KARENIA: The biology and ecology of a toxic genus

Five Karenia species along the Chinese coast: With the description of a new species, Karenia hui sp. nov. (Kareniaceae, Dinophyta)

Dinoflagellates within the genus Karenia are well known for their potential to cause harmful algal blooms and induce detrimental ecological consequences. In this study, five Karenia species, Karenia longicanalis, Karenia papilionacea, Karenia mikimotoi, Karenia selliformis, and a new species, Karenia hui sp. nov., were isolated from Chinese coastal waters. The new species exhibits the typical characteristics of the genus Karenia, including a linear apical groove and butanoyl-oxyfucoxanthin as the major accessory pigment. It is distinguished from the other Karenia species by a wide-open sulcal intrusion onto the epicone, a conical epicone with an apical crest formed by the rim of the apical groove, and a hunchbacked hypocone. It is most closely related to Karenia cristata, with a genetic divergence of 3.16 % (22 bp out of 883 bp of LSU rDNA). Acute toxicity tests indicated that the five Karenia species from China are all toxic to marine medaka Oryzias melastigma. Karenia selliformis and K. hui were very toxic to O. melastigma, resulting in 100 % mortality within 4 h and 24 h, respectively. Further analysis by high-performance liquid chromatography revealed that four species, K. selliformis, K. longicanalis, K. papilionacea and K. mikimotoi were capable of producing Gymnodimine-A (GYM-A). The highest GYM-A content was in K. selliformis (strain HK-43), in which the value was 889 fg/cell. No GYM-A was detected in the new species K. hui, however and its toxin remains unknown. Below we provide a comprehensive report of the morphology, phylogeny, pigment composition, and toxicity profiles of Karenia species along the Chinese coast. These findings contribute new insights for monitoring of Karenia species, with important toxicological and ecological implications.

Morpho-molecular and environmental evidence of the ocurrence of Karenia longicanalis (Dinophyceae: Kareniaceae) as a bloom former in the Eastern Pacific Ocean

Karenia longicanalis, an athecate dinoflagellate, was first described during a bloom in Victoria Harbour (Hong Kong, China). This study confirms the presence of K. longicanalis as a bloom former in the eastern Pacific Ocean. Specimens were collected in March 2019 at three sampling stations in Acapulco Bay, Mexico. Water temperature, salinity, dissolved oxygen, and pH were measured in situ at the time of sample collection. Cell morphology was examined by optical and scanning electron microscopy. A molecular analysis based on the amplification of the large subunit (LSU) rDNA region revealed that the LSU sequences formed a monophyletic group with other GenBank sequences belonging to K. longicanalis. The resulting phylogeny demonstrates that Karenia is closely related to Asterodinium, Gertia, and Shimiella. The morphology of the specimens was consistent with previous descriptions.

Precautions for seafood consumers: An updated review of toxicity, bioaccumulation, and rapid detection methods of marine biotoxins

Seafood products are globally consumed, and there is an increasing demand for the quality and safety of these products among consumers. Some seafoods are easily contaminated by marine biotoxins in natural environments or cultured farming processes. When humans ingest different toxins accumulated in seafood, they may exhibit different poisoning symptoms. According to the investigations, marine toxins produced by harmful algal blooms and various other marine organisms mainly accumulate in the body organs such as liver and digestive tract of seafood animals. Several regions around the world have reported incidents of seafood poisoning by biotoxins, posing a threat to human health. Thus, most countries have legislated to specify the permissible levels of these biotoxins in seafood. Therefore, it is necessary for seafood producers and suppliers to conduct necessary testing of toxins in seafood before and after harvesting to prohibit excessive toxins containing seafood from entering the market, which therefore can reduce the occurrence of seafood poisoning incidents. In recent years, some technologies which can quickly, conveniently, and sensitively detect biological toxins in seafood, have been developed and validated, these technologies have the potential to help seafood producers, suppliers and regulatory authorities. This article reviews the seafood toxins sources and types, mechanism of action and bioaccumulation of marine toxins, as well as legislation and rapid detection technologies for biotoxins in seafood for official and fishermen supervision.

