Fate of green plastids in Dinophysis caudata following ingestion of the benthic ciliate Mesodinium coatsi: Ultrastructure and psbA gene

Phototrophic Dinophysis species are known to acquire plastids of the cryptophyte Teleaulax amphioxeia through feeding on the ciliate Mesodinium rubrum or M. cf. rubrum. In addition, several molecular studies have detected plastid encoding genes of various algal taxa within field populations of Dinophysis species. The trophic pathway by which Dinophysis species acquire plastids from algae other than the cryptophyte genus Teleaulax, however, is unknown. In this study, we examined the fate of prey organelles and plastid genes obtained by Dinophysis caudata through ingestion of Mesodinium coatsi, a benthic ciliate that retains green plastids of Chroomonas sp. Transmission electron microscopy and molecular analysis revealed relatively rapid digestion of prey-derived plastids. Following digestion of M. coatsi, however, photodamaged D. caudata cells having olive-green rather than reddish-brown plastids were able to recover some of their original reddish-brown pigmentation. Results further suggest that plastid genes of various algal taxa detected in field populations of Dinophysis species may reflect prey diversity rather than sequestration of multiple plastid types. Ingestion and digestion of prey other than M. rubrum or M. cf. rubrum may also provide nutritional requirements needed to repair and perhaps maintain sequestered T. amphioxeia plastids.     

The fate of cryptophyte cell organelles in the ciliate Mesodinium cf. rubrum subjected to starvation

Mesodinium rubrum Lohmann is a mixotrophic ciliate and one of the best studied species exhibiting acquired phototrophy. To investigate the fate of cryptophyte organelles in the ciliate subjected to starvation, we conducted ultrastructural studies of a Korean strain of M. cf. rubrum during a 10 week starvation experiments. Ingested cells of the cryptophyte Teleaulax amphioxeia were first enveloped by ciliate membrane, and then prey organelles, including ejectisomes, flagella, basal bodies and flagellar roots, were digested. Over time, prey nuclei protruded into the cytoplasm of the ciliate, their size and volume increased, and their number decreased, suggesting that the cryptophyte nuclei likely fused with each other in the ciliate cytoplasm. At 4 weeks of starvation, M. cf. rubrum cells without cryptophyte nuclei started to appear. At 10 weeks of starvation, only two M. cf. rubrum cells still possessing a cryptophyte nucleus had relatively intact chloroplast-mitochondria complexes (CMCs), while M. cf. rubrum cells without cryptophyte nuclei had a few damaged CMCs. This is the first ultrastructural study demonstrating that cryptophyte nuclei undergo a dramatic change inside M. cf. rubrum in terms of size, shape, and number following their acquisition.     

psbA based molecular analysis of cross-feeding experiments suggests that Dinophysis acuta does not harbour permanent plastids

Toxic marine dinoflagellate species of the genus Dinophysis Ehrenberg are obligate mixotrophs that require feeding on the ciliate Mesodinium rubrum and light to achieve growth. It is now well known that they harbour plastids of cryptophyte origin, particularly of the genus Teleaulax, Plagioselmis or Geminigera group (TPG clade). Nevertheless, whether these plastids are permanent, or periodically acquired from M. rubrum prey, need additional studies in different phototrophic Dinophysis species. The origin of plastids from Dinophysis acuta Ehrenberg, one of the main agents of diarrhetic shellfish poisoning (DSP) outbreaks in Western Europe, was investigated here. Cross feeding-starvation experiments were carried out with cultures of D. acuta using M. rubrum as prey, the latter fed with two cryptophyte species, Teleaulax amphioxeia Hill and Teleaulax gracilis, belonging to the TPG clade in addition to Falcomonas sp. and Hemiselmis sp. The fate of cryptophyte plastids transferred to D. acuta through its ciliate prey was investigated using the plastid psbA gene as a tracer.     

