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 ≥15 μm 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 <13 μm 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 336–620 ng C mL⁻¹. The maximum specific growth rates (mixotrophic growth) of T. helix on A. lusitanicum and A. tamarense were 0.272 and 0.268 d⁻¹, respectively, at 20 °C under a 14:10 h light/dark cycle of 20 μE m⁻² s⁻¹ illumination, while its growth rates (phototrophic growth) under the same light conditions without added prey were 0.152 and 0.094 d⁻¹, respectively. The maximum ingestion rates of T. helix on A. lusitanicum and A. tamarense were 1.23 and 0.48 ng 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.     

Grazing impact of heterotrophic dinoflagellates and ciliates on common red-tide euglenophyte Eutreptiella gymnastica in Masan Bay, Korea

The euglenophyte Eutreptiella gymnastica is a common red tide causative species. However, there have been no studies on the grazing impact of heterotrophic protists on this species. To investigate the grazing impact of heterotrophic protists on E. gymnastica, we measured daily the abundances of E. gymnastica and co-occurring potential heterotrophic protistan grazers in Masan Bay, Korea, in August 2004 when an E. gymnastica red tide occurred. In addition, we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes, and Stoeckeria algicida and the naked ciliates Strobilidium sp. (30–40 μm in cell length) and Strombidinopsis sp. (70–100 μm in cell length) were able to feed on E. gymnastica. We also measured their growth and ingestion rates on E. gymnastica as a function of prey concentration. Finally, we calculated the grazing coefficients by combining field data on the abundance of the heterotrophic dinoflagellate and ciliate grazers and co-occurring E. gymnastica with laboratory data on ingestion rates obtained in this study. The maximum abundance of E. gymnastica in Masan Bay in August, 2004 was 7575 cells ml⁻¹, while those of Gyrodinium spp., P. kofoidii, P. bipes, the naked ciliates (≤50 μm in cell length), and naked ciliates (>50 μm in cell length) were 50, 9, 58, 32, and 3 cells ml⁻¹, respectively. The maximum growth rate of G. dominans on E. gymnastica (1.13 d⁻¹) was higher than that of O. marina (0.81 d⁻¹) or P. bipes (0.77 d⁻¹). However, E. gymnastica did not support positive growth of P. kofoidii, Strobilidium sp., and Strombidinopsis sp. (−0.04 ∼ −2.8 d⁻¹). The maximum ingestion rates of G. dominans, P. kofoidii, P. bipes, O. marina, and Strobilidium sp. on E. gymnastica (2.1–2.7 ng C predator⁻¹ d⁻¹) were similar, but they were much lower than that of Strombidinopsis sp. (156 ng C predator⁻¹ d⁻¹). The calculated grazing coefficients for P. bipes, small heterotrophic Gyrodinium spp. (25–35 μm in cell length), naked ciliates (≤50 μm in cell length), P. kofoidii, and naked ciliates (>50 μm in cell length) on E. gymnastica were up to 0.77, 0.61, 0.22, 0.07 and 0.03 d⁻¹, respectively (i.e., up to 54%, 46%, 20%, 7%, and 3% of E. gymnastica populations were removed by the population of each of these heterotrophic protistan grazers in 1 d, respectively). The results of the present study suggest that P. bipes, small heterotrophic Gyrodinium spp., and naked ciliates (≤50 μm in cell length) sometimes have considerable potential grazing impact on the populations of E. gymnastica.     