An Unprecedented Bloom of Oceanic Dinoflagellates (Karenia spp.) Inside a Fjord within a Highly Dynamic Multifrontal Ecosystem in Chilean Patagonia

At the end of summer 2020, a moderate (~105 cells L-1) bloom of potential fish-killing Karenia spp. was detected in samples from a 24 h study focused on Dinophysis spp. in the outer reaches of the Pitipalena-Añihue Marine Protected Area. Previous Karenia events with devastating effects on caged salmon and the wild fauna of Chilean Patagonia had been restricted to offshore waters, eventually reaching the southern coasts of Chiloé Island through the channel connecting the Chiloé Inland Sea to the Pacific Ocean. This event occurred at the onset of the COVID-19 lockdown when monitoring activities were slackened. A few salmon mortalities were related to other fish-killing species (e.g., Margalefidinium polykrikoides). As in the major Karenia event in 1999, the austral summer of 2020 was characterised by negative anomalies in rainfall and river outflow and a severe drought in March. Karenia spp. appeared to have been advected in a warm (14-15 °C) surface layer of estuarine saline water (S > 21). A lack of daily vertical migration patterns and cells dispersed through the whole water column suggested a declining population. Satellite images confirmed the decline, but gave evidence of dynamic multifrontal patterns of temperature and chl a distribution. A conceptual circulation model is proposed to explain the hypothetical retention of the Karenia bloom by a coastally generated eddy coupled with the semidiurnal tides at the mouth of Pitipalena Fjord. Thermal fronts generated by (topographically induced) upwelling around the Tic Toc Seamount are proposed as hot spots for the accumulation of swimming dinoflagellates in summer in the southern Chiloé Inland Sea. The results here provide helpful information on the environmental conditions and water column structure favouring Karenia occurrence. Thermohaline properties in the surface layer in summer can be used to develop a risk index (positive if the EFW layer is thin or absent).

Potential geographical distribution of harmful algal blooms caused by the toxic dinoflagellate Karenia mikimotoi in the China Sea

The fish-killing dinoflagellate Karenia mikimotoi frequently blooms in China and poses a threat to food safety and human health. To better understand harmful algal blooms (HABs) caused by K. mikimotoi and predict the risk of HABs under climate change, the combined effect of nitrate and norfloxacin (NOR) on the growth of K. mikimotoi was tested. A growth model was used to test the effects of nutrients and pollutants on the carrying capacity of the unicellular algae. The carrying capacity increased with increasing concentrations of nitrate and NOR, reaching a maximum at 62.2 μmol L-1 of nitrate and 9.03 mg L-1 of NOR. The calculated carrying capacity of K. mikimotoi in the China Sea showed a declining trend from nearshore to offshore, with a value >30 × 106 cells L-1 in the estuary of the Changjiang River and Hangzhou Bay. The HAB index proposed in this study as a measurement of HAB risk was constructed using the carrying capacity and relative abundance from the MaxEnt (maximum entropy) model. The index showed that HABs caused by K. mikimotoi consecutively occurred in Zhejiang and Fujian coastal waters and predicted that they will continue until 2100, regardless of the greenhouse gas emission scenario. The center of the integrated area moved northward, with a range of 120-900 km. The HAB index integrates the characteristics of the carrying capacity and suitability of habitats, and expresses the information contained in the intensive and extensive variables that affect HAB occurrence. This index is a promising predictor of HAB risk in coastal waters.

Acute Effects of Brevetoxin-3 Administered via Oral Gavage to Mice

Brevetoxins (BTXs) constitute a family of lipid-soluble toxic cyclic polyethers mainly produced by Karenia brevis, which is the main vector for a foodborne syndrome known as neurotoxic shellfish poisoning (NSP) in humans. To prevent health risks associated with the consumption of contaminated shellfish in France, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) recommended assessing the effects of BTXs via an acute oral toxicity study in rodents. Here, we investigated the effect of a single oral administration in both male and female mice with several doses of BTX-3 (100 to 1,500 µg kg-1 bw) during a 48 h observation period in order to provide toxicity data to be used as a starting point for establishing an acute oral reference dose (ARfD). We monitored biological parameters and observed symptomatology, revealing different effects of this toxin depending on the sex. Females were more sensitive than males to the impact of BTX-3 at the lowest doses on weight loss. For both males and females, BTX-3 induced a rapid, transient and dose-dependent decrease in body temperature, and a transient dose-dependent reduced muscle activity. Males were more sensitive to BTX-3 than females with more frequent observations of failures in the grip test, convulsive jaw movements, and tremors. BTX-3's impacts on symptomatology were rapid, appearing during the 2 h after administration, and were transient, disappearing 24 h after administration. The highest dose of BTX-3 administered in this study, 1,500 µg kg-1 bw, was more toxic to males, leading to the euthanasia of three out of five males only 4 h after administration. BTX-3 had no effect on water intake, and affected neither the plasma chemistry parameters nor the organs' weight. We identified potential points of departure that could be used to establish an ARfD (decrease in body weight, body temperature, and muscle activity).