PREDATION BY THE TOXIC DINOFLAGELLATE DINOPHYSIS FORTII ON THE CILIATE MYRIONECTA RUBRA AND OBSERVATION OF SEQUESTRATION OF CILIATE CHLOROPLASTS1

This is the first report of the propagation of the toxic dinoflagellate Dinophysis fortii Pavill. under laboratory conditions when fed on the marine ciliate Myrionecta rubra grown with the cryptophyte Teleaulax amphioxeia (W. Conrad) D. R. A. Hill. In contrast, reduced growth of D. fortii (max. of 3–4 divisions) and formation of small cells were observed in the absence of the ciliate or when provided with T. amphioxeia only as prey, showing that D. fortii cannot utilize T. amphioxeia as prey. In the TEM observation of D. fortii cells, which had fully fed on the ciliate prey, well‐developed chloroplasts (5–12 μm in length) were seen and three thylakoids were usually arranged in most of the chloroplasts observed, but chloroplasts having two thylakoids were sometimes confirmed. In cells starved for 4 weeks, decrease of chloroplast numbers and disappearance of large chloroplasts were observed, and only a few small chloroplasts (0.5–2 μm in length) remained in the marginal regions. In the observation of the sequestration process of the chloroplasts ingested from M. rubra by D. fortii, within 15 min after D. fortii captured M. rubra, incorporation of almost all of the chloroplasts was observed, while most of the other cell contents still remained in the M. rubra cell. After that, dispersion of the ingested chloroplasts toward the marginal regions was confirmed, suggesting that chloroplasts of M. rubra are ingested and dispersed in D. fortii cells in advance of the ingestion of the other cell contents to prevent them from being digested in food vacuoles. The ingested chloroplasts can also function as kleptoplastids.     

Growth and Chloroplast Replacement of the Benthic Mixotrophic Ciliate Mesodinium coatsi

While the ecophysiology of planktonic Mesodinium rubrum species complex has been relatively well studied, very little is known about that of benthic Mesodinium species. In this study, we examined the growth response of the benthic ciliate Mesodinium coatsi to different cryptophyte prey using an established culture of this species. M. coatsi was able to ingest all of the offered cryptophyte prey types, but not all cryptophytes supported its positive, sustained growth. While M. coatsi achieved sustained growth on all of the phycocyanin‐containing Chroomonas spp. it was offered, it showed different growth responses to the phycoerythrin‐containing cryptophytes Rhodomonas spp., Storeatula sp., and Teleaulax amphioxeia. M. coatsi was able to easily replace previously ingested prey chloroplasts with newly ingested ones within 4 d, irrespective of prey type, if cryptophyte prey were available. Once retained, the ingested prey chloroplasts seemed to be photosynthetically active. When fed, M. coatsi was capable of heterotrophic growth in darkness, but its growth was enhanced significantly in the light (14:10 h light:dark cycle), suggesting that photosynthesis by ingested prey chloroplast leads to a significant increase in the growth of M. coatsi. Our results expand the knowledge of autecology and ecophysiology of the benthic M. coatsi.     

Preferential Plastid Retention by the Acquired Phototroph Mesodinium chamaeleon

The ciliate genus Mesodinium contains species that rely to varying degrees on photosynthetic machinery stolen from cryptophyte algal prey. Prey specificity appears to scales inversely with this reliance: The predominantly phototrophic M. major/rubrum species complex exhibits high prey specificity, while the heterotrophic lineages M. pulex and pupula are generalists. Here, we test the hypothesis that the recently described mixotroph M. chamaeleon, which is phylogenetically intermediate between M. major/rubrum and M. pulex/pupula, exhibits intermediate prey preferences. Using a series of feeding and starvation experiments, we demonstrate that M. chamaeleon grazes and retains plastids at rates which often exceed those observed in M. rubrum, and retains plastids from at least five genera of cryptophyte algae. Despite this relative generality, M. chamaeleon exhibits distinct prey preferences, with higher plastid retention, mixotrophic growth rates and efficiencies, and starvation tolerance when offered Storeatula major, a cryptophyte that M. rubrum does not appear to ingest. These results suggest that niche partitioning between the two acquired phototrophs may be mediated by prey identity. M. chamaeleon appears to represent an intermediate step in the transition to strict reliance on acquired phototrophy, indicating that prey specificity may evolve alongside degree of phototrophy.     