Interactions between the mixotrophic dinoflagellate Takayama helix and common heterotrophic protists

The phototrophic dinoflagellate Takayama helix that is known to be harmful to abalone larvae has recently been revealed to be mixotrophic. Although mixotrophy elevates the growth rate of T. helix by 79%–185%, its absolute growth rate is still as low as 0.3 d⁻¹. Thus, if the mortality rate of T. helix due to predation is high, this dinoflagellate may not easily prevail. To investigate potential effective protistan grazers on T. helix, feeding by diverse heterotrophic dinoflagellates such as engulfment-feeding Oxyrrhis marina, Gyrodinium dominans, Gyrodinium moestrupii, Polykrikos kofoidii, and Noctiluca scintillans, peduncle-feeding Aduncodinium glandula, Gyrodiniellum shiwhaense, Luciella masanensis, and Pfiesteria piscicida, pallium-feeding Oblea rotunda and Protoperidinium pellucidum, and the naked ciliates Pelagostrobilidium sp. (ca. 40 μm in cell length) and Strombidinopsis sp. (ca. 150 μm in cell length) on T. helix was explored. Among the tested heterotrophic protists, O. marina, G. dominans, G. moestrupii, A. glandula, L. masanensis, P. kofoidii, P. piscicida, and Strombidinopsis sp. were able to feed on T. helix. The growth rates of all these predators except Strombidinopsis sp. with T. helix prey were lower than those without the prey. The growth rate of Strombidinopsis sp. on T. helix was almost zero although the growth rate of Strombidinopsis sp. with T. helix prey was higher than those without the prey. Moreover, T. helix fed on O. marina and P. pellucidum and lysed the cells of P. kofoidii and G. shiwhaense. With increasing the concentrations of T. helix, the growth rates of O. marina and P. kofoidii decreased, but those of G. dominans and L. masanensis largely did not change. Therefore, reciprocal predation, lysis, no feeding, and the low ingestion rates of the common protists preying on T. helix may result in a low mortality rate due to predation, thereby compensating for this species’ low growth rate.     

Feeding by the newly described heterotrophic dinoflagellate Stoeckeria changwonensis: A comparison with other species in the family Pfiesteriaceae

The feeding ecology of the newly described heterotrophic dinoflagellate Stoeckeria changwonensis was explored. The feeding behavior of S. changwonensis, and the kinds of prey species that it feeds on were investigated with several different types of microscopes and high-resolution video-microscopy. Additionally, the growth and ingestion rates of S. changwonensis as a function of prey concentration for perch (Lateolabrax japonicus) blood cells, the raphidophyte Heterosigma akashiwo, the cryptophytes Rhodomonas salina and Teleaulax sp., and the phototrophic dinoflagellate Amphidinium carterae prey were measured. S. changwonensis feeds on prey through a peduncle, after anchoring the prey by using a tow filament. This type of feeding behavior is similar to that of Stoeckeria algicida, Pfiesteria piscicida, and Luciella masanensis in the family Pfiesteriaceae; however, S. changwonensis feeds on various kinds of prey species different from those of the other heterotrophic dinoflagellates. S. changwonensis ingested perch blood cells and diverse algal species, in particular, the large thecate dinoflagellate Lingulodinium polyedrum which are not eaten by the other peduncle feeders. H. akashiwo and the perch blood cells supported positive growth of S. changwonensis, but R. salina, Teleaulax sp., and A. carterae which support positive growth of P. piscicida and L. masanensis did not support positive growth of S. changwonensis. With increasing mean prey concentration the growth rates for S. changwonensis on H. akashiwo and the perch blood cells increased rapidly and then slowly or became saturated. The maximum growth rates of S. changwonensis on H. akashiwo and the perch blood cells were 0.376 and 0.354 d⁻¹, respectively. Further, the maximum ingestion rates of S. changwonensis on H. akashiwo and the perch blood cells were 0.35 ng C predator⁻¹ d⁻¹ (3.5 cells predator⁻¹ d⁻¹) and 0.27 ng C predator⁻¹ d⁻¹ (29 cells predator⁻¹ d⁻¹), respectively. These maximum growth and ingestion rates of S. changwonensis on H. akashiwo, the perch blood cells, R. salina, Teleaulax sp., and A. carterae differed considerably from those of S. algicida, P. piscicida, and L. masanensis on the same prey species. Thus, the feeding behavior of S. changwonensis may differ from that of other species in the family Pfiesteriaceae.     

