Feeding by the newly described mixotrophic dinoflagellate Paragymnodinium shiwhaense: feeding mechanism, prey species, and effect of prey concentration

Comparative analyses of nutritional strategies among the species within the genus Paragymnodinium (Gymnodiniales, Dinophyceae)

The genus Paragymnodinium exhibits various nutritional strategies despite the fact that each species has pigmented plastids. This provides an excellent opportunity to investigate the evolution of the mode of nutrition and the function of the plastids through comparative studies. In this study, we analyzed the growth, pigment composition, absorption spectra, variable chlorophyll a fluorescence, and photosynthetic carbon fixation capabilities of P. stigmaticum, P. asymmetricum, and P. inerme. The autotrophic species P. asymmetricum and P. inerme without resorting to any nutrition from prey organisms displayed high PSII activity and carbon fixation rates. The pigment compositions of these two species were identical to those of other typical peridinin-containing type dinoflagellates. On the contrary, the phagotrophic species P. stigmaticum showed heterotrophic growth, and the variable chlorophyll a fluorescence properties and carbon fixation rates indicated significantly lower photosynthetic competence relative to those of the above two species. Paragymnodinium stigmaticum also contained peridinin, but pigment content ratios of peridinin, chlorophyll c_2, and β-carotene were significantly different from those of other two species. The absorption spectrum analysis revealed a redshift in the peak of the Q_y band of chlorophyll a in P. stigmaticum, presumably due to a change in chlorophyll-protein complexes. We conclude that such distinct differences in nutritional strategies between members of the genus Paragymnodinium would provide a platform for the hypothetical loss of photosynthetic function leading to colorless dinoflagellates.

Phytoplankton and particle size spectra indicate intense mixotrophic dinoflagellates grazing from summer to winter

Mixotrophic dinoflagellates (MTD) are a diverse group of organisms often responsible for the formation of harmful algal blooms. However, the development of dinoflagellate blooms and their effects on the plankton community are still not well explored. Here we relate the species succession of MTD with parallel changes of phytoplankton size spectra during periods of MTD dominance. We used FlowCAM analysis to acquire size spectra in the range 2-200 μm every one or two weeks from July to December 2007 at Helgoland Roads (Southern North Sea). Most size spectra of dinoflagellates were bimodal, whereas for other groups, e.g. diatoms and autotrophic flagellates, the spectra were unimodal, which indicates different resource use strategies of autotrophs and mixotrophs. The biomass lost in the size spectrum correlates with the potential grazing pressure of MTD. Based on size-based analysis of trophic linkages, we suggest that mixotrophy, including detritivory, drives species succession and facilitates the formation of bimodal size spectra. Bimodality in particular indicates niche differentiation through grazing of large MTD on smaller MTD. Phagotrophy of larger MTD may exceed one of the smaller MTD since larger prey was more abundant than smaller prey. Under strong light limitation, a usually overlooked refuge strategy may derive from detritivory. The critical role of trophic links of MTD as a central component of the plankton community may guide future observational and theoretical research.

Feeding diverse prey as an excellent strategy of mixotrophic dinoflagellates for global dominance

Microalgae fuel food webs and biogeochemical cycles of key elements in the ocean. What determines microalgal dominance in the ocean is a long-standing question. Red tide distribution data (spanning 1990 to 2019) show that mixotrophic dinoflagellates, capable of photosynthesis and predation together, were responsible for ~40% of the species forming red tides globally. Counterintuitively, the species with low or moderate growth rates but diverse prey including diatoms caused red tides globally. The ability of these dinoflagellates to trade off growth for prey diversity is another genetic factor critical to formation of red tides across diverse ocean conditions. This finding has profound implications for explaining the global dominance of particular microalgae, their key eco-evolutionary strategy, and prediction of harmful red tide outbreaks.

