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Many dinoflagellates produce bioluminescence. To estimate the intensity of bioluminescence produced by populations of the heterotrophic dinoflagellates Noctiluca scintillans and Polykrikos kofoidii and autotrophic dinoflagellate Alexandrium mediterraneum in Korean waters, we measured cellular bioluminescence intensity as a function of water temperature and calculated population bioluminescence intensity with cell abundances and water temperature. The mean 200-second-integrated bioluminescence intensity per cell (BLcell) of N. scintillans satiated with the chlorophyte Dunaliella salina decreased continuously with increasing water temperature from 5 to 25°C. However, the BLcell of P. kofoidii satiated with the mixotrophic dinoflagellate Alexandrium minutum continuously increased from 5 to 15°C but decreased at temperatures exceeding this (to 30°C). Similarly, the BLcell of A. mediterraneum continuously increased from 10 to 20°C but decreased between 20 and 30°C. The difference between highest and lowest BLcell of N. scintillans, P. kofoidii, and A. mediterraneum at the tested water temperatures was 3.5, 11.8, and 21.0 times, respectively, indicating that water temperature clearly affected BLcell. The highest estimated population bioluminescence intensity (BLpopul) of N. scintillans in Korean waters in 1998-2022 was 4.22 × 1013 relative light unit per liter (RLU L-1), which was 1,850 and 554,000 times greater than that of P. kofoidii and A. mediterraneum, respectively. This indicates that N. scintillans populations produced much brighter bioluminescence in Korean waters than the populations of P. kofoidii or A. mediterraneum. [ABSTRACT FROM AUTHOR]

Some species in the dinoflagellate genus Alexandrium are bioluminescent. Of the 33 formally described Alexandrium species, the bioluminescence capability of only nine species have been tested, and eight have been reported to be bioluminescent. The present study investigated the bioluminescence capability of seven Alexandrium species that had not been tested. Alexandrium mediterraneum, A. pohangense, and A. tamutum were bioluminescent, but A. andersonii, A. hiranoi, A. insuetum, and A. pseudogonyaulax were not. We also measured the bioluminescent intensity of A. affine, A. fraterculus, A. mediterraneum, A. ostenfeldii, A. pacificum, A. pohangense, A. tamarense, and A. tamutum. The mean 200-second-integrated bioluminescence intensity per cell ranged from 0.02 to 32.2 χ 104 relative luminescence unit per cell (RLU cell-1), and the mean maximum bioluminescence intensity per cell per second (BLMax) ranged from 0.01 to 10.3 χ 104 RLU cell-1 s-1. BLMax was significantly correlated with the maximum growth rates of Alexandrium species, except for A. tamarense. A phylogenetic tree based on large subunit ribosomal DNA (LSU rDNA) showed that the bioluminescent species A. affine, A. catenella, A. fraterculus, A. mediterraneum, A. pacificum, and A. tamarense formed a large clade. However, the toxicity or mixotrophic capability of these species was split. Thus, their bioluminescence capability in this clade was more consistent than their toxicity or mixotrophic capability. Phylogenetic trees based on LSU rDNA and the luciferase gene of Alexandrium were consistent except for A. pohangense. The results of the present study can provide a basis for understanding the interspecific diversity in bioluminescence of Alexandrium. [ABSTRACT FROM AUTHOR]

This case study examined how an Indigenous Kalasha participant in northern Pakistan makes sense of his multi-faceted identity. The interpretative phenomenological analysis method was used to analyze data from an in-depth, single person case study. Our selected participant was a 36-year-old male postgraduate student from an Indigenous marginalized tribe, which is an ethnic and religious minority group in the northern Hindukush mountain region of Pakistan. Results are presented in four superordinate themes: (1) Identity: admixture of a number of identities, (2) Changing culture and its psychological impact, (3) Cultural protective factors against psychological problems and (4) Mental health perspective. The findings explained the construct of "identity" in an indigenous way that takes varying forms ranging from admixture of identities (ethnic and national identities) to how globalization, transition, and adaptation processes have been disrupting the practice of Kalasha cultural traditions. The findings also revealed the factors deeply enrooted in Kalasha culture and traditions that have helped people maintain their psychological resilience and buffered against psychological problems. This paper is presented as an appropriate method for advancing psychological understanding of "cultural identity" and exploring the connections between the two constructs of "mental health" and "identity." [ABSTRACT FROM AUTHOR]

