarely found under dense vegetation.     

Correlation of Encystment and Division in Schizopyrenus russelli*

SYNOPSIS. Schizopyrenus russelli, a free-living soil ameba, grows and encysts in the presence of bacteria. The encystment occurs with decline in the division rate. This is accompanied by incorporation of [U-¹⁴C] glucose into cyst cellulose. The degree of multiplication (but not of encystment) is a function of bacterial concentration. Berenil, a trypanocidal drug, while allowing excystment, completely inhibited multiplication of emerged amebae and their encystment. Addition of this drug after 24 hr, when amebae had gone into a phase of active division failed to check encystment, although it still inhibited further multiplication of the amebae. The findings suggest that a phase of cell division may be a prerequisite for encystment.     

Encystment and Excystment of the Heterotrichous Ciliate Blepharisma stoltei Isquith*

SYNOPSIS. The structure and cytochemistry of encystment and excystment of Blepharisma stoltei Isquith are described. The encystment process may be subdivided into 4 stages: (i) in the precystic stage the buccal apparatus overlaps about the posterior, (ii) in early encystment, the buccal apparatus is resorbed and an ectocyst is secreted, (iii) an interwall space, endocyst, and plug are secreted during late encystment, and (iv) the resting cyst stage typically has disc-like structures on the ectocyst, and a vacuole in the macronucleus. In excystment, 6 distinct stages may be defined: (i) partial kineties are formed in early excystment, (ii) permanent kineties give rise to anlagen of the buccal apparatus during stomatogenesis, (iii) the organism elongates and reforms the vegetative shape in late excystment, (iv) some cysts then divide, (v) the redeveloped organism is liberated thru the plug pore, and (vi) the postcystic stage resembles the vegetative form except for its size and lack of pigmentation. Cortical structures, extracellular membranes, and the macronuclear membrane are composed of protein-lipids. Unbound protein and RNA are found in the cytoplasm thruout the cystic cycle. DNA is present only in the nuclei. Polysaccharides, 1st found in the cytoplasm, are shifted to the plug in encystment. The plug material disappears during excystment, while PAS positive granules appear in the cytoplasm.     

In vitro excystment of the metacercaria of Maritrema arenaria (Digenea: Microphallidae)

Metacercarial cysts of Mantrema arenaria were subjected to a solution containing trypsin and bile salts at 41°C. This treatment induced intense metacercarial activity and after 15 min metacercariae burst through their cyst walls and emerged. Electron microscopy demonstrated that during the process of excystment the inner layer of the cyst wall changed from a compact to a loose fibrous state. Experiments showed that only cysts containing viable metacercariae underwent this change whereas cysts which had been forcibly vacated before treatment did not. This indicated that the structural change of the inner layer of the cyst wall could not be attributed to the excystment medium. Also there was much less acid phosphatase activity in and on the surface of newly excysted metacercariae compared with encapsulated specimens. It was concluded that the excystment medium induced physical activity in, and the release of enzymic material by, the metacercariae. Together these activities rendered the cyst wall soft and susceptible to rupture by physical pressure.     

AN ENCYSTMENT STAGE,BEARING A NEW SCALE TYPE,OF THE ANTARCTIC PRASINOPHYTE PYRAMIMONAS GELIDICOLA AND ITS PALEOLIMNOLOGICAL AND TAXONOMIC SIGNIFICANCE1

Cysts of the Antarctic prasinophyte Pyramimonas gelidicola McFadden were found in water samples from a fjord and a saline lake in the Vestfold Hills, Antarctica Unialgal cultures of P. gelidicola from Ace Lake produced cysts. After ca. five weeks, tile cysts settled and adhered to the bottom of the culture flask. The cyst wall was covered by a scale type not seen on the flagellated cells; however, the base of the cyst scale was similar to the box scales of P. gelidicola motile cells. Cyst scales were also found off the continental shelf in Prydz Bay. In a 1.7 m sediment core taken from Ace Lake, both cyst scales and box scales of P. gelidicola occurred at most depths. Differences in the ratio of these two scale types at different depths in the core may indicate past ecological changes in the lake. Upper sediments of the core were dated at 5310 ± 90 yrs B.P., indicating that prasinophyte scales may be recognizably preserved for extended periods. P. gelidicola was widely distributed in saline lakes of the Vestfold Hills with salinities of 3.2–133% and temperatures ranging from – 5.0 to 10.4°C. This is the first report of encystment of P. gelidicola and, to our knowledge, is the first record of a prasinophyte with two distinctly different scale types occurring on cells during different stages of the life history.