Ultrastructure and Systematics of Two New Species of Dinoflagellate, Paragymnodinium Asymmetricum sp. nov. and Paragymnodinium Inerme sp. nov. (Gymnodiniales, Dinophyceae)1

The genus Paragymnodinium currently includes two species, P. shiwhaense and P. stigmaticum, that are characterized by mixotrophic nutrition and the possession of nematocysts. In this study, two new dinoflagellates belonging to this genus were described based on observations using LM, SEM, and TEM together with a molecular analysis. Cells of P. asymmetricum sp. nov., isolated from Nha Trang Beach, Vietnam, were 7.9-12.6 μm long and 4.7-9.0 μm wide. The species showed no evidence of feeding behavior and was able to sustain itself phototrophically. Paragymnodinium asymmetricum shared many features with P. shiwhaense, including presence of nematocysts, absence of an eyespot, and a planktonic lifestyle, but was clearly distinguished by the asymmetric shape of the hyposome, possession of a single chloroplast, and its nutritional mode. Cells of P. inerme sp. nov., isolated from Jogashima, Kanagawa Pref, Japan, were 15.3-23.7 μm long and 10.9-19.6 μm wide. This species also showed no evidence of feeding behavior. Paragymnodinium inerme was similar to cells of P. shiwhaense in shape and planktonic lifestyle, but its nutritional mode was different. The presence of incomplete nematocysts was also a unique feature. A phylogenetic analysis inferred from concatenated SSU and LSU rDNA sequences recovered the two dinoflagellates in a robust clade with Paragymnodinium spp., within the clade of Gymnodinium sensu stricto. This evidence, together with their morphological similarities, made it reasonable to conclude that these two dinoflagellates are new species of Paragymnodinium.

Contrasting effects of high-intensity photosynthetically active radiation on two bloom-forming dinoflagellates

While light limitation can inhibit bloom formation in dinoflagellates, the potential for high-intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well-studied. We measured the effects of high-intensity PAR on the bloom-forming dinoflagellates Alexandrium fundyense and Heterocapsa rotundata. Various physiological parameters (photosynthetic efficiency F_v /F_m , cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll-a content) were measured before and after exposure to high-intensity natural sunlight in short-term light stress experiments. In addition, photosynthesis-irradiance (P-E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species-specific responses to high PAR. While high light decreased F_v /F_m in both species, A. fundyense showed little additional evidence of light stress in short-term experiments, although increased membrane permeability and intracellular DMSP indicated a response to handling. P-E responses further indicated a high light-adapted species with Chl-a inversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per-cell photosynthesis rates suggest a trade-off between photoprotection and C fixation in high light-acclimated cells. Heterocapsa rotundata cells, in contrast, swelled in response to high light and sometimes lysed in short-term experiments, releasing DMSP. P-E responses confirmed a low light-adapted species with high photosynthetic efficiencies associated with trade-offs in the form of substantial photoinhibition and a lack of plasticity in Chl-a content. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species-specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.

Effects of light intensity, temperature, and salinity on the growth and ingestion rates of the red-tide mixotrophic dinoflagellate Paragymnodinium shiwhaense

Among mixotrophic dinoflagellates, the maximum mixotrophic growth rate of the red-tide dinoflagellate Paragymnodinium shiwhaense is relatively high, whereas mortality due to predation is low. To investigate the effects of major environmental parameters on P. shiwhaense, growth and ingestion rates of one strain of P. shiwhaense on the algal prey species Amphidinium carterae (also a dinoflagellate) were determined under various light intensities (0-500 μE m^-2s^-1), water temperatures (5-30 °C), and salinities (5-40). Cells of P. shiwhaense did not grow well in darkness but grew well at light intensities ≥ 10 μE m^-2s^-1. There were no significant differences in either growth or ingestion rates of P. shiwhaense fed A. carterae at light intensities between 10 and 500 μE m^-2s^-1. Furthermore, P. shiwhaense did not grow at 5 °C or ≥ 28 °C. Its growth rates between 7 and 26 °C were significantly affected by temperature, and the optimal temperature for maximal growth was 25 °C. With increasing salinity from 5 to 20, the growth rate of P. shiwhaense fed A. carterae increased and became saturated at salinities between 20 and 40, while the ingestion rate at salinities between 10 and 40 did not significantly change. Thus, overall, the growth and ingestion rates of P. shiwhaense fed A. carterae were affected by temperature and salinity, but not by light intensity other than darkness. These findings provide a beginning basis for understanding the ecology of this potentially harmful algal species in marine coastal ecosystems.