Combined Effects of Temperature,Irradiance, and pH on Teleaulax amphioxeia (Cryptophyceae) Physiology and Feeding Ratio For Its Predator Mesodinium rubrum (Ciliophora)1

The cryptophyte Teleaulax amphioxeia is a source of plastids for the ciliate Mesodinium rubrum and both organisms are members of the trophic chain of several species of Dinophysis. It is important to better understand the ecology of organisms at the first trophic levels before assessing the impact of principal factors of global change on Dinophysis spp. Therefore, combined effects of temperature, irradiance, and pH on growth rate, photosynthetic activity, and pigment content of a temperate strain of T. amphioxeia were studied using a full factorial design (central composite design 2³*) in 17 individually controlled bioreactors. The derived model predicted an optimal growth rate of T. amphioxeia at a light intensity of 400 μmol photons · m⁻² · s⁻¹, more acidic pH (7.6) than the current average and a temperature of 17.6°C. An interaction between temperature and irradiance on growth was also found, while pH did not have any significant effect. Subsequently, to investigate potential impacts of prey quality and quantity on the physiology of the predator, M. rubrum was fed two separate prey: predator ratios with cultures of T. amphioxeia previously acclimated at two different light intensities (100 and 400 μmol photons · m⁻² s⁻¹). M. rubrum growth appeared to be significantly dependent on prey quantity while effect of prey quality was not observed. This multi-parametric study indicated a high potential for a significant increase of T. amphioxeia in future climate conditions but to what extent this would lead to increased occurrences of Mesodinium spp. and Dinophysis spp. should be further investigated.     

Inducible Mixotrophy in the Dinoflagellate Prorocentrum minimum

Prorocentrum minimum is a neritic dinoflagellate that forms seasonal blooms and red tides in estuarine ecosystems. While known to be mixotrophic, previous attempts to document feeding on algal prey have yielded low grazing rates. In this study, growth and ingestion rates of P. minimum were measured as a function of nitrogen (‐N) and phosphorous (‐P) starvation. A P. minimum isolate from Chesapeake Bay was found to ingest cryptophyte prey when in stationary phase and when starved of N or P. Prorocentrum minimum ingested two strains of Teleaulax amphioxeia at higher rates than six other cryptophyte species. In all cases ‐P treatments resulted in the highest grazing. Ingestion rates of ‐P cells on T. amphioxeia saturated at ~5 prey per predator per day, while ingestion by ‐N cells saturated at 1 prey per predator per day. In the presence of prey, ‐P treated cells reached a maximum mixotrophic growth rate (μ_max) of 0.5 d^?1, while ‐N cells had a μ_max of 0.18 d^?1. Calculations of ingested C, N, and P due to feeding on T. amphioxeia revealed that phagotrophy can be an important source of all three elements. While P. minimum is a proficient phototroph, inducible phagotrophy is an important nutritional source for this dinoflagellate.     

Sequestered plastids in Mesodinium rubrum are functionally active up to 80 days of phototrophic growth without cryptomonad prey