Efficient modulation of cellular phosphorus components in response to phosphorus deficiency in the dinoflagellate Karenia mikimotoi

IMPORTANCE

Dinoflagellates are the most common phytoplankton group and account for more than 75% of harmful algal blooms in coastal waters. In recent decades, dinoflagellates seem to prevail in phosphate-depleted waters. However, the underlying acclimation mechanisms and competitive strategies of dinoflagellates in response to phosphorus deficiency are poorly understood, especially in terms of intracellular phosphorus modulation and recycling. Here, we focused on the response of intracellular phosphorus metabolism to phosphorus deficiency in the model dinoflagellate Karenia mikimotoi. Our work reveals the strong capability of K. mikimotoi to efficiently regulate intracellular phosphorus resources, particularly through membrane phospholipid remodeling and miRNA regulation of energy metabolism. Our research improved the understanding of intracellular phosphorus metabolism in marine phytoplankton and underscored the advantageous strategies of dinoflagellates in the efficient modulation of internal phosphorus resources to maintain active physiological activity and growth under unsuitable phosphorus conditions, which help them outcompete other species in coastal phosphate-depleted environments.

Variation in intracellular polyphosphate and associated gene expression in response to different phosphorus conditions in the dinoflagellate Karenia mikimotoi

Polyphosphate (polyP) has long been recognized as a crucial intracellular reservoir for phosphorus in microorganisms. However, the dynamics of polyP and its regulatory mechanism in eukaryotic phytoplankton in response to variations in external phosphorus conditions remain poorly understood. A comprehensive investigation was conducted to examine the intracellular polyP-associated metabolic response of the dinoflagellate Karenia mikimotoi, a harmful algal bloom species, through integrated physiological, biochemical, and transcriptional analyses under varying external phosphorus conditions. Comparable growth curves and Fv/Fm between phosphorus-replete conditions and phosphorus-depleted conditions suggested that K. mikimotoi has a strong capability to mobilize the intracellular phosphorus pool for growth under phosphorus deficiency. Intracellular phosphate (IPi) and polyP contributed approximately 6-23 % and 1-3 %, respectively, to the overall particulate phosphorus (PP) content under different phosphorus conditions. The significant decrease in PP and increase in polyP:PP suggested that cellular phosphorus components other than polyP are preferred for utilization under phosphorus deficiency. Genes involved in polyP synthesis and hydrolysis were upregulated to maintain phosphorus homeostasis in K. mikimotoi. These findings provide novel insights into the specific cellular strategies for phosphorus storage and the transcriptional response in intracellular polyP metabolism in K. mikimotoi. Additionally, these results also indicate that polyP may not play a crucial role in cellular phosphorus storage in phytoplankton, at least in dinoflagellates.

In hot water: Interactions of temperature, nitrogen form and availability and photosynthetic and nitrogen uptake responses in natural Karenia brevis populations

During 2020-2021, an unusually prolonged bloom of the toxigenic dinoflagellate Karenia brevis persisted for more than 12 months along the Gulf coast of Florida, resulting in severe environmental effects. Motivated by the possibility that unusual nutrient conditions existed during summer 2021, the short-term interactions of temperature, nitrogen (N) forms (ammonium (NH4+), nitrate (NO3-), and urea) and availability on photosynthesis-irradiance responses and N uptake rates were examined in summer 2021 and compared to such responses from the earlier winter. Winter samples were exposed to temperatures of 15, 20, 25, 30 °C while summer samples were incubated at 15, 25, 30, 33 °C, representing the maximum range the cells might experience throughout the water column due to daytime surface heating or extreme weather events. Depending on thermal history of the cells, photosynthetic performance differed when cells were exposed to the same temperature, showing a capacity for thermal acclimation in this species. Although blooms generally do not persist throughout the summer, bloom biomass was remarkably higher in summer than during the winter. However, most of the photosynthetic parameters and N uptake rates, as well as total carbon (C) and N cell-1 were significantly lower in the summer populations, showing that the summer populations were photosynthetically and nutritionally stressed. When the summer cells were treated with urea, however, uptake rates and total C and N cell-1 were higher than with the other N substrates, especially in warmer waters, showing differential thermal responses depending on N forms.