Despite the widespread use of social networking sites (SNS) such as Facebook, an adequate understanding of their impact on the users' mental health is still lacking. The present study intends to expand on the current understanding of the linkage between social networking site use and mental health. Our study explored how Facebook use may be associated with obsessive-compulsive disorder (OCD), with the inclusion of obsessive-compulsive beliefs as a mediator. One hundred and fifty-six undergraduate students completed the questionnaire given. Overall, Facebook use contributed to OCD severity through obsessive-compulsive beliefs as the mediator. The present study also yielded other relevant results for cyberpsychology that may help pave the way for future studies. [ABSTRACT FROM AUTHOR]

Noctiluca scintillans is a heterotrophic dinoflagellate that causes red-colored oceans during the day (red tides) and glowing oceans at night (bioluminescence). This species feeds on diverse prey, including phytoplankton, heterotrophic protists, and eggs of metazoans. Thus, many scientists have conducted studies on the ecophysiology of this species. It is easy to cultivate N. scintillans at a scale of <1 L, but it is difficult to cultivate them at a scale of >100 L because N. scintillans cells usually stay near the surface, while prey cells stay below the surface in large water tanks. To obtain mass-cultured N. scintillans cells, we developed an automatic system for cultivating N. scintillans on a scale of 100 L. The system consisted of four tanks containing fresh nutrients, the chlorophyte Dunaliella salina as prey, N. scintillans for growth, and N. scintillans for storage, respectively. The light intensities supporting the high growth rates of D. salina and N. scintillans were 300 and 20 µmol photons m-2 s-1, respectively. Twenty liters of D. salina culture from the prey culture tank were transferred to the predator culture tank, and subsequently 20 L of nutrients from the nutrient tank were transferred to the prey culture tank every 2 d. When the volume of N. scintillans in the predator culture tank reached 90 L 6 d later, 70 L of the culture were transferred to the predator storage tank. To prevent N. scintillans cells from being separated from D. salina cells in the predator culture tank, the culture was mixed using an air pump, a sparger, and a stirrer. The highest abundance of N. scintillans in the predator culture tank was 45 cells mL-1, which was more than twice the highest abundance when this dinoflagellate was cultivated manually. This automatic system supplies 100 L of N. scintillans pure culture with a high density every 10 d for diverse experiments on N. scintillans. [ABSTRACT FROM AUTHOR]

Hypoxia can indeed impact the survival of protists, which play a crucial role in marine ecosystems. To better understand the protistan community structure and species that can thrive in hypoxic waters, we collected samples from both the surface and bottom waters during the hypoxic period in Jinhae and Masan Bays and the non-hypoxic period in Jinhae Bay. Subsequently, we utilized metabarcoding techniques to identify the protistan species. During hypoxia, with dissolved oxygen concentrations of 0.8 mg -1 in Jinhae Bay and 1.8 mg -1 in Masan Bay within the bottom waters, the phylum Dinoflagellata exhibited the highest amplicon sequence variants richness among the identified protist phyla. Following the Dinoflagellata, Ochrophyta and Ciliophora also displayed notable presence. In hypoxic waters of Jinhae and Masan Bays, we identified a total of 36 dinoflagellate species that exhibited various trophic modes. These included one autotrophic species, 14 mixotrophic species, 9 phototrophic species with undetermined trophic modes (either autotrophic or mixotrophic), 2 kleptoplastidic species, and 10 heterotrophic species. Furthermore, the hypoxic bottom water exhibited a greater number of heterotrophic dinoflagellate species compared to the non-hypoxic surface water within the same water column or the non-hypoxic bottom water. Therefore, feeding by mixotrophic and heterotrophic dinoflagellates may be partially responsible for their dominance in terms of the number of species surviving in hypoxic waters. This study not only introduces the initial documentation of 26 dinoflagellate species surviving in hypoxic conditions but also establishes a foundation for a more comprehensive understanding of the ecophysiology of dinoflagellates in hypoxic marine environments. [ABSTRACT FROM AUTHOR]