The red tide ciliate Mesodinium rubrum is an obligate mixotroph which requires feeding on cryptomonad prey mainly to retain its photosynthetic apparatus. Functionality of the sequestered plastids has been known to be lowered within a few weeks. The upper limit of the functionally active duration for the newly retained plastid, however, has been rarely estimated or determined. In parallel with genetic analysis, we investigated dynamics of population density, orange fluorescence of the plastids, and DCMU ((3-(3,4-dichlorophenyl)-1,1-dimethylurea) photosynthetic capacity of phototrophically growing M. rubrum (strain MR-MAL01) for 100 days. M. rubrum populations continued their phototrophic growth for the first 6 weeks, with gradually decreasing growth rates. Rapid decline of population density began from the 8th week. The photosynthetic capacity remained quite stable, ranging from 0.7 during the 1st week down to 0.5 during the 11th week. On day 87, the photosynthetic capacity steeply decreased to 0.05. The orange fluorescence of the retained plastids became very weak during the 4th week, to be almost undetectable on day 98. Only plastid 16S rRNA gene kept strong band intensity of PCR products throughout the whole period of 100 day experiment. Interestingly, the band intensities from psaA and psbA genes all become dramatically weakened after day 77. After new prey cryptomonads (strain CR-MAL03) were offered to M. rubrum starved for 80 days, ‘CR-MAL03 type’ 1192-bp PCR product of plastid 16S rRNA gene was detected in most experimental single M. rubrum cells. Here, we demonstrate that M. rubrum can grow for ∼6 weeks in the absence of cryptomonad prey, and photosynthetic capacity of M. rubrum can be maintained active for ∼11 weeks without prey. Additionally, M. rubrum starved for 80 days was shown to be physiologically healthy enough to ingest cryptomonad preys and retain new plastids.     

Ultrastructure of the Oral Apparatus of Mesodinium rubrum from Korea

Mesodinium rubrum Lohmann is a photosynthetic marine ciliate that has functional chloroplasts of cryptophyte origin. Little is known about the oral ultrastructure of M. rubrum compared with several reports on the sequestration of nuclei and plastids from prey organisms, such as Geminigera cryophila and Teleaulax species. Here, we describe the fine structure of the oral apparatus of a M. rubrum strain from Gomso Bay, Korea. The cytopharynx was cone‐shaped and supported by 20–22 ribbons of triplet microtubules. At the anterior end of the cytopharynx, an annulus anchored small cylinders composed of 11 microtubules. The small cylinders were spaced at regular intervals, each reinforced by one set of the triplet microtubules. At the opening of the cytostome, larger 14‐membered microtubular cylinders were set adjacent to the small, 11‐membered microtubular cylinders, each pair surrounded by separate membranes, however, only the large cylinders extended into the oral tentacles. There were 20–22 oral tentacles each having one to five extrusomes at its tip. At the anterior end of the oral apparatus, microtubular bands supporting the cytostome curved posteriad, extending beneath the cell cortex to the kinetosomes of the somatic cirri. The microtubular bands were connected by striated fibers and originated from kinetosomes anchored by fibers. Each cirrus consisted of eight cilia associated with 16 kinetosomes. The ultrastructure of M. rubrum from Korea provides information useful for taxonomic characterization of the genus Mesodinium and relevant to developing a better understanding of the acquisition of foreign organelles through phagocytosis by M. rubrum.     

A Dinoflagellate Amylax triacantha with Plastids of the Cryptophyte Origin: Phylogeny,Feeding Mechanism,and Growth and Grazing Responses

The gonyaulacalean dinoflagellates Amylax spp. were recently found to contain plastids of the cryptophyte origin, more specifically of Teleaulax amphioxeia. However, not only how the dinoflagellates get the plastids of the cryptophyte origin is unknown but also their ecophysiology, including growth and feeding responses as functions of both light and prey concentration, remain unknown. Here, we report the establishment of Amylax triacantha in culture, its feeding mechanism, and its growth rate using the ciliate prey Mesodinium rubrum (= Myrionecta rubra) in light and dark, and growth and grazing responses to prey concentration and light intensity. The strain established in culture in this study was assigned to A. triacantha, based on morphological characteristics (particularly, a prominent apical horn and three antapical spines) and nuclear SSU and LSU rDNA sequences. Amylax triacantha grew well in laboratory culture when supplied with the marine mixotrophic ciliate M. rubrum as prey, reaching densities of over 7.5 × 10³ cells/ml. Amylax triacantha captured its prey using a tow filament, and then ingested the whole prey by direct engulfment through the sulcus. The dinoflagellate was able to grow heterotrophically in the dark, but the growth rate was approximately two times lower than in the light. Although mixotrophic growth rates of A. triacantha increased sharply with mean prey concentrations, with maximum growth rate being 0.68/d, phototrophic growth (i.e. growth in the absence of prey) was ?0.08/d. The maximum ingestion rate was 2.54 ng C/Amylax/d (5.9 cells/Amylax/d). Growth rate also increased with increasing light intensity, but the effect was evident only when prey was supplied. Increased growth with increasing light intensity was accompanied by a corresponding increase in ingestion. In mixed cultures of two predators, A. triacantha and Dinophysis acuminata, with M. rubrum as prey, A. triacantha outgrew D. acuminata due to its approximately three times higher growth rate, suggesting that it can outcompete D. acuminata. Our results would help better understand the ecophysiology of dinoflagellates retaining foreign plastids.     