Control of the harmful alga Cochlodinium polykrikoides by the naked ciliate Strombidinopsis jeokjo in mesocosm enclosures

Red tides dominated by the harmful dinoflagellate Cochlodinium polykrikoides have caused annual losses of USD $5–60 million to the Korean aquaculture industry annually since 1995 and a loss of USD $3 million during a 1999 net-pen fish mortality event in Canada. In order to evaluate the potential to control C. polykrikoides red tides dominated by using mass-cultured heterotrophic protistan grazers, we monitored the abundance of Strombidinopsis jeokjo (a naked ciliate) and C. polykrikoides after mass-cultured S. jeokjo was introduced into mesocosms (ca. 60 l) deployed in situ and containing natural red tide waters dominated by C. polykrikoides. Water temperature, salinity, and pH, as well as the abundance of co-occurring other protists and metazooplankton were measured concurrently. To compare the growth and ingestion rates of S. jeokjo feeding on cultured versus natural populations of C. polykrikoides, we also monitored the abundance of cultured C. polykrikoides and S. jeokjo in bottles during laboratory grazing experiments. S. jeokjo introduced into the mesocosms grew well, effectively reducing natural populations of C. polykrikoides from approximately 1000 cells ml⁻¹ to below 10 cells ml⁻¹ within 2 days. The growth and ingestion rates of cultured S. jeokjo on natural populations of C. polykrikoides in the mesocosms for the first 30 h (0.72 day⁻¹ and 51 ng C grazer⁻¹ day⁻¹) were 84% and 44%, respectively, of those measured in the laboratory during bottle incubations with similar initial prey concentrations. The calculated grazing impact of S. jeokjo on natural populations of C. polykrikoides suggests that large-scale cultures of this ciliate could be used for controlling red tides by C. polykrikoides in small areas.     

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. 8 μm 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 = 5.6 μm). With increasing mean prey concentrations, both specific growth and ingestion rates of Y. yeosuensis increased rapidly before saturating at a mean Pyramimonas concentration of 109 ng C mL⁻¹ (2725 cells mL⁻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 20 μE m⁻² s⁻¹ was 1.32 d⁻¹, whereas the growth rate of Y. yeosuensis without added prey was 0.026 d⁻¹. The maximum ingestion rate of Y. yeosuensis fed with Pyramimonas sp. was 0.37 ng C predator⁻¹ d⁻¹ (9.3 cells predator⁻¹ d⁻¹). At a Teleaulax concentration of 1130 ng C mL⁻¹ (66,240 cells mL⁻¹), growth and ingestion rates of Y. yeosuensis fed with Teleaulax sp. were 1.285 d⁻¹ and 0.38 ng C predator⁻¹ d⁻¹ (22.4 cells predator⁻¹ d⁻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 (1572 μm s⁻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.     

Killing potential protist predators as a survival strategy of the newly described dinoflagellate Alexandrium pohangense

Blooms caused by some species belonging to the dinoflagellate genus Alexandrium are known to cause large-scale mortality of fish. Thus, the dynamics of these species is important and of concern to scientists, officials, and people in the aquaculture industry. To understand the dynamics of such species, their growth and mortality due to predation need to be assessed. The newly described dinoflagellate Alexandrium pohangense is known to grow slowly, with a maximum autotrophic growth rate of 0.1 d⁻¹. Thus, it may not form bloom patches if its mortality due to predation is high. Therefore, to explore the mortality of A. pohangense due to predation, feeding on this species by the common heterotrophic dinoflagellates Gyrodinium dominans, Gyrodinium moestrupii, Luciella masanensis, Noctiluca scintillans, Oxyrrhis marina, Oblea rotunda, Polykrikos kofoidii, and Pfiesteria piscicida, as well as by the ciliate Tiarina fusus, was examined. None of these potential predators was able to feed on A. pohangense. In contrast, these potential predators were killed and their bodies were dissolved when incubated with A. pohangense cells or cell-free culture filtrates. The survival of G. moestrupii, O. marina, P. kofoidii, and T. fusus on incubation with 10 cells ml⁻¹ of A. pohangense was 20–60%, while that at the equivalent culture filtrates was 20–70%. With increasing A. pohangense cell-concentration (up to 1000 cells ml⁻¹ or equivalent culture filtrates), the survival rate of G. moestrupii, O. marina, P. kofoidii, and T. fusus rapidly decreased. The lethal concentration (LC₅₀) for G. moestrupii, O. marina, P. kofoidii, and T. fusus at the elapsed time of 24 h with A. pohangense cells (cultures of 11.4, 13.3, 1.6, and 3.3 cells ml⁻¹, respectively) was lower than that with A. pohangense filtrates (culture filtrates of 35.5, 30.6, 5.5, and 5.0 cells ml⁻¹, respectively). Furthermore, most of the ciliates and heterotrophic dinoflagellates in the water collected from the coast of Tongyoung, Korea, were killed when incubated with cultures of 1000 A. pohangense cells ml⁻¹ and equivalent culture filtrates. The relatively slow growing A. pohangense may form blooms by reducing mortality due to predation through killing potential protist predators.     