A dinoflagellate bloom caused by multiple species of Kareniaceae in the coastal waters of Fujian in June 2022 and its adverse impacts on Brachionus plicatilis and Artemia salina

Recently, dinoflagellate blooms have frequently occurred in the coastal waters of Fujian, East China Sea. In June 2022, a fish-killing bloom of Kareniaceae species occurred in this region. In this study, four species of Kareniaceae, namely, Karenia longicanalis, K. papilionacea, Karlodinium veneficum, and Karl. digitatum were identified from this bloom event based on the results of single-cell PCR and clone libraries, and intraspecies genetic diversity was found in the Karl. veneficum population. The results of acute toxicity assays of the bloom water to two zooplankton species (Brachionus plicatilis and Artemia salina) demonstrated this bloom event strongly inhibited their swimming capacities and survival. The results of this study suggested that the bloom events caused by multiple species of Kareniaceae in the Fujian coastal waters had adverse impacts on the local fishery resources and zooplankton community.

Oxidative Stress and DNA Damage in Pagrus major by the Dinoflagellate Karenia mikimotoi

Karenia mikimotoi is a common species of red tide dinoflagellate that causes the mass mortality of marine fauna in coastal waters of Republic of Korea. Despite continuous studies on the ecophysiology and toxicity of K. mikimotoi, the underlying molecular mechanisms remain poorly understood. Red sea bream, Pagrus major, is a high-value aquaculture fish species, and the coastal aquaculture industry of red sea bream has been increasingly affected by red tides. To investigate the potential oxidative effects of K. mikimotoi on P. major and the molecular mechanisms involved, we exposed the fish to varying concentrations of K. mikimotoi and evaluated its toxicity. Our results showed that exposure to K. mikimotoi led to an accumulation of reactive oxygen species (ROS) and oxidative DNA damage in the gill tissue of P. major. Furthermore, we found that K. mikimotoi induced the activation of antioxidant enzymes, such as superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, in the gill tissue of P. major, with a significant increase in activity at concentrations above 5000 cells/mL. However, the activity of glutathione S-transferase did not significantly increase at the equivalent concentration. Our study confirms that oxidative stress and DNA damage is induced by acute exposure to K. mikimotoi, as it produces ROS and hypoxic conditions in P. major. In addition, it was confirmed that gill and blood samples can be used as biomarkers to detect the degree of oxidative stress in fish. These findings have important implications for the aquaculture of red sea bream, particularly in the face of red tide disasters.

Establishment of colloidal gold immunochromatography strip for rapid detection of Karen mikimotoi and its application

In China, harmful algal blooms (HABs) are one of the most prominent ecological disasters in the coastal areas. Among the harmful algae species that cause HABs, Karen mikimotoi is a kind of algae that appear frequently. It can secrete hemolytic toxins and fish toxins such as glycolipids and unsaturated fatty, posing a significant threat to marine life. In order to establish a fast and effective detection technology for Karen mikimotoi that can be promoted and applied on site, we have developed a test strip which is based on monoclonal antibody technology and the colloidal gold immune-chromatography assay (GICA). The experimental results show that this test strip can detect different growth stages including growth, and stable and recession period of Karen mikimotoi. The detection results can be displayed within 3-15 min. It had high sensitivity and specificity, with a detection limit of 754 cells/mL. A colorimetric card was made to further determine the concentration of algae detected. What is more, we had developed a method that can be used for on-site enrichment of algae cells using a syringe to detect lower concentrations of Karen mikimotoi, with a minimum detection concentration of 100 cells/mL. Also the test strip was used for on-site testing along the coast of China. This test strip not only serves as a warning for the outbreak of red tide, but also provides a new approach for the development of rapid detection technology for red tide algae.