Acquired phototrophy in Mesodinium and Dinophysis – A review of cellular organization,prey selectivity,nutrient uptake and bioenergetics

Acquired phototrophy, i.e. the use of chloroplasts from ingested prey, can be found among some species of dinoflagellates and ciliates. The best studied examples of this phenomenon in these groups are within the ciliate genus Mesodinium and the dinoflagellate genus Dinophysis, both ecologically important genera with a worldwide distribution. Mesodinium species differ considerably in their carbon metabolism. Some species rely almost exclusively on food uptake, while other species rely mostly on photosynthesis. In Mesodinium with acquired phototrophy, a number of prey organelles in addition to chloroplasts may be retained, and the host ciliate has considerable control over the acquired chloroplasts; Mesodinium rubrum is capable of dividing its acquired chloroplasts and can also photoacclimate. In Dinophysis spp., the contents of ciliate prey are sucked out, but only the chloroplasts are retained from the ingested prey. Some chloroplast house-keeping genes have been found in the nucleus of Dinophysis and some preliminary evidence suggests that Dinophysis may be capable for photoacclimation. Both genera have been claimed to take up inorganic nutrients, including NO₃⁻, indicating that processes beyond photosynthesis have been acquired. M. rubrum seems to depend upon prey species within the Teleaulax/Plagioselmis/Geminigera clade of marine cryptophytes. Up until now, Dinophysis species have only been maintained cultured on M. rubrum as food, but other ciliates may also be ingested. Dinophysis spp. and M. rubrum are obligate mixotrophs, depending upon both prey and light for sustained growth. However, while M. rubrum only needs to ingest 1–2% of its carbon demand per day to attain maximum growth, Dinophysis spp. need to obtain about half of their carbon demand from ingestion for maximum growth. Both Mesodinium and Dinophysis spp. can survive for months in the light without food. The potential role for modeling in exploring the complex balance of phototrophy and phago-heterotrophy, and its ecological implications for the mixotroph and their prey, is discussed.     

Mixotrophy in the marine red-tide cryptophyte Teleaulax amphioxeia and ingestion and grazing impact of cryptophytes on natural populations of bacteria in Korean coastal waters