Environmental control of harmful dinoflagellates and diatoms in a fjordic system

Fjordic coastlines provide an ideal protected environment for both finfish and shellfish aquaculture operations. This study reports the results of a cruise to the Scottish Clyde Sea, and associated fjordic sea lochs, that coincided with blooms of the diarrhetic shellfish toxin producing dinoflagellate Dinophysis acuta and the diatom genus Chaetoceros, that can generate finfish mortalities. Unusually, D. acuta reached one order of magnitude higher cell abundance in the water column (2840 cells L⁻¹) than the more common Dinophysis acuminata (200 cells L⁻¹) and was linked with elevated shellfish toxicity (maximum 601 ± 237 μg OA eq/kg shellfish flesh) which caused shellfish harvesting closures in the region. Significant correlations between D. acuta abundance and that of Mesodinium rubrum were also observed across the cruise transect potentially supporting bloom formation of the mixotrophic D. acuta. Significant spatial variability in phytoplankton that was related to physical characteristics of the water column was observed, with a temperature-driven frontal region at the mouth of Loch Fyne being important in the development of the D. acuta, but not the Chaetoceros bloom. The front also provided important protection to the aquaculture located within the loch, with neither of the blooms encroaching within it. Analysis based on a particle-tracking model confirms the importance of the front to cell transport and shows significant inter-annual differences in advection within the region, that are important to the harmful algal bloom risk therein.     

Expansion of harmful brown tides caused by the pelagophyte,Aureoumbra lagunensis DeYoe et Stockwell,to the US east coast

Brown tides caused by the pelagophyte Aureoumbra lagunensis DeYoe et Stockwell have formed ecosystem disruptive algal blooms in shallow lagoons of Texas (TX), USA, for more than two decades but have never been reported elsewhere. During the summer of 2012, a dense brown tide occurred in the Mosquito Lagoon and northern Indian River Lagoon along the east coast of Florida (FL), USA. While chlorophyll a levels in this system have averaged 5 μg L⁻¹ during the past two decades, concentrations during this brown tide reached ∼200 μg L⁻¹. Concurrently, levels of nitrate were significantly lower than average and levels of dissolved organic nitrogen were significantly higher than average (p < 0.001 for both). Sequences of the 18S rRNA gene of the bloom community and of single cell isolates were identical to those of Aureoumbra lagunensis DeYoe et Stockwell from TX. The A. lagunensis brown tide in FL bloomed to densities exceeding 10⁶ cells mL⁻¹ (quantified with a species-specific immuno-label) from July through September, began to dissipate in October, but maintained densities exceeding 10⁵ cells mL⁻¹ in some regions through December of 2012. The decline of the bloom was associated with near-hypoxic conditions and more than 30 fish kills reported within the Mosquito Lagoon in September 2012, a number far exceeding all prior monthly reports in this system dating to 1996. Wild northern quahog populations (a.k.a. hard clam, Mercenaria mercenaria) suffered mass die offs during the brown tide and eastern oysters (Crassostrea virginica) that settled during 2012 were significantly smaller than prior years. Clearance rates of hard clams and eastern oyster were significantly reduced in the presence of Mosquito Lagoon bloom water and A. lagunensis monocultures isolated from the Mosquito Lagoon at densities of ∼10⁶ cells L⁻¹. The expansion of harmful brown tides caused by A. lagunensis to these estuaries represents a new threat to the US southeast coast.     