Feeding by common heterotrophic protists on the mixotrophic alga Gymnodinium smaydae (Dinophyceae), one of the fastest growing dinoflagellates

Gymnodinium smaydae is one of the fastest growing dinoflagellates. However, its population dynamics are affected by both growth and mortality due to predation. Thus, feeding by common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Oblea rotunda, Oxyrrhis marina, and Polykrikos kofoidii, and the naked ciliate Pelagostrobilidium sp. on G. smaydae was investigated in the laboratory. Furthermore, growth and ingestion rates of O. marina, G. dominans, and Pelagostrobilidium sp. on G. smaydae in response to prey concentration were also determined. Oxyrrhis marina, G. dominans, G. moestrupii, and Pelagostrobilidium sp. were able to feed on G. smaydae, but P. kofoidii and O. rotunda did not feed on this dinoflagellate. The maximum growth rate of O. marina on G. smaydae was 0.411 per day. However, G. smaydae did not support the positive growth of Pelagostrobilidium sp. The maximum ingestion rates of O. marina and Pelagostrobilidium sp. on G. smaydae were 0.27 and 6.91 ng C · predator^-1  · d^-1 , respectively. At the given mean prey concentrations, the highest growth and ingestion rates of G. dominans on G. smaydae were 0.114 per day and 0.04 ng C · predator^-1  · d^-1 , respectively. The maximum growth and ingestion rates of O. marina on G. smaydae are lower than those on most of the other algal prey species. Therefore, O. marina may be an effective predator of G. smaydae, but G. smaydae may not be the preferred prey for supporting high growth of the predator in comparison to other species as inferred from a literature survey.

Feeding by the heterotrophic nanoflagellate Katablepharis remigera on algal prey and its nationwide distribution in Korea

Heterotrophic nanoflagellates are ubiquitous in natural waters, and most heterotrophic nanoflagellates are known to grow on bacteria. Recently, the heterotrophic nanoflagellate Katablepharis japonica has been reported to be an effective predator of diverse toxic or harmful algal prey. To date, 7 Katablepharis species have been identified, and therefore important questions arise as to whether other Katablepharis species can feed on algal prey, and further whether the types of prey of other Katablepharis species differ from those of K. japonica. To answer these important questions, feeding by Katablepharis remigera on diverse algal prey was examined. Specific growth and ingestion rates of K. remigera feeding on the raphidophytes Heterosigma akashiwo and Chattonella subsalsa were determined. Furthermore, the abundance of K. remigera at 28 stations along the coastline of Korea from January 2015 to October 2017 was quantified using qPCR method and newly designed specific primer-probe sets. Among 25 potential algal prey tested, K. remigera fed on only H. akashiwo and C. subsalsa; however, it did not feed on a diatom, a prymnesiophyte, a prasinophyte, cryptophytes, dinoflagellates, Mesodinium rubrum, a mixotrophic ciliate, and another raphidophyte Fibrocapsa japonica. The number of prey types on which K. remigera could feed (2 species) was considerably smaller than that of K. japonica (14 species). With the increase in the mean prey concentration, the specific growth rates of K. remigera on H. akashiwo and C. subsalsa increased as well before becoming saturated. The maximum specific growth rates of K. remigera on H. akashiwo and C. subsalsa were 0.717 and 0.129 d^-1, respectively. In addition, the maximum ingestion rates of K. remigera on H. akashiwo and C. subsalsa were 0.333 and 0.661 ng C predator^-1 d^-1 (3.33 and 0.23 cells predator^-1 d^-1), respectively. The results of this study clearly indicate that K. remigera is an effective predator of 2 red tide-causing raphidophyte species, and additionally, the feeding activity of K. remigera differs greatly from that of K. japonica. The abundance of K. remigera was ≥0.1 cells mL^-1 at 24 stations located in the East, West, and South Sea of Korea. Thus, K. remigera has a nationwide distribution in Korea. The highest abundance of K. remigera in Korean waters was 24.9 cells mL^-1 in March 2017, when there was no red tide caused by H. akashiwo or Chattonella spp. In most regions where red tides caused by H. akashiwo or Chattonella spp. occurred in 2000-2017, K. remigera was detected. Thus, the abundance of K. remigera may increase during red tides caused by H. akashiwo and Chattonella spp.

