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Cyanobacteria are ubiquitous in marine environments and play an important role as primary producers. Some cyanobacteria, the so-called cyanobionts (cyanobacterial symbionts), have a symbiotic relationship with unicellular organisms. Among these relationships, in particular, the nature (e.g., genetic diversity, host or cyanobiont specificity, and cyanobiont seasonality) of the cyanobiont-dinoflagellate host consortia remains poorly understood. In this study, 16S rDNA of cyanobionts in 138 single host cells isolated over four seasons from temperate waters were sequenced using the MiSeq platform. Genetic analysis of cyanobionts from the dinoflagellate host Ornithocercus revealed that three genetic types of Synechococcales cyanobionts occurred in a wide range of water temperatures (11–24 °C), and their distribution seemed to be closely associated with variations in salinity. Furthermore, a certain degree of host (or cyanobiont) specificity in cyanobionts (or the host) among Ornithocercus species as well as among other dinophysoid species (i.e. Amphisolenia, Citharistes, and Histioneis), was observed. In addition to the Synechococcales cyanobionts, this study identified OTU sequences affiliated with Vampirovibrionales and Chroococcidiopsidales in some Ornithocercus cells, suggesting that Ornithocercus species are an additional habitat for these bacterial groups. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A mixotrophic dinoflagellate Fragilidium duplocampanaeforme has been recently reported to specifically feed on mixotrophic dinoflagellates Dinophysis spp., which specifically feed on a mixotrophic ciliate Mesodinium rubrum (= Myrionecta rubra ), which specifically feeds on a cryptophyte. Using established cultures, in this study we aimed (1) to examine the pattern of phased cell division of a Korean strain of F . duplocampanaeforme , (2) to explore its mixotrophic characteristics, and (3) to address food web interactions among top three protists within the trophic series (i.e., M . rubrum , Dinophysis spp. and F . duplocampanaeforme ). During phototrophic growth, F . duplocampanaeforme displayed the phased cell division, with cell division starting 4 h after light was on and finished before the light off. Motile vegetative cell stage of F . duplocampanaeforme , which can ingest the prey, persisted over the dark period. When offered two Dinophysis species ( Dinophysis acuminata and Dinophysis caudata ) as prey, Korean F . duplocampanaeforme strain did not display any prey preference. The dinoflagellate F . duplocampanaeforme grew well phagotrophically in the dark if offered with plentiful prey (i.e., a facultative mixotroph). The dinoflagellate F . duplocampanaeforme grew well at growth rates of 0.21–0.26 d −1 when supplied with D . acuminata or D . caudata as prey in the dark, but its growth rates were 2.3–3.0 times slower than those when grown mixotrophically in the light. Nonetheless, the growth rates in the dark were similar to those in purely phototrophic growth. Our results from the mixed cultures of three protists demonstrated that the food web dynamics from cryptophyte through ciliate M. rubrum and dinoflagellate Dinophysis spp. to dinoflagellate F . duplocampanaeforme could be affected by predator–prey interaction as well as allelopathic interaction, depending on Dinophysis species included in the trophic series. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Several Dinoflagellates are known to practice acquired phototrophy by either hosting intact algal endosymbionts or retaining plastids. The acquisition of phototrophy in Dinoflagellates appears to occur independently over a variety of orders, rather than being restricted to any specific order(s). While Dinoflagellates with intact algal cells host endosymbionts of cyanobacteria, pelagophyte, prasinophyte or dictyochophyte, most organelle-retaining Dinoflagellates acquire plastids from cryptophytes. In Dinoflagellates with acquired phototrophy, the mechanism by which symbionts or plastids are obtained has not been well studied at sub-cellular or ultrastructural level, and thus little is known regarding their mechanism to sequester and maintain photosynthetic structures, except for three cases, Amphidinium poecilochroum, Gymnodinium aeruginosum, and Dinophysis caudata with peduncle feeding. Dinoflagellates with acquired phototrophy display different degrees of reduction of the retained endosymbiont and organelles, ranging from those which contain intact whole algal cells (e.g. green Noctiluca scintillans), to those which have retained almost a full complement of organelles (e.g., Amphidinium poecilochroum and Podolampas bipes), to those in which only the plastids remain (e.g., Amphidinium wigrense and Dinophysis spp.). A series of events leading to acquisition and subsequent degeneration of a whole-cell endosymbiont have been widely recognized as evolutionary pathway of the acquisition of plastids. However, recent work on D. caudata suggests that acquisition of phototrophy by predation (i.e. kleptoplastidy) may be a mechanism and evolutionary pathway through which plastids originated in Dinoflagellates with 'foreign' plastids other than the 'typical' peridinin-type plastids. Most organelle-retaining Dinoflagellates are facultative mixotrophs, with Dinophysis species and an undescribed Antarctic dinoflagellate being the only obligate mixotrophs known so far. The establishment of Dinoflagellates with acquired phototrophy in cultures and careful research using the cultures would help improve our knowledge of the evolution of the dinoflagellate plastids and their ecophysiology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