Molecular cloning reveals co-occurring species behind red tide blooms of the harmful dinoflagellate Cochlodinium polykrikoides

Red tides caused by the marine dinoflagellate Cochlodinium polykrikoides Margalef pose significant environmental problems worldwide. Recently, the existence of severe blooms attributable to a single Cochlodinium Schütt species has been questioned by many researchers. Herein we investigated the dinoflagellate composition of harmful algal blooms (HABs) attributed to C. polykrikoides in Korean coastal waters at nine different stations (St.). The component species of Cochlodinium blooms were examined by using microscopic and gene-cloning methods. In the nine study areas, C. polykrikoides was the predominant species of HABs in St. 2, 4, 7, and St. 9. Based on the morphological identification, the bloom was initially thought to be caused only by C. polykrikoides; however, we detected additional bloom-forming dinoflagellates (Polykrikos schwartzii Bütschli and Polykrikos kofoidii Chatton), and diatoms (Pseudo-nitzschia americana (Hasle) Fryxell) along with C. polykrikoides. The parasitic dinoflagellates Amoebophrya Koeppen and Euduboscquella Coats, Bachvaroff & Delwiche were found to be co-located with Cochlodinium in our study, and for the first time, Cochlodinium fulvescens Iwataki, Kawami & Matsuoka was detected in Korea (west coast). These results suggest co-existence of multiple dinoflagellates in bloom populations of Cochlodinium and describe the composition of other dinoflagellate blooms (e.g., Polykrikos spp.) in Korean coastal regions. This co-occurrence may be considered during efforts to monitor and control HABs.     

Factors regulating excystment of Alexandrium in Puget Sound,WA, USA

Factors regulating excystment of a toxic dinoflagellate in the genus Alexandrium were investigated in cysts from Puget Sound, Washington State, USA. Experiments were carried out in the laboratory using cysts collected from benthic seedbeds to determine if excystment is controlled by internal or environmental factors. The results suggest that the timing of germination is not tightly controlled by an endogenous clock, though there is a suggestion of a cyclical pattern. This was explored using cysts that had been stored under cold (4 °C), anoxic conditions in the dark and then incubated for 6 weeks at constant favorable environmental conditions. Excystment occurred during all months of the year, with variable excystment success ranging from 31–90%. When cysts were isolated directly from freshly collected sediments every month and incubated at the in situ bottom water temperature, a seasonal pattern in excystment was observed that was independent of temperature. This pattern may be consistent with secondary dormancy, an externally modulated pattern that prevents excystment during periods that are not favorable for sustained vegetative growth. However, observation over more annual cycles is required and the duration of the mandatory dormancy period of these cysts must be determined before the seasonality of germination can be fully characterized in Alexandrium from Puget Sound. Both temperature and light were found to be important environmental factors regulating excystment, with the highest rates of excystment observed for the warmest temperature treatment (20 °C) and in the light.     

Feeding by ciliates on two harmful algal bloom species, Prymnesium parvum and Prorocentrum minimum