Feeding by the harmful phototrophic dinoflagellate Takayama tasmanica (Family Kareniaceae)

The trophic mode of a phototrophic dinoflagellate is a critical factor in the dynamics of its harmful algal bloom. Recent discoveries of the mixotrophic capabilities of phototrophic dinoflagellates have changed the traditional view of bloom dynamics and prediction models. Here, mixotrophy in the harmful phototrophic dinoflagellate Takayama tasmanica was examined. Moreover, growth and ingestion rates of T. tasmanica on each of Alexandrium minutum CCMP1888 and Alexandrium tamarense CCMP1493, suitable prey, were determined as a function of prey concentration. This study reported for the first time that T. tasmanica is a mixotrophic species. Among the phytoplankton species offered as prey, T. tasmanica fed on all prey species whose equivalent spherical diameter (ESD) was greater than 30 μm, but also A. minutum whose ESD was 19 μm. In contrast, T. tasmanica did not feed on the phototrophic dinoflagellates Heterocapsa triquetra, Gymnodinium aureolum, Scrippsiella acuminata (previously S. trochoidea), Cochlodinium polykrikoides, Alexandrium affine, Alexandrium insuetum, and Alexandrium pacificum that its sister species Takayama helix is able to feed on. With increasing mean prey concentration, ingestion rates of T. tasmanica on A. minutum increased, but became saturated at the prey concentrations of >2130 cells mL^-1 (1070 ng C mL^-1). The maximum ingestion rate (MIR) of T. tasmanica on A. minutum was 0.5 ng C predator^-1 d^-1 (1.0 cells predator^-1 d^-1) which is only 64% of the body carbon of a T. tasmanica cell. Growth rates of T. tasmanica on A. minutum were not affected by prey concentrations. Thus, the low maximum ingestion rate is likely to be responsible for the small increases of its growth rate through mixotrophy. In addition, neither growth nor ingestion rates of T. tasmanica feeding on Alexandrium tamarense were affected by prey concentrations. The maximum ingestion rate of T. tasmanica on A. minutum was considerably lower than that of T. helix on the same prey species. Therefore, the mixotrophic ability of T. tasmanica is weaker than that of T. helix, and also T. tasmanica may have an ecological niche different from that of T. helix in marine ecosystems.

Interactions between the Newly Described Small- and Fast-Swimming Mixotrophic Dinoflagellate Yihiella yeosuensis and Common Heterotrophic Protists

The mixotroph Yihiella yeosuensis is a small- and fast-swimming dinoflagellate. To investigate its protistan predators, interactions between Y. yeosuensis and 11 heterotrophic protists were explored. No potential predators were able to feed on actively swimming Y. yeosuensis cells, which escaped via rapid jumps, whereas Aduncodinium glandula, Oxyrrhis marina, and Strombidinopsis sp. (approximately 150 μm in cell length) were able to feed on weakly swimming cells that could not jump. Furthermore, Gyrodinium dominans, Luciella masanensis, and Pfiesteria piscicida were able to feed on heat-killed Yihiella cells, whereas Gyrodinium moestrupii, Noctiluca scintillans, Oblea rotunda, Polykrikos kofoidii, and Strombidium sp. (20 μm) did not feed on them. Thus, the jumping behavior of Y. yeosuensis might be primarily responsible for the observed lack of predation. With increasing Yihiella concentration, the growth rate of O. marina decreased, whereas that of Strombidinopsis did not change. However, with increasing Yihiella concentration (up to 530 ng C/ml), the ingestion rate of Strombidinopsis on Yihiella increased linearly. The highest ingestion rate was 24.1 ng C per predator per d. The low daily carbon acquisition from Yihiella relative to the body carbon content of Strombidinopsis might be responsible for its negligible growth. Thus, Y. yeosuensis might have an advantage over its competitors due to its low mortality rate.

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^-1h^-1 and 0.012-0.033d^-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.