Mixotrophic dinoflagellates act as primary producers, prey, and predators in marine planktonic food webs, whereas exclusively autotrophic dinoflagellates are primary producers and prey. Species of the dinoflagellate genus Scrippsiella are commonly found in marine ecosystems and sometimes cause harmful red tides. Among the 28 formally described Scrippsiella species, S. acuminata has been found to be mixotrophic and two unidentified species have been found to be mixotrophic. To determine whether the other species in this genus are similarly mixotrophic, the mixotrophic ability of S. donghaiensis SDGJ1703, S. lachrymosa SLBS1703, S. masanensis SSMS0908, S. plana SSSH1009A, and S. ramonii VGO1053 was explored using 15 potential prey items, including 2-µm fluorescently labeled microspheres (FLM) and heterotrophic bacteria (FLB), the cyanobacterium Synechococcus sp., and various microalgal prey species. The ability of S. acuminata to feed on FLM and FLB was also investigated. We found that S. donghaiensis, S. lachrymosa, S. masanensis, S. plana, and S. ramonii did not feed on any potential prey tested in this study, indicating a lack of mixotrophy. However, S. acuminata fed on both FLM and FLB, confirming its mixotrophic ability. These results lowered the proportion of mixotrophic species relative to the total number of tested Scrippsiella species for mixotrophy from 100% to 29-38%. Owing to its mixotrophic ability, S. acuminata occupies an ecological niche that is distinct from that of S. donghaiensis, S. lachrymosa, S. masanensis, S. plana, and S. ramonii. [ABSTRACT FROM AUTHOR]

The mortality rate of red-tide dinoflagellates owing to predation is a major parameter that affects their population dynamics. The dinoflagellates Ansanella granifera and Ansanella sp. occasionally cause red tides. To understand the interactions between common heterotrophic protists and A. granifera, we explored the feeding occurrence of nine heterotrophic protists on A. granifera and the growth and ingestion rates of the heterotrophic dinoflagellate Gyrodinium dominans on A. granifera as a function of prey concentration and those of Oxyrrhis marina at a single high prey concentration. The heterotrophic dinoflagellates Aduncodinium glandula, G. dominans, Gyrodinium moestrupii, Luciella masanensis, Oblea rotunda, O. marina, Polykrikos kofoidii, and Pfiesteria piscicida and the naked ciliate Strombidium sp. were able to feed on A. granifera. With increasing mean prey concentrations, the growth and ingestion rates of G. dominans feeding on A. granifera rapidly increased and became saturated or slowly increased. The maximum growth and ingestion rates of G. dominans on A. granifera were 0.305 d-1 and 0.42 ng C predator-1 d-1 (3.8 cells predator-1 d-1), respectively. Furthermore, the growth and ingestion rates of O. marina on A. granifera at 1,700 ng C mL-1 (15,454 cells mL-1) were 0.037 d-1 and 0.19 ng C predator-1 d-1 (1.7 cells predator-1 d-1), respectively. The growth and ingestion rates of G. dominans and O. marina feeding on A. granifera were almost the lowest among those on the dinoflagellate prey species. Therefore, G. dominans and O. marina may prefer A. granifera less than other dinoflagellate prey species. The low mortality rate of A. granifera may positively affect its bloom formation. [ABSTRACT FROM AUTHOR]

