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Phytoplankton blooms can cause imbalances in marine ecosystems leading to great economic losses in diverse industries. Better understanding and prediction of blooms one week in advance would help to prevent massive losses, especially in areas where aquaculture cages are concentrated. This study has aimed to develop a method to predict the magnitude and timing of phytoplankton blooms using nutrient and chlorophyll-a concentrations. We explored variations in nutrient and chlorophyll-a concentrations in incubated seawater collected from the coastal waters off Yeosu, South Korea, seven times between May and August 2019. Using the data from a total of seven bottle incubations, four different linear regressions for the magnitude of bloom peaks and four linear regressions for the timing were analyzed. To predict the bloom magnitude, the chlorophyll-a peak or peak-to-initial ratio was analyzed against the initial concentrations of NO3 or the ratio of the initial NO3 to chlorophyll-a. To predict the timing, the chlorophyll-a peak timing or the growth rate against the natural log of NO3 or the natural log of the ratio of the initial NO3 to chlorophyll-a was analyzed. These regressions were all significantly correlated. From these regressions, we developed the best-fit equations to predict the magnitude and timing of the bloom peak. The results from these equations led to the predicted bloom magnitude and timing values showing significant correlations with those of natural seawater in other regions. Therefore, this method can be applied to predict bloom magnitude and timing one week in advance and give aquaculture farmers time to harvest fish in cages early or move the cages to safer regions.

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]

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]

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]

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]

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]

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]