We have focused on ciliates as potential grazers on toxic phytoplankton because they are major herbivores in aquatic food webs. Ciliates may exert top down control on toxic phytoplankton blooms, potentially suppressing or shortening the duration of harmful algal blooms (HABs). We measured the growth rates of several ciliate species on uni-algal and mixed diets of both HAB and non-HAB algae. The tintinnids Favella ehrenbergii, Eutintinnus pectinis and Metacylis angulata and the non-loricate ciliates Strombidinopsis sp. and Strombidium conicum were isolated from Long Island Sound (LIS), and fed HAB species including the prymnesiophyte Prymnesium parvum (strain 97-20-01) and the dinoflagellate Prorocentrum minimum (strains Exuv and JA 98-01). Ciliates were fed algal prey from cultures at various growth phases and at varying concentrations. We observed no harmful effects of P. minimum (Exuv) on any of the ciliates. However in a comparison of strains, P. minimum (Exuv) supported high growth rates, whereas P. minimum (JA 98-01) supported only nominal growth. P. parvum was acutely toxic to ciliates at high concentrations (2×10⁴–3×10⁴ cells ml⁻¹). At low concentrations (5×10³–1×10⁴ cells ml⁻¹), or in culture filtrate, ciliates survived for at least several hours. In mixed diet experiments, as long as a non-toxic alga was available, ciliates survived and at times grew well at concentrations of P. parvum (5×10²–3×10⁴ cells ml⁻¹) that would otherwise have killed them. The present study suggests that prior to the onset of toxicity and bloom formation ciliates may exert grazing pressure on these HAB species, potentially contributing to the suppression or decline of P. minimum and P. parvum blooms.     

The interactive roles of nutrient loading and zooplankton grazing in facilitating the expansion of harmful algal blooms caused by the pelagophyte,Aureoumbra lagunensis,to the Indian River Lagoon,FL, USA

Confined to Texas, USA, for more than 20 years, brown tides caused by Aureoumbra lagunensis emerged in the Indian River Lagoon and Mosquito Lagoon, Florida, USA, during 2012 and 2013, affording the opportunity to assess whether hypotheses developed regarding the occurrence of these blooms are ecosystem-specific. To examine the extent to which top-down (e.g. grazing) and bottom-up (e.g. nutrients) processes controlled the development of Aureoumbra blooms in Florida, nitrogen (N) uptake, nutrient amendment, and seawater-dilution, zooplankton grazing experiments were performed and the responses of Aureoumbra and competing phytoplankton were evaluated. During the study, Aureoumbra comprised up to 98% of total phytoplankton biomass, achieved cell densities exceeding 2 × 10⁶ cells mL⁻¹, and contained isotopically lighter N compared to non-bloom plankton populations, potentially reflecting the use of recycled N. Consistent with this hypothesis, N-isotope experiments revealed that urea and ammonium accounted for >90% of N uptake within bloom populations whereas nitrate was a primary N source for non-bloom populations. Low levels (10 μM) of experimental ammonium enrichment during blooms frequently enhanced the growth of Aureoumbra and resulted in the growth rates of Aureoumbra exceeding those of phycoerythrin-containing, but not phycocyanin-containing, cyanobacteria. A near absence of grazing pressure on Aureoumbra further enabled this species to out-grow other phytoplankton populations. Given this alga is generally known to resist zooplankton grazing under hypersaline conditions, these findings collectively suggest that moderate loading rates of reduced forms of nitrogenous nutrients (e.g ammonium, urea) into other subtropical, hypersaline lagoons could make them susceptible to future brown tides caused by Aureoumbra.     

Control of ichthyotoxic Cochlodinium polykrikoides using the mixotrophic dinoflagellate Alexandrium pohangense: A potential effective sustainable method