Metabarcoding reveals high genetic diversity of harmful algae in the coastal waters of Texas, Gulf of Mexico

Environmental-DNA (eDNA) for metabarcoding is a rapid and effective means to investigate microplankton community composition and species diversity. The objective of this study was to examine the genetic diversity of the phytoplankton community in the Gulf of Mexico, with particular emphasis on harmful algal bloom species. Samples were collected at stations along the coast of Texas in September-October 2017 that were inundated by low salinity waters in the aftermath of Hurricane Harvey. Metabarcodes were generated from the eDNA targeting both the V4 and V8-V9 regions of the 18S rDNA gene. Evaluation of the metabarcodes revealed an unexpectedly high number of harmful algal species during this short period, including five that had not been documented in this region previously. A total of 36 harmful algal species could be differentiated based on V4 and V8-V9 metabarcode markers. Using a phylogenetic approach, the taxonomic resolution of each marker differed and not all species could be differentiated using solely one marker. The V4 region resolved species within some genera (e.g., Heterocapsa), while the V8-V9 marker was necessary to resolve species within other genera (e.g., Chattonella). In other cases, species differentiation within a genus required a combination of both markers (e.g., Prorocentrum, Karenia), or another marker will be needed to resolve all species (e.g., Alexandrium, Dinophysis). We conclude that no single marker can delineate all species, so it is recommended HAB monitoring programs use more than one marker. Overall, the observed diversity of HAB species along the Texas coast using metabarcoding exceeded reports from other parts of the world.

Karenia selliformis bloom dynamics and growth rate estimation in the Sfax harbour (Tunisia), by using automated flow cytometry equipped with image in flow, during autumn 2019

A Karenia selliformis bloom event in the Gulf of Gabès (Mediterranean Sea), was monitored over 9 days at high frequency during fall 2019, by using an automated flow cytometer (Cytosense, Cytobuoy b.v.) with an image-in-flow attachment. The instrument recorded the shape of the optical signals that lead to the resolution of six cell groups of pico-, nano- and microphytoplankton, during the Harmful Algal Bloom (HAB). K. selliformis cell dimensions derived from the hourly records, enabled to estimate the daily division rate over the bloom period. Results revealed that K. selliformis was the only bloom-forming species and it reached its highest mean abundance the fourth day of the survey. A shift in the nutrient composition occurred with a potential P limitation during the bloom growth and N limitation during the bloom collapse. The co-inertia analysis revealed opposite patterns for K. selliformis and heterotrophic prokaryotes suggesting trophic interactions and possible mixotrophic behaviour of K. selliformis at the end of the bloom. K. selliformis exhibited low growth rates generally < 1 division day^-1, which could not explain the observed high abundance. The tide played a crucial role in the dynamics of K. selliformis at a semi-diurnal scale and at spring-neap tide scale and was probably enhancing K. selliformis accumulation.

A massive bloom of Karenia species (Dinophyceae) off the Kamchatka coast, Russia, in the fall of 2020

In the fall of 2020, a long-lasting and massive harmful algal bloom (HAB) with extensive fields of yellow sea foam was observed in relatively cold waters (7-13 °C) off the coasts of the Kamchatka Peninsula, Russia. According to the estimates based on bio-optical parameters in satellite imagery, the Kamchatka bloom 2020 lasted for two months and covered a vast area of more than 300 × 100 km. An abundance of dead fish and invertebrates, including sea urchins, sea anemones, chitons, cephalopods, bivalves were found on shore during the bloom. Animals suffered almost 100% mortality within a depth range between 5 and 20 m. To identify the causative microalgal species, light and scanning electron microscopy, Raman spectroscopy, and molecular phylogenetic approaches were used. The HAB area was estimated by the spectral analysis of satellite-derived imagery. The causative organisms were unarmored dinoflagellates of Karenia species. Their density and biomass reached 100-620 cells·mL^-1 and 1300-7700 mg·m^-3, respectively, which accounted for 31-99% of the total cell density and 82-99% of the total phytoplankton biomass in late September to mid-October. The dominant species was Karenia selliformis, and the other co-occurring kareniacean species were K. cf. cristata, K. mikimotoi, K. papilionacea, K. longicanalis, and two unidentified morphotypes of Karenia spp. The molecular phylogeny inferred from LSU rDNA and ITS region showed that K. selliformis from Kamchatka in 2020 belonged to the cold-water group I and was identical to K. selliformis strains from Hokkaido, Japan, identified in 2021. This is the first HAB event caused by K. selliformis recorded from Russian coastal waters.