Newly discovered role of the heterotrophic nanoflagellate Katablepharis japonica, a predator of toxic or harmful dinoflagellates and raphidophytes

Heterotrophic nanoflagellates are ubiquitous and known to be major predators of bacteria. The feeding of free-living heterotrophic nanoflagellates on phytoplankton is poorly understood, although these two components usually co-exist. To investigate the feeding and ecological roles of major heterotrophic nanoflagellates Katablepharis spp., the feeding ability of Katablepharis japonica on bacteria and phytoplankton species and the type of the prey that K. japonica can feed on were explored. Furthermore, the growth and ingestion rates of K. japonica on the dinoflagellate Akashiwo sanguinea-a suitable algal prey item-heterotrophic bacteria, and the cyanobacteria Synechococcus sp., as a function of prey concentration were determined. Among the prey tested, K. japonica ingested heterotrophic bacteria, Synechococcus sp., the prasinophyte Pyramimonas sp., the cryptophytes Rhodomonas salina and Teleaulax sp., the raphidophytes Heterosigma akashiwo and Chattonella ovata, the dinoflagellates Heterocapsa rotundata, Amphidinium carterae, Prorocentrum donghaiense, Alexandrium minutum, Cochlodinium polykrikoides, Gymnodinium catenatum, A. sanguinea, Coolia malayensis, and the ciliate Mesodinium rubrum, however, it did not feed on the dinoflagellates Alexandrium catenella, Gambierdiscus caribaeus, Heterocapsa triquetra, Lingulodinium polyedra, Prorocentrum cordatum, P. micans, and Scrippsiella acuminata and the diatom Skeletonema costatum. Many K. japonica cells attacked and ingested a prey cell together after pecking and rupturing the surface of the prey cell and then uptaking the materials that emerged from the ruptured cell surface. Cells of A. sanguinea supported positive growth of K. japonica, but neither heterotrophic bacteria nor Synechococcus sp. supported growth. The maximum specific growth rate of K. japonica on A. sanguinea was 1.01 d^-1. In addition, the maximum ingestion rate of K. japonica for A. sanguinea was 0.13ngC predator^-1d^-1 (0.06 cells predator^-1d^-1). The maximum ingestion rate of K. japonica for heterotrophic bacteria was 0.019ngC predator^-1d^-1 (266 bacteria predator^-1d^-1), and the highest ingestion rate of K. japonica for Synechococcus sp. at the given prey concentrations of up to ca. 10⁷ cells ml^-1 was 0.01ngC predator^-1d^-1 (48 Synechococcus predator^-1d^-1). The maximum daily carbon acquisition from A. sanguinea, heterotrophic bacteria, and Synechococcus sp. were 307, 43, and 22%, respectively, of the body carbon of the predator. Thus, low ingestion rates of K. japonica on heterotrophic bacteria and Synechococcus sp. may be responsible for the lack of growth. The results of the present study clearly show that K. japonica is a predator of diverse phytoplankton, including toxic or harmful algae, and may also affect the dynamics of red tides caused by these prey species.

Differential interactions between the nematocyst-bearing mixotrophic dinoflagellate Paragymnodinium shiwhaense and common heterotrophic protists and copepods: Killer or prey