Red tides dominated by Cochlodinium polykrikoides often lead to great economic losses and some methods of controlling these red tides have been developed. However, due to possible adverse effects and the short persistence of their control actions, safer and more effective sustainable methods should be developed. The non-toxic dinoflagellate Alexandrium pohangense is known to grow well mixotrophically feeding on C. polykrikoides, and populations are also maintained by photosynthesis. Thus, compared with other methods, the use of mass-cultured A. pohangense is safer and the effects can be maintained in the long term. To develop an effective method, the concentrations of A. pohangense cells and culture filtrate resulting in the death of C. polykrikoides cells were determined by adding the cells or filtrates to cultured and natural populations of C. polykrikoides. Cultures containing 800 A. pohangense cells ml⁻¹ eliminated almost all cultured C. polykrikoides cells at a concentration of 1000 cells ml⁻¹ within 24 h. Furthermore, the addition of A. pohangense cultures at a concentration of 800 cells ml⁻¹ to C. polykrikoides populations from a red-tide patch resulted in the death of most C. polykrikoides cells (99.8%) within 24 h. This addition of A. pohangense cells also lowered the abundances of total phototrophic dinoflagellates excluding C. polykrikoides, but did not lower the abundance of total diatoms. Filtrate from 800 cells ml⁻¹ A. pohangense cultures reduced the population of cultured C. polykrikoides by 80% within 48 h. This suggests that A. pohangense cells eliminate C. polykrikoides by feeding and releasing extracellular compounds. Over time, A. pohangense concentrations gradually increased when incubated with C. polykrikoides. Thus, an increase in the concentration of A. pohangense by feeding may lead to A. pohangense cells eliminating more C. polykrikoides cells in larger volumes. Based on the results of this study, a 1 m³ stock culture of A. pohangense at 4000 cells ml⁻¹ is calculated to remove all C. polykrikoides cells in ca. 200 m³ within 6 days. Furthermore, maintenance of A. pohangense populations through photosynthesis prepared A. pohangense to eliminate C. polykrikoides cells in future red-tide patches. Moreover, incubation of A. pohangense at 2000 cells ml⁻¹ with juvenile olive flounder Paralichthys olivaceus for 3 days did not result in the death of fish. Therefore, the method developed in this study is a safe and effective way of controlling C. polykrikoides populations and can be easily applied to aqua-tanks on land.     

The value of harmful algal bloom predictions to the nearshore commercial shellfish fishery in the Gulf of Maine

In this study, we develop a framework for measuring the value of harmful algal bloom (HAB) predictions. The framework captures the effects of both private and public responses to HABs. Using data from the New England nearshore commercial shellfish fishery and impact estimates for a large-scale HAB event in 2005, we illustrate how the potential value of HAB forecasts may be estimated. The results of our study suggest that the long-term value of a HAB prediction and tracking system for the Gulf of Maine is sensitive to the frequency of HAB events, the accuracy of predictions, the choice of HAB impact measures, and the effectiveness of public and private responses.     

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.     

Mixotrophy in the phototrophic harmful alga Cochlodinium polykrikoides (Dinophycean): prey species, the effects of prey concentration, and grazing impact

We first reported here that the harmful alga Cochlodinium polykrikoides, which had been previously known as an autotrophic dinoflagellate, was a mixotrophic species. We investigated the kinds of prey species and the effects of the prey concentration on the growth and ingestion rates of C. polykrikoides when feeding on an unidentified cryptophyte species (Equivalent Spherical Diameter, ESD = 5.6 microm). We also calculated grazing coefficients by combining field data on abundances of C. polykrikoides and co-occurring cryptophytes with laboratory data on ingestion rates obtained in the present study. Cocholdinium polykrikoides fed on prey cells by engulfing the prey through the sulcus. Among the phytoplankton prey offered, C. polykrikoides ingested small phytoplankton species that had ESD's < or = 11 microm (e.g. the prymnesiophyte Isochrysis galbana, an unidentified cryptophyte, the cryptophyte Rhodomonas salina, the raphidophyte Heterosigma akashiwo, and the dinoflagellate Amphidinium carterae). It did not feed on larger phytoplankton species that had ESD's > or = 12 microm (e.g. the dinoflagellates Heterocapsa triquetra, Prorocentrum minimum, Scrippsiella sp., Alexandrium tamarense, Prorocentrum micans, Gymnodinium catenatum, Akashiwo sanguinea, and Lingulodinium polyedrum). Specific growth rates of C. polykrikoides on a cryptophyte increased with increasing mean prey concentration, with saturation at a mean prey concentration of approximately 270 ng C ml(-1) (i.e. 15,900 cells ml(-1)). The maximum specific growth rate (mixotrophic growth) of C. polykrikoides on a cryptophyte was 0.324 d(-1), under a 14:10 h light-dark cycle of 50 microE m(-2) s(-1), while its growth rate (phototrophic growth) under the same light conditions without added prey was 0.166 d(-1). Maximum ingestion and clearance rates of C. polykrikoides on a cryptophyte were 0.16 ng C grazer(-1)d(-1) (9.4 cells grazer(-1)d(-1)) and 0.33 microl grazer(-1)h(-1), respectively. Calculated grazing coefficients by C. polykrikoides on cryptophytes were 0.001-0.745 h(-1) (i.e. 0.1-53% of cryptophyte populations were removed by a C. polykrikoides population in 1 h). The results of the present study suggest that C. polykrikoides sometimes has a considerable grazing impact on populations of cryptophytes.     