Exploring mixotrophy of dinoflagellate species is critical to understanding red-tide dynamics and dinoflagellate evolution. Some species in the dinoflagellate genus Karenia have caused harmful algal blooms. Among 10 Karenia species, the mixotrophic ability of only two species, Karenia mikimotoi and Karenia brevis, has been investigated. These species have been revealed to be mixotrophic; however, the mixotrophy of the other species should be explored. Moreover, although K. mikimotoi was previously known to be mixotrophic, only a few potential prey species have been tested. We explored the mixotrophic ability of Karenia bicuneiformis, Karenia papilionacea, and Karenia selliformis and the prey spectrum of K. mikimotoi by incubating them with 16 potential prey species, including a cyanobacterium, diatom, prymnesiophyte, prasinophyte, raphidophyte, cryptophytes, and dinoflagellates. Cells of K. bicuneiformis, K. papilionacea, and K. selliformis did not feed on any tested potential prey species, indicating a lack of mixotrophy. The present study newly discovered that K. mikimotoi was able to feed on the common cryptophyte Teleaulax amphioxeia. The phylogenetic tree based on the large subunit ribosomal DNA showed that the mixotrophic species K. mikimotoi and K. brevis belonged to the same clade, but K. bicuneiformis, K. papilionacea, and K. selliformis were divided into different clades. Therefore, the presence or lack of a mixotrophic ability in this genus may be partially related to genetic characterizations. The results of this study suggest that Karenia species are not all mixotrophic, varying from the results of previous studies. [ABSTRACT FROM AUTHOR]

Water temperature affects plankton survival and growth. The dinoflagellate Shimiella gracilenta survives using the plastids of ingested prey, indicating kleptoplastidy. However, studies on the effects of water temperature on kleptoplastidic dinoflagellates are lacking. We explored the growth and ingestion rates of S. gracilenta as a function of water temperature. Furthermore, using data on its spatiotemporal distribution in Korean coastal waters during 2015-2018, we predicted its distribution under elevated temperature conditions of +2, +4, and +6°C. Growth rates of S. gracilenta with and without Teleaulax amphioxeia prey as well as ingestion rates were significantly affected by water temperature. Growth rates of S. gracilenta with and without prey were positive or zero at 5-25°C but were negative at =30°C. The maximum growth rate of S. gracilenta with T. amphioxeia was 0.85 d-1, achieved at 25°C, and 0.21 d-1 at 20°C without prey. The ingestion rate of S. gracilenta on T. amphioxeia at 25°C (0.05 ng C predator-1 d-1) was greater than that at 20°C (0.04 ng C predator-1 d-1). Thus, feeding may shift the optimal temperature for the maximum growth rate of S. gracilenta from 20 to 25°C. In spring and winter, the distributions of S. gracilenta under elevated temperature conditions were predicted not to differ from those during 2015-2018. However, S. gracilenta was predicted not to survive at some additional stations under elevated temperature conditions of +2, +4, and +6°C in summer or under elevated temperature conditions of +6°C in autumn. Therefore, global warming may affect the distribution of S. gracilenta. [ABSTRACT FROM AUTHOR]