To investigate interactions between the nematocyst-bearing mixotrophic dinoflagellate Paragymnodinium shiwhaense and different heterotrophic protist and copepod species, feeding by common heterotrophic dinoflagellates (Oxyrrhis marina and Gyrodinium dominans), naked ciliates (Strobilidium sp. approximately 35μm in cell length and Strombidinopsis sp. approximately 100μm in cell length), and calanoid copepods Acartia spp. (A. hongi and A. omorii) on P. shiwhaense was explored. In addition, the feeding activities of P. shiwhaense on these heterotrophic protists were investigated. Furthermore, the growth and ingestion rates of O. marina, G. dominans, Strobilidium sp., Strombidinopsis sp., and Acartia spp. as a function of P. shiwhaense concentration were measured. O. marina, G. dominans, and Strombidinopsis sp. were able to feed on P. shiwhaense, but Strobilidium sp. was not. However, the growth rates of O. marina, G. dominans, Strobilidium sp., and Strombidinopsis sp. feeding on P. shiwhaense were very low or negative at almost all concentrations of P. shiwhaense. P. shiwhaense frequently fed on O. marina and Strobilidium sp., but did not feed on Strombidinopsis sp. and G. dominans. G. dominans cells swelled and became dead when incubated with filtrate from the experimental bottles (G. dominans+P. shiwhaense) that had been incubated for one day. The ingestion rates of O. marina, G. dominans, and Strobilidium sp. on P. shiwhaense were almost zero at all P. shiwhaense concentrations, while those of Strombidinopsis sp. increased with prey concentration. The maximum ingestion rate of Strombidinopsis sp. on P. shiwhaense was 5.3ngC predator^-1d^-1 (41 cells predator^-1d^-1), which was much lower than ingestion rates reported in the literature for other mixotrophic dinoflagellate prey species. With increasing prey concentrations, the ingestion rates of Acartia spp. on P. shiwhaense increased up to 930ngCml^-1 (7180cellsml^-1) at the highest prey concentration. The highest ingestion rate of Acartia spp. on P. shiwhaense was 4240ngC predator^-1d^-1 (32,610 cells predator^-1d^-1), which is comparable to ingestion rates from previous studies on other dinoflagellate prey species calculated at similar prey concentrations. Thus, P. shiwhaense might play diverse ecological roles in marine planktonic communities by having an advantage over competing phytoplankton in anti-predation against potential protistan grazers.

Mixotrophy in the newly described dinoflagellate Yihiella yeosuensis: A small, fast dinoflagellate predator that grows mixotrophically, but not autotrophically

To investigate tropical roles of the newly described Yihiella yeosuensis (ca. 8μm in cell size), one of the smallest phototrophic dinoflagellates in marine ecosystems, its trophic mode and the types of prey species that Y. yeosuensis can feed upon were explored. Growth and ingestion rates of Y. yeosuensis on its optimal prey, Pyramimonas sp. (Prasinophyceae), as a function of prey concentration were measured. Additionally, growth and ingestion rates of Y. yeosuensis on the other edible prey, Teleaulax sp. (Cryptophyceae), were also determined for a single prey concentration at which both these rates of Y. yeosuensis on Pyramimonas sp. were saturated. Among bacteria and diverse algal prey tested, Y. yeosuensis fed only on small Pyramimonas sp. and Teleaulax sp. (both cell sizes=5.6μm). With increasing mean prey concentrations, both specific growth and ingestion rates of Y. yeosuensis increased rapidly before saturating at a mean Pyramimonas concentration of 109ngCmL^-1 (2725cellsmL^-1). The maximum growth rate (mixotrophic growth) of Y. yeosuensis fed with Pyramimonas sp. at 20°C under a 14:10-h light-dark cycle of 20μEm^-2s^-1 was 1.32d^-1, whereas the growth rate of Y. yeosuensis without added prey was 0.026d^-1. The maximum ingestion rate of Y. yeosuensis fed with Pyramimonas sp. was 0.37ngCpredator^-1d^-1 (9.3cellspredator^-1d^-1). At a Teleaulax concentration of 1130ngCmL^-1 (66,240cellsmL^-1), growth and ingestion rates of Y. yeosuensis fed with Teleaulax sp. were 1.285d^-1 and 0.38ngCpredator^-1d^-1 (22.4cellspredator^-1d^-1), respectively. Thus, Y. yeosuensis rarely grows without mixotrophy, and mixotrophy supports high growth rates in Y. yeosuensis. Y. yeosuensis has the highest maximum mixotrophic growth rate with the exception of Ansanella graniferaamong engulfment feeding mixotrophic dinoflagellates. However, the high swimming speed of Y. yeosuensis (1572μms^-1), almost the highest among phototrophic dinoflagellates, may prevent autotrophic growth. This evidence suggests that Y. yeosuensis may be an effective mixotrophic dinoflagellate predator on Pyramimonas and Teleaulax, and occurs abundantly during or after blooms of these two prey species.