Cryptophytes are ubiquitous and one of the major phototrophic components in marine plankton communities. They often cause red tides in the waters of many countries. Understanding the bloom dynamics of cryptophytes is, therefore, of great importance. A critical step in this understanding is unveiling their trophic modes. Prior to this study, several freshwater cryptophyte species and marine Cryptomonas sp. and Geminifera cryophila were revealed to be mixotrophic. The trophic mode of the common marine cryptophyte species, Teleaulax amphioxeia has not been investigated yet. Thus, to explore the mixotrophic ability of T. amphioxeia by assessing the types of prey species that this species is able to feed on, the protoplasms of T. amphioxeia cells were carefully examined under an epifluorescence microscope and a transmission electron microscope after adding each of the diverse prey species. Furthermore, T. amphioxeia ingestion rates heterotrophic bacteria and the cyanobacterium Synechococcus sp. were measured as a function of prey concentration. Moreover, the feeding of natural populations of cryptophytes on natural populations of heterotrophic bacteria was assessed in Masan Bay in April 2006. This study reported for the first time, to our knowledge, that T. amphioxeia is a mixotrophic species. Among the prey organisms offered, T. amphioxeia fed only on heterotrophic bacteria and Synechococcus sp. The ingestion rates of T. amphioxeia on heterotrophic bacteria or Synechococcus sp. rapidly increased with increasing prey concentrations up to 8.6 × 10⁶ cells ml⁻¹, but slowly at higher prey concentrations. The maximum ingestion rates of T. amphioxeia on heterotrophic bacteria and Synechococcus sp. reached 0.7 and 0.3 cells predator⁻¹ h⁻¹, respectively. During the field experiments, the ingestion rates and grazing coefficients of cryptophytes on natural populations of heterotrophic bacteria were 0.3–8.3 cells predator⁻¹ h⁻¹ and 0.012–0.033 d⁻¹, respectively. Marine cryptophytes, including T. amphioxeia, are known to be favorite prey species for many mixotrophic and heterotrophic dinoflagellates and ciliates. Cryptophytes, therefore, likely play important roles in marine food webs and may exert a considerable potential grazing impact on the populations of marine bacteria.     

PHOTOACCLIMATION IN THE PHOTOTROPHIC MARINE CILIATE MESODINIUM RUBRUM (CILIOPHORA)1

Mesodinium rubrum (=Myrionecta rubra), a marine ciliate, acquires plastids, mitochondria, and nuclei from cryptophyte algae. Using a strain of M. rubrum isolated from McMurdo Sound, Antarctica, we investigated the photoacclimation potential of this trophically unique organism at a range of low irradiance levels. The compensation growth irradiance for M. rubrum was 0.5 μmol quanta · m⁻² · s⁻¹, and growth rate saturated at ∼20 μmol quanta · m⁻² · s⁻¹. The strain displayed trends in photosynthetic efficiency and pigment content characteristic of marine phototrophs. Maximum chl a–specific photosynthetic rates were an order of magnitude slower than temperate strains, while growth rates were half as large, suggesting that a thermal limit to enzyme kinetics produces a fundamental limit to cell function. M. rubrum acclimates to light‐ and temperature‐limited polar conditions and closely regulates photosynthesis in its cryptophyte organelles. By acquiring and maintaining physiologically viable, plastic plastids, M. rubrum establishes a selective advantage over purely heterotrophic ciliates but reduces competition with other phototrophs by exploiting a very low‐light niche.     

Ultrastructure and Ecology of Perispira ovum (Ciliophora: Litostomatea): An Aerobic, Planktonic Ciliate that Sequesters the Chloroplasts, Mitochondria, and Paramylon of Euglena proxima in a Micro-oxic Habitat

ABSTRACT. High resolution sampling of the stratified water column in a fjord-like ecosystem revealed a green-pigmented planktonic ciliate that was found to be a ravenous predator of Euglena proxima. The vertical distributions of both predator and prey were coincident, and maximum populations occurred across the transition from oxic to anoxic water. This ciliate was identified as Perispira ovum (family Spathidiidae; Order Haptorida). P. ovum was observed by transmission electron microscopy to retain not only the chloroplasts, but also the mitochondria and paramylon reserve of its algal prey. A mechanism of sequestration of algal organelles is demonstrated for the first time. This mechanism includes: recognition, capture, and ingestion of prey; rupture and release of algal cell contents; and enrobing of individual organelles and paramylon by the host vacuolar membrane. The structural integrity, peripheral location, and association with host endoplasmic reticulum suggests the sequestered organelles may be functional within P. ovum. The occurrence and high biomass of this aerobic ciliate in an oxygen-limited environment also suggests that the sequestered chloroplasts are photosynthetically active and may provide additional substrates (such as oxygen) and metabolic capabilities that are crucial for its survival.     