To explore the ecophysiological characteristics of the kleptoplastidic dinoflagellate Shimiella gracilenta, we determined its spatiotemporal distribution in Korean coastal waters and growth and ingestion rates as a function of prey concentration. The abundance of S. gracilenta at 28 stations from 2015 to 2018 was measured using quantitative realtime polymerase chain reaction. Cells of S. gracilenta were detected at least once at all the stations and in each season, when temperature and salinity were 1.7--26.4°C and 9.9--35.6, respectively. Moreover, among the 28 potential prey species tested, S. gracilenta SGJH1904 fed on diverse prey taxa. However, the highest abundance of S. gracilenta was only 3 cells mL-1 during the study period. The threshold Teleaulax amphioxeia concentration for S. gracilenta growth was 5,618 cells mL-1, which was much higher than the highest abundance of T. amphioxeia (667 cells mL-1). Thus, T. amphioxeia was not likely to support the growth of S. gracilenta in the field during the study period. However, the maximum specific growth and ingestion rates of S. gracilenta on T. amphioxeia, the optimal prey species, were 1.36 d-1 and 0.04 ng C predator- 1 d-1, respectively. Thus, if the abundance of T. amphioxeia was much higher than 5,618 cells mL-1, the abundance of S. gracilenta could be much higher than the highest abundance observed in this study. Eurythermal and euryhaline characteristics of S. gracilenta and its ability to feed on diverse prey species and conduct kleptoplastidy are likely to be responsible for its common spatiotemporal distribution. [ABSTRACT FROM AUTHOR]

The mixotrophic dinoflagellate Tripos furca causes red tides in the waters of many countries. To understand its population dynamics, mortality due to predation as well as growth rate should be assessed. Prior to the present study, the heterotrophic dinoflagellates Noctiluca scintillans, Polykrikos kofoidii, Protoperidinium steinii, and mixotrophic dinoflagellate Fragilidium subglobosum were known to ingest T. furca. However, if other common heterotrophic protists are able to feed on T. furca has not been tested. We explored interactions between T. furca and nine heterotrophic dinoflagellates and one naked ciliate. Furthermore, we investigated the abundance of T. furca and common heterotrophic protists in coastal-offshore waters off Yeosu, southern Korea, on Jul 31, 2020, during its red tide. Among the tested heterotrophic protists, the heterotrophic dinoflagellates Aduncodinium glandula, Luciella masanensis, and Pfiesteria piscicida were able to feed on T. furca. However, the heterotrophic dinoflagellates Gyrodiniellum shiwhaense, Gyrodinium dominans, Gyrodinium jinhaense, Gyrodinium moestrupii, Oblea rotunda, Oxyrrhis marina, and the naked ciliate Rimostrombidium sp. were unable to feed on it. However, T. furca did not support the growth of A. glandula, L. masanensis, or P. piscicida. Red tides dominated by T. furca prevailed in the South Sea of Korea from Jun 30 to Sep 5, 2020. The maximum abundance of heterotrophic dinoflagellates in the waters off Yeosu on Jul 31, 2020, was as low as 5.0 cells mL-1, and A. glandula, L. masanensis, and P. piscicida were not detected. Furthermore, the abundances of the known predators F. subglobosum, N. scintillans, P. kofoidii, and Protoperidinium spp. were very low or negligible. Therefore, no or low abundance of effective predators might be partially responsible for the long duration of the T. furca red tides in the South Sea of Korea in 2020. [ABSTRACT FROM AUTHOR]

Heterotrophic dinoflagellates Gyrodinium spp. are one of the major grazers of phytoplankton in many coastal waters. Gyrodinium dominans, G. jinhaense, and G. moestrupii have similar morphologies but different edible prey species. To explore the variations in the ecological niches of these three species, we investigated their spatial-temporal distributions in Korean waters. Because of the high similarity in morphology among these three Gyrodinium species, we used real-time polymerase chain reactions to quantify their abundance in water samples that were seasonally collected from 28 stations along the Korean Peninsula from April 2015 to October 2018. Cells of G. dominans were found at all sampling stations, G. jinhaense at 26 stations, and G. moestrupii at 22 stations, indicating that all three species were widely distributed in Korea. Furthermore, all three species displayed strong seasonal distributions. The largest numbers of the stations where G. dominans and G. jinhaense cells were present were found during the summer (26 and 23 stations, respectively), but that for G. moestrupii was found in the autumn (15 stations). The abundance of G. dominans was positively correlated with that of G. jinhaense, but not with that of G. moestrupii. The highest abundances of G. dominans (202.5 cells mL-1) and G. jinhaense (20.2 cells mL-1) were much greater than that of G. moestrupii (1.2 cells mL-1). The highest abundances of G. dominans and G. jinhaense were found in July, whereas that of G. moestrupii was found in March. The abundances of G. dominans and G. jinhaense, but not G. moestrupii, were positively correlated with water temperature. Therefore, the spatial-temporal distributions of G. dominans and G. jinhaense were closer than those of G. moestrupii and G. dominans or G. jinhaense. This differs from results based on the relative differences in ribosomal DNA sequences and the types of edible prey reported in the literature. Thus, the variations in spatial-temporal distributions and prey species of these three Gyrodinium species suggest that they may have different ecological niches in Korean coastal waters. [ABSTRACT FROM AUTHOR]