Mixotrophy in the phototrophic dinoflagellate Takayama helix (family Kareniaceae): Predator of diverse toxic and harmful dinoflagellates

Takayama spp. are phototrophic dinoflagellates belonging to the family Kareniaceae and have caused fish kills in several countries. Understanding their trophic mode and interactions with co-occurring phytoplankton species are critical steps in comprehending their ecological roles in marine ecosystems, bloom dynamics, and dinoflagellate evolution. To investigate the trophic mode and interactions of Takayama spp., the ability of Takayama helix to feed on diverse algal species was examined, and the mechanisms of prey ingestion were determined. Furthermore, growth and ingestion rates of T. helix feeding on the dinoflagellates Alexandrium lusitanicum and Alexandrium tamarense, which are two optimal prey items, were determined as a function of prey concentration. T. helix ingested large dinoflagellates ≥15μm in size, except for the dinoflagellates Karenia mikimotoi, Akashiwo sanguinea, and Prorocentrum micans (i.e., it fed on Alexandrium minutum, A. lusitanicum, A. tamarense, A. pacificum, A. insuetum, Cochlodinium polykrikoides, Coolia canariensis, Coolia malayensis, Gambierdiscus caribaeus, Gymnodinium aureolum, Gymnodinium catenatum, Gymnodinium instriatum, Heterocapsa triquetra, Lingulodinium polyedrum, and Scrippsiella trochoidea). All these edible prey items are dinoflagellates that have diverse eco-physiology such as toxic and non-toxic, single and chain forming, and planktonic and benthic forms. However, T. helix did not feed on small flagellates and dinoflagellates <13μm in size (i.e., the prymnesiophyte Isochrysis galbana; the cryptophytes Teleaulax sp., Storeatula major, and Rhodomonas salina; the raphidophyte Heterosigma akashiwo; the dinoflagellates Heterocapsa rotundata, Amphidinium carterae, Prorocentrum minimum; or the small diatom Skeletonema costatum). T. helix ingested Heterocapsa triquetra by direct engulfment, but sucked materials from the rest of the edible prey species through the intercingular region of the sulcus. With increasing mean prey concentration, the specific growth rates of T. helix on A. lusitanicum and A. tamarense increased continuously before saturating at prey concentrations of 336-620ngC mL^-1. The maximum specific growth rates (mixotrophic growth) of T. helix on A. lusitanicum and A. tamarense were 0.272 and 0.268d^-1, respectively, at 20°C under a 14:10 h light/dark cycle of 20μE m^-2 s^-1 illumination, while its growth rates (phototrophic growth) under the same light conditions without added prey were 0.152 and 0.094d^-1, respectively. The maximum ingestion rates of T. helix on A. lusitanicum and A. tamarense were 1.23 and 0.48ng C predator^-1d^-1, respectively. The results of the present study suggest that T. helix is a mixotrophic dinoflagellate that is able to feed on a diverse range of toxic species and, thus, its mixotrophic ability should be considered when studying red tide dynamics, food webs, and dinoflagellate evolution.

Mixotrophic ability of the phototrophic dinoflagellates Alexandrium andersonii, A. affine, and A. fraterculus

The dinoflagellate Alexandrium spp. have received much attention due to their harmful effects on diverse marine organisms, including commercially important species. For minimizing loss due to red tides or blooms of Alexandrium spp., it is very important to understand the eco-physiology of each Alexandrium species and to predict its population dynamics. Its trophic mode (i.e., exclusively autotrophic or mixotrophic) is one of the most critical parameters in establishing prediction models. However, among the 35 Alexandrium species so far described, only six Alexandrium species have been revealed to be mixotrophic. Thus, mixotrophic ability of the other Alexandrium species should be explored. In the present study, whether each of three Alexandrium species (A. andersonii, A. affine, and A. fraterculus) isolated from Korean waters has or lacks mixotrophic ability, was investigated. When diets of diverse algal prey, cyanobacteria, and bacteria sized micro-beads were provided, A. andersonii was able to feed on the prasinophyte Pyramimonas sp., the cryptophyte Teleaulax sp., and the dinoflagellate Heterocapsa rotundata, whereas neither A. affine nor A. fraterculus fed on any prey item. Moreover, mixotrophy elevated the growth rate of A. andersonii. The maximum mixotrophic growth rates of A. andersonii on Pyramimonas sp. under a 14:10h light/dark cycle of 20μEm^-2s^-1 was 0.432d^-1, while the autotrophic growth rate was 0.243d^-1. With increasing mean prey concentration, the ingestion rate of A. andersonii increased rapidly at prey concentrations <650ngCml^-1 (ca. 16,240 cellsml^-1), but became saturated at the higher prey concentrations. The maximum ingestion rate by A. andersonii of Pyramimonas sp. was 1.03ngC predator^-1d^-1 (25.6 cells predator^-1d^-1). This evidence suggests that the mixotrophic ability of A. andersonii should be taken into consideration in predicting the outbreak, persistence, and decline of its harmful algal blooms.