'Phototrophic' Dinophysis Ehrenberg species are well known to have chloroplasts of a cryptophyte origin, more specifically of the cryptophyte genus complex Teleaulax/ Geminigera. Nonetheless, whether chloroplasts of 'phototrophic' Dinophysis are permanent plastids or periodically derived kleptoplastids (stolen chloroplasts) has not been confirmed. Indeed, molecular sequence data and ultrastructural data lead to contradictory interpretations about the status of Dinophysis plastids. Here, we used established cultures of D. caudata strain DC-LOHABE01 and M. rubrum strain MR-MAL01 to address the status of Dinophysis plastids. Our approach was to experimentally generate D. caudata with 'green' plastids and then follow the ingestion and fate of 'reddish-brown' prey plastids using light microscopy, time-lapse videography, and single-cell TEM. Our results for D. caudata resolve the apparent discrepancy between morphological and molecular data by showing that plastids acquired when feeding on M. rubrum are structurally modified and retained as stellate compound chloroplasts characteristic of Dinophysis species. [ABSTRACT FROM AUTHOR]

'Phototrophic' Dinophysis Ehrenberg species are well known to have chloroplasts of a cryptophyte origin, more specifically of the cryptophyte genus complex Teleaulax/ Geminigera. Nonetheless, whether chloroplasts of 'phototrophic' Dinophysis are permanent plastids or periodically derived kleptoplastids (stolen chloroplasts) has not been confirmed. Indeed, molecular sequence data and ultrastructural data lead to contradictory interpretations about the status of Dinophysis plastids. Here, we used established cultures of D. caudata strain DC-LOHABE01 and M. rubrum strain MR-MAL01 to address the status of Dinophysis plastids. Our approach was to experimentally generate D. caudata with 'green' plastids and then follow the ingestion and fate of 'reddish-brown' prey plastids using light microscopy, time-lapse videography, and single-cell TEM. Our results for D. caudata resolve the apparent discrepancy between morphological and molecular data by showing that plastids acquired when feeding on M. rubrum are structurally modified and retained as stellate compound chloroplasts characteristic of Dinophysis species. [ABSTRACT FROM AUTHOR]

To survive, the marine dinoflagellate Dinophysis caudata Saville-Kent must feed on the plastidic ciliate Myrionecta rubra (= Mesodinium rubrum), itself a consumer of cryptophytes . Whether D.  caudata has its own permanent chloroplasts or retains plastids from its ciliate prey, however, remains unresolved. Further, how long D.  caudata plastids (or kleptoplastids) persist and remain photosynthetically active in the absence of prey remains unknown. We addressed those issues here, using the first established culture of D.  caudata. Phylogenetic analyses of the plastid 16S rRNA and psbA gene sequences directly from the three organisms ( D.  caudata, M.  rubra, and a cryptophyte) revealed that the sequences of both genes from the three organisms are almost identical to each other, supporting that the plastids of D.  caudata are kleptoplastids. A 3-month starvation experiment revealed that D.  caudata can remain photosynthetically active for ∼2 months when not supplied with prey. D.  caudata cells starved for more than 2 months continued to keep the plastid 16S rRNA gene but lost the photosynthesis-related genes (i.e., psaA and psbA genes). When the prey was available again, however, D.  caudata cells starved for more than 2 months were able to reacquire plastids and slowly resumed photosynthetic activity. Taken all together, the results indicate that the nature of the relationship between D.  caudata and its plastids is not that of permanent cellular acquisitions. D.  caudata is an intriguing protist that would represent an interesting evolutionary adaptation with regard to photosynthesis as well as help us to better understand plastid evolution in eukaryotes. [ABSTRACT FROM AUTHOR]