The article offers information on the implications for explaining the global dominance of particular microalgae, their key eco-evolutionary strategy, and prediction of harmful red tide outbreaks. It mentions that feeding diverse prey as an excellent strategy of mixotrophic dinoflagellates for global dominance.

Omega-3 fatty acids, including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are polyunsaturated fatty acids beneficial to human health. A limited number of microalgae have been used for commercial omega-3 production, which necessitates the identification of new microalgae with high omega-3 contents. We explored the fatty acid composition and EPA and DHA contents of the mixotrophic dinoflagellate Gymnodinium smaydae fed with the optimal algal prey species Heterocapsa rotundata. Cells of G. smaydae were found to be rich in omega-3 fatty acids. In particular, the DHA content of G. smaydae was 21 mg g-1 dry weight, accounting for 43% of the total fatty acid content. The percentage of DHA in the total fatty acid content of G. smaydae was the highest among the reported microalgae except for Crypthecodinium cohnii. Moreover, to determine if the prey supply interval affected the growth rate of G. smaydae and its fatty acid content, three different prey supply intervals (daily, once every 2 d, and once for 4 d) were tested. Daily prey supply yielded the highest total fatty acid and DHA contents in G. smaydae. Furthermore, we successfully produced high-density G. smaydae cultures semi-continuously for 43 d with daily prey supply. During the semi-continuous cultivation period, the highest density of G. smaydae was 57,000 cells mL-1, with an average growth rate of 0.7 d-1. Taken together, the percentage of EPA and DHA in the total fatty acid content was maintained in the range of 54.2-56.9%. The results of this study support G. smaydae as a promising microalgal candidate for commercial DHA production and demonstrate that daily supply of prey can efficiently produce high-density G. smaydae cultures for more than a month. [ABSTRACT FROM AUTHOR]

Water temperature is known to affect the growth and feeding of marine dinoflagellates. Each dinoflagellate species grows well at a certain optimal temperature but dies at very cold and hot temperatures. Thus, changes in water temperatures driven by global warming and extremely high or low temperatures can affect the distribution of dinoflagellates. Yihiella yeosuensis is a mixotrophic dinoflagellate that can feed on only the cryptophyte Teleaulax amphioxeia and the chlorophyte Pyramimonas sp. Furthermore, it grows fast mixotrophically but rarely grows photosynthetically. We explored the direct and indirect effects of water temperature on the growth and ingestion rates of Y. yeosuensis feeding on T. amphioxeia and the growth rates of T. amphioxeia and Pyramimonas sp. under 7 different water temperatures (5- 35°C). Both the autotrophic and mixotrophic growth rates of Y. yeosuensis on T. amphioxeia were significantly affected by temperature. Under the mixotrophic and autotrophic conditions, Y. yeosuensis survived at 10-25°C, but died at 5°C and ≥30°C. The maximum mixotrophic growth rate of Y. yeosuensis on T. amphioxeia (1.16 d-1) was achieved at 25°C, whereas the maximum autotrophic growth rate (0.16 d-1) was achieved at 15°C. The maximum ingestion rate of Y. yeosuensis on T. amphioxeia (0.24 ng C predator-1 d-1) was achieved at 25°C. The cells of T. amphioxeia survived at 10-25°C, but died at 5 and ≥30°C. The cells of Pyramimonas sp. survived at 5-25°C, but died at 30°C. The maximum growth rate of T. amphioxeia (0.72 d-1) and Pyramimonas sp. (0.75 d-1) was achieved at 25°C. The abundance of Y. yeosuensis is expected to be high at 25°C, at which its two prey species have their highest growth rates, whereas Y. yeosuensis is expected to be rare or absent at 5°C or ≥30°C at which its two prey species do not survive or grow. Therefore, temperature can directly or indirectly affect the population dynamics and distribution of Y. yeosuensis. [ABSTRACT FROM AUTHOR]