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 × 10(3)  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.

First report of the epiphytic benthic dinoflagellates Coolia canariensis and Coolia malayensis in the waters off Jeju Island, Korea: morphology and rDNA sequences

Coolia spp. are epiphytic and benthic dinoflagellates. Herein, we report for the first time, the occurrence of Coolia canariensis and Coolia malayensis in Korean waters. The morphology of the Korean strains of C. canariensis and C. malayensis isolated from the waters off Jeju Island, Korea was similar to that of the original Canary lslands strains and Malaysian strains, respectively. We found several pores and a line of small knobs on the pore plate, and perforations within the large pores of both C. canariensis and C. malayensis. The plates of the Korean strains of C. canariensis and C. malayensis were arranged in a Kofoidian series of Po, 3', 7'', 6c, 6s, 5''', and 2'''', and Po, 3', 7'', 7c, 6-7s, 5''', and 2'''', respectively. When properly aligned, the large subunit (LSU) rDNA sequence of the Korean strain of C. canariensis was identical to that of the Biscayan strains, but it was 2-3% different from the Canary lslands strain VGO0775 and the Australian strain. In addition, the sequences of small subunit (SSU) and/or LSU rDNA from the two Korean strains of C. malayensis were < 1% different from the Malaysian strains of C. malayensis and the Florida strain CCMP1345 and New Zealand strain CAWD39 ("Coolia monotis"). In phylogenetic trees based on LSU rDNA sequences, the Korean strains of C. malayensis belonged to a clade including the Malaysian strains and these two strains. Therefore, based on genealogical analyses, we suggest that the Korean strain of C. canariensis is closely related to two Atlantic strains and the Australian strain, whereas the Korean strains of C. malayensis are related to the Malaysian strains of C. malayensis and the Florida and New Zealand strains.

Selective algicidal action of peptides against harmful algal bloom species

Recently, harmful algal bloom (HAB), also termed “red tide”, has been recognized as a serious problem in marine environments according to climate changes worldwide. Many novel materials or methods to prevent HAB have not yet been employed except for clay dispersion, in which can the resulting sedimentation on the seafloor can also cause alteration in marine ecology or secondary environmental pollution. In the current study, we investigated that antimicrobial peptide have a potential in controlling HAB without cytotoxicity to harmless marine organisms. Here, antimicrobial peptides are proposed as new algicidal compounds in combating HAB cells. HPA3 and HPA3NT3 peptides which exert potent antimicrobial activity via pore forming action in plasma membrane showed that HPA3NT3 reduced the motility of algal cells, disrupted their plasma membrane, and induced the efflux of intracellular components. Against raphidoflagellate such as Heterosigma akashiwo, Chattonella sp., and C. marina, it displayed a rapid lysing action in cell membranes at 1∼4 µM within 2 min. Comparatively, its lysing effects occurred at 8 µM within 1 h in dinoflagellate such as Cochlodium polykrikoides, Prorocentrum micans, and P. minimum. Moreover, its lysing action induced the lysis of chloroplasts and loss of chlorophyll a. In the contrary, this peptide was not effective against Skeletonema costatum, harmless algal cell, even at 256 µM, moreover, it killed only H. akashiwo or C. marina in co-cultivation with S. costatum, indicating to its selective algicidal activity between harmful and harmless algal cells. The peptide was non-hemolytic against red blood cells of Sebastes schlegeli, the black rockfish, at 120 µM. HAB cells were quickly and selectively lysed following treatment of antimicrobial peptides without cytotoxicity to harmless marine organisms. Thus, the antibiotic peptides examined in our study appear to have much potential in effectively controlling HAB with minimal impact on marine ecology.