Physiological Responses of Mesodinium major to Irradiance,Prey Concentration and Prey Starvation

Ciliates within the Mesodinium rubrum/Mesodinium major species complex harbor chloroplasts and other cell organelles from specific cryptophyte species. Mesodinium major was recently described, and new studies indicate that blooms of M. major are just as common as blooms of M. rubrum. Despite this, the physiology of M. major has never been studied and compared to M. rubrum. In this study, growth, food uptake, chlorophyll a and photosynthesis were measured at six different irradiances, when fed the cryptophyte, Teleaulax amphioxeia. The results show that the light compensation point for growth of M. major was significantly higher than for M. rubrum. Inorganic carbon uptake via photosynthesis contributed by far most of total carbon uptake at most irradiances, similar to M. rubrum. Mesodinium major cells contain ~four times as many chloroplast as M. rubrum leading to up to ~four times higher rates of photosynthesis. The responses of M. major to prey starvation and refeeding were also studied. Mesodinium major was well adapted to prey starvation, and 51 d without prey did not lead to mortality. Mesodinium major quickly recovered from prey starvation when refed, due to high ingestion rates of > 150 prey/predator/d.     

The Effect of Starvation on Plastid Number and Photosynthetic Performance in the Kleptoplastidic Dinoflagellate Amylax triacantha

The dinoflagellate Amylax triacantha is known to retain plastids of cryptophyte origin by engulfing the mixotrophic ciliate Mesodinium rubrum, itself a consumer of cryptophytes. However, there is no information on the fate of the prey's organelles and the photosynthetic performance of the newly retained plastids in A. triacantha. In this study, we conducted a starvation experiment to observe the intracellular organization of the prey's organelles and temporal changes in the photosynthetic efficiency of acquired plastids in A. triacantha. The ultrastructural observations revealed that while the chloroplast‐mitochondria complexes and nucleus of cryptophyte were retained by A. triacantha, other ciliate organelles were digested in food vacuoles. Acquired plastids were retained in A. triacantha for about 1 mo and showed photosynthetic activities for about 18 d when measured by a pulse‐amplitude modulation fluorometer.     

Harmful effects of Dinophysis to the ciliate Mesodinium rubrum: Implications for prey capture

Toxigenic Dinophysis spp. are obligate mixotrophic dinoflagellates that require a constant supply of prey—Mesodinium rubrum—to achieve long-term growth by means of kleptoplasty. Mesodinium rubrum is, however, a fast moving, jumping ciliate exhibiting an effective escape response from suspensivorous predators. In the present study, a series of laboratory experiments evaluating the motility and survival of M. rubrum in the presence of Dinophysis cells and/or substances contained in their culture medium was designed, in order to assess the mechanisms involved in prey capture by Dinophysis spp. Cell abundance of M. rubrum decreased in the presence of Dinophysis cf. ovum cells producing okadaic acid (OA; up to 7.94 ± 2.67 pg cell⁻¹) and smaller amounts of dinophysistoxin-1 (DTX-1) and pectenotoxin-2 (PTX-2). Prey capture was often observed after the ciliate had been attached to adhesive “mucus traps”, which only appeared in the presence of Dinophysis cells. Before being attached to the mucus traps, M. rubrum cells reduced significantly their swimming frequency (from ∼41 to 19 ± 3 jumps min⁻¹) after only 4 h of initial contact with D. cf. ovum cells. M. rubrum survival was not affected in contact with purified OA, DTX-1 and PTX-2 solutions, but decreased significantly when the ciliate was exposed to cell-free or filtered culture medium from both D. cf. ovum and D. caudata, the latter containing moderate concentrations of free eicosapentaenoic acid and docosahexaenoic acid. The results thus indicate that Dinophysis combines the release of toxic compounds other than shellfish toxins, possibly free PUFAs, and a “mucus trap” to enhance its prey capture success by immobilizing and subsequently arresting M. rubrum cells.