• The two morphotypes of D. acuminata and D. ovum were observed in cultures established from a single morphotype D. cf. ovum. • The mitochondrial cox1 gene does not consistently discriminate between species within the clade of " D. acuminata complex". • D. acuminata and D. ovum may be the same species, with their morphological variability associated with several factors (e.g. the cell cycle phase, the nutrient status, the geographical origins, seasonal changes, and environmental conditions). Marine dinoflagellates of the genus Dinophysis are well known for producing diarrhetic shellfish poisoning (DSP) toxins and/or pectenotoxins which have a significant impact on public health as well as on marine aquaculture. Out of more than 80 Dinophysis species recorded so far, D. cf. acuminata is the most commonly observed in coastal areas worldwide. Due to their highly similar morphological features, however, an accurate discrimination of the various D. cf. acuminata species such as D. acuminata , D. ovum , and D. sacculus under light microscopy has proven to be a difficult task to accomplish. Hence, these species have thus far been referred to as the " Dinophysis acuminata complex". Recent studies showed a discrimination between local strains of D. acuminata and D. ovum from Galician, northwestern Spain, using the mitochondrial cox1 gene as a genetic marker in addition to commonly used morphological features such as size and contour of the large hypothecal plates, shape of the small cells formed as part of their polymorphic life-cycle, development of the left sulcal list and ribs, and length of the right sulcal list. In the present study, attempts were made to discriminate between D. acuminata and D. ovum following single-cell isolation of 54 " D. acuminata complex" collected from Korean coastal waters, based on the abovementioned traits. Morphological data showed that all the traits analyzed overlapped between the two species. The mitochondrial cox1 (cytochrome c oxidase subunit I) gene sequences of every isolate were also determined, but a genetic distinction between D. acuminata and D. ovum could not be confirmed, suggesting that the cox1 gene is not a suitable genetic marker for discrimination between the two species. The results of this study suggest that the morphological variations observed within the " D. acuminata complex" may have been caused by several factors (e.g. different geographical locations, seasonal changes, and different environmental conditions), and that D. acuminata and D. ovum may be the same species. [ABSTRACT FROM AUTHOR]

• Parasite-mediation was investigated in predator-prey interactions of planktonic protists. • Akashiwo sanguinea, Luciella masanensis , and Parvilucifera infectans were used as model organisms. • Parasite mediation permitted L. masanensis to exploit A. sanguinea prey that was previously not used or preferred. • P. infectans infection cycle seems to influence parasite-mediated exploitation. • Presence of P. infectans at low density led to co-occurrence of the three protists. In planktonic predator-prey interactions, predation pressure could be affected by several factors associated with prey, including cell size, shape, nutritional quality, presence of chemical deterrents, and movement behaviors. In addition, parasitism may also play an important role in predator-prey interaction by infecting one or both partners involved in the biological interaction. In this study, parasite mediation in predator-prey interactions of planktonic protists was addressed using model organisms commonly observed in temperate coastal environments, namely, a phototrophic dinoflagellate Akashiwo sanguinea as a potential host and prey, a heterotrophic dinoflagellate predator Luciella masanensis , and a dinoflagellates-infecting parasitoid Parvilucifera infectans. Parasite mediation permitted L. masanensis to exploit the prey A. sanguinea that the predator was otherwise unable to use or did not prefer. However, parasite-mediated exploitation seems to be dependent on the infection cycle of P. infectans. Although zoospores and mature sporangium produced from infected A. sanguinea were not ingested by the predator L. masanensis , both newly infected (i.e., cells that zoospores had just penetrated) and infected (those containing early to late trophocytes) A. sanguinea cells attracted numerous L. masanensis cells and were rapidly ingested by the predator, leading to the predator's positive growth. The results from mixed culture experiments of the three protists showed that the presence of P. infectans at low density led to co-occurrence of the three protists. Thus, the parasitoid seems to play the role of a mediator in either inedible prey- or predator-dominated environments, leading to co-occurrence of the three protists. [ABSTRACT FROM AUTHOR]