Species belonging to the dinoflagellate genus Prorocentrum are known to cause red tides or harmful algal blooms. To understand the dynamics of a Prorocentrum sp., its growth and mortality due to predation need to be assessed. However, there are only a few Prorocentrum spp. for which heterotrophic protist predators have been reported. We explored feeding by the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Oblea rotunda, and Polykrikos kofoidii and the naked ciliate Strombidinopsis sp. (approx. 90 µm cell length) on the planktonic species Prorocentrum triestinum, P. cordatum, P. donghaiense, P. rhathymum, and P. micans as well as the benthic species P. lima and P. hoffmannianum. All heterotrophic protists tested were able to feed on the planktonic prey species. However, O. marina and O. rotunda did not feed on P. lima and P. hoffmannianum, while G. dominans, P. kofoidii, and Strombidinopsis sp. did. The growth and ingestion rates of G. dominans and P. kofoidii on one of the seven Prorocentrum spp. were significantly different from those on other prey species. G. dominans showed the top three highest growth rates when it fed on P. triestinum, P. cordatum, and P. donghaiense, however, P. kofoidii had negative growth rates when fed on these three prey species. In contrast, P. kofoidii had a positive growth rate only when fed on P. hoffmannianum. This differential feeding on Prorocentrum spp. between G. dominans and P. kofoidii may provide different ecological niches and reduce competition between these two common heterotrophic protist predators. [ABSTRACT FROM AUTHOR]

The dinoflagellate genus Alexandrium is known to often form harmful algal blooms causing human illness and largescale mortality of marine organisms. Therefore, the population dynamics of Alexandrium species are of primary concern to scientists and aquaculture farmers. The growth rate of the Alexandrium species is the most important parameter in prediction models and nutrient conditions are critical parameters affecting the growth of phototrophic species. In Korean coastal waters, Alexandrium affine and Alexandrium fraterculus, of similar sizes, often form red-tide patches together. Thus, to understand bloom dynamics of A. affine and A. fraterculus, growth rates and nitrate uptake of each species as a function of nitrate (NO3) concentration at 100 µmol photons m-2 s-1 under 14-h light : 10-h dark and continuous light conditions were determined using a nutrient repletion method. With increasing NO3 concentration, growth rates and NO3 uptake of A. affine or A. fraterculus increased, but became saturated. Under light : dark conditions, the maximum growth rates of A. affine and A. fraterculus were 0.45 and 0.42 d-1, respectively. However, under continuous light conditions, the maximum growth rate of A. affine slightly increased to 0.46 d-1, but that of A. fraterculus largely decreased. Furthermore, the maximum nitrate uptake of A. affine and A. fraterculus under light : dark conditions were 12.9 and 30.1 pM cell-1 d-1, respectively. The maximum nitrate uptake of A. affine under continuous light conditions was 16.4 pM cell-1 d-1. Thus, A. affine and A. fraterculus have similar maximum growth rates at the given NO3 concentration ranges, but they have different maximum nitrate uptake rates. A. affine may have a higher conversion rate of NO3 to body nitrogen than A. fraterculus. Moreover, a longer exposure time to the light may confer an advantage to A. affine over A. fraterculus. [ABSTRACT FROM AUTHOR]