Improved real-time PCR method for quantification of the abundance of all known ribotypes of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides by comparing 4 different preparation methods

Red tides by the ichthyotoxic dinoflagellate Cochlodinium polykrikoides have caused large scaled mortality of fish and great loss in aquaculture industry in many countries. Detecting and quantifying the abundance of this species are the most critical step in minimizing the loss. The conventional quantitative real-time PCR (qPCR) method has been used for quantifying the abundance of this species. However, when analyzing > 500 samples collected during huge C. polykrikoides red tides in South Sea of Korea in 2014, this conventional method and the previously developed specific primer and probe set for C. polykrikoides did not give reasonable abundances when compared with cell counting data. Thus improved qPCR methods and a new specific primer and probe set reflecting recent discovery of 2 new ribotypes have to be developed. A new species-specific primer and probe set for detecting all 3 ribotypes of C. polykrikoides was developed and provided in this study. Furthermore, because the standard curve between cell abundance and threshold cycle value (Ct) is critical, the efficiencies of 4 different preparation methods used to determine standard curves were comparatively evaluated. The standard curves were determined by using the following 4 different preparations: (1) extraction of DNA from a dense culture of C. polykrikoides followed by serial dilution of the extracted DNA (CDD method), (2) extraction of DNA from each of the serially diluted cultures with different concentrations of C. polykrikoides cultures (CCD method), (3) extraction of DNA from a dense field sample of C. polykrikoides collected from natural seawater and then dilution of the extracted DNA in serial (FDD method), and (4) extraction of DNA from each of the serially diluted field samples having different concentrations of C. polykrikoides (FCD method). These 4 methods yielded different results. The abundances of C. polykrikoides in the samples collected from the coastal waters of South Sea, Korea, in 2014–2015, obtained using the standard curves determined by the CCD and the FCD methods, were the most similar (0.93–1.03 times) and the second closest (1.16–1.33 times) to the actual cell abundances obtained by enumeration of cells. Thus, our results suggest that the CCD method is a more effective tool to quantify the abundance of C. polykrikoides than the conventional method, CDD, and the FDD and FCD methods.     

Dinoflagellate cysts from surface sediments of Saldanha Bay, South Africa: an indication of the potential risk of harmful algal blooms

The distribution and abundance of dinoflagellate cysts from recent coastal sediments in Saldanha Bay, was investigated, and compared to the cyst assemblages of the adjacent coastal upwelling system as reflected in the sediments off Lambert's Bay on the southern Namaqua shelf. Twenty-two cyst types were identified from three sample sites off Lambert's Bay with recorded abundances between 1726 and 1863 cysts ml⁻¹ wet sediment. At least 21 distinctive cyst types were identified from 32 sample sites within Saldanha Bay. Cyst abundance in Saldanha Bay was relatively low, averaging 116 cysts ml⁻¹ wet sediment. The region off Lambert's Bay is especially susceptible to the formation of harmful algal blooms attributed to high biomass dinoflagellate blooms. Owing to these blooms and the retentive circulation characteristics of this area, cyst formation and deposition is high. Blooms can be advected into Saldanha Bay, but their development and duration in the Bay is restricted by the system of exchange that operates between the Bay and the coastal upwelling system, in that there is a net export of surface waters from the Bay. Consequently, fewer cysts are formed and deposited within the Bay thereby reducing the likelihood of in situ bloom development initiated from the excystment of cysts.