Reduced Fitness and Elevated Oxidative Stress in the Marine Copepod Tigriopus japonicus Exposed to the Toxic Dinoflagellate Karenia mikimotoi

Blooms of the toxic dinoflagellate Karenia mikimotoi cause devastation to marine life, including declines of fitness and population recruitment. However, little is known about the effects of them on benthic copepods. Here, we assessed the acute and chronic effects of K. mikimotoi on the marine benthic copepod Tigriopus japonicus. Results showed that adult females maintained high survival (>85%) throughout 14-d incubation, but time-dependent reduction of survival was detected in the highest K. mikimotoi concentration, and nauplii and copepodites were more vulnerable compared to adults. Ingestion of K. mikimotoi depressed the grazing of copepods but significantly induced the generation of reactive oxygen species (ROS), total antioxidant capacity, activities of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), and acetylcholinesterase. Under sublethal concentrations for two generations, K. mikimotoi reduced the fitness of copepods by prolonging development time and decreasing successful development rate, egg production, and the number of clutches. Our findings suggest that the bloom of K. mikimotoi may threaten copepod population recruitment, and its adverse effects are associated with oxidative stress.

Multi-Omics Profiling Reveals Resource Allocation and Acclimation Strategies to Temperature Changes in a Marine Dinoflagellate

Temperature is a critical environmental factor that affects the cell growth of dinoflagellates and bloom formation. To date, the molecular mechanisms underlying the physiological responses to temperature variations are poorly understood. Here, we applied quantitative proteomic and untargeted metabolomic approaches to investigate protein and metabolite expression profiles of a bloom-forming dinoflagellate Prorocentrum shikokuense at different temperatures. Of the four temperatures (19, 22, 25, and 28°C) investigated, P. shikokuense at 25°C exhibited the maximal cell growth rate and maximum quantum efficiency of photosystem II (Fv/Fm) value. The levels of particulate organic carbon (POC) and nitrogen (PON) decreased with increasing temperature, while the POC/PON ratio increased and peaked at 25°C. Proteomic analysis showed proteins related to photoreaction, light harvesting, and protein homeostasis were highly expressed at 28°C when cells were under moderate heat stress. Metabolomic analysis further confirmed reallocated amino acids and soluble sugars at this temperature. Both omic analyses showed glutathione metabolism that scavenges the excess reactive oxygen species, and transcription and lipid biosynthesis that compensate for the low translation efficiency and plasma membrane fluidity were largely upregulated at suboptimal temperature. Higher accumulations of glutathione, glutarate semialdehyde, and 5-KETE at 19°C implied their important roles in low-temperature acclimation. The strikingly active nitrate reduction and nitrogen flux into asparagine, glutamine, and aspartic acid at 19°C indicated these three amino acids may serve as nitrogen storage pools and help cells cope with low temperature. Our study provides insights into the effects of temperature on dinoflagellate resource allocation and advances our knowledge of dinoflagellate bloom formation in marine environments. IMPORTANCE Marine phytoplankton is one of the most important nodes in global biogeochemical cycle. Deciphering temperature-associated marine phytoplankton cell stoichiometric changes and the underlying molecular mechanisms are therefore of great ecological concerns. However, knowledge of how phytoplankton adjust the cell stoichiometry to sustain growth under temperature changes is still lacking. This study investigates the variations of protein and metabolite profiles in a marine dinoflagellate across temperatures at which the field blooms usually occur and highlights the temperature-dependent molecular traits and key metabolites that may be associated with rapid cell growth and temperature stress acclimation.

Cell death responses to acute high light mediated by non-photochemical quenching in the dinoflagellate Karenia brevis

Programmed cell death (PCD) can be induced in microalgae by many abiotic challenges via generation of reactive oxygen species (ROS). Marine phytoplankton live in a highly variable light environment, yet the potential for excess photosynthetically available radiation to trigger PCD has not been examined. On the other hand, photoprotective non-photochemical quenching (NPQ) is hypothesized to counteract intracellular ROS, potentially preventing cell death. The main objective of this study is to investigate high-light-induced death processes and their relationship with photosynthesis in bloom-forming dinoflagellate Karenia brevis. Here, we characterized the prevalence of ROS, caspase-like enzyme activity and cell death as well as photosynthetic status under acute irradiance of 500, 750 or 1000 µmol m^-2 s^-1. PCD only occurred at the largest light shift. Although depressed photosynthetic capacities and oxidative stress were apparent across the stress gradient, they did not necessarily lead to cell death. NPQ exhibited dose-dependent activation with increasing light stress, which enabled cells to resist or delay PCD. These results highlight the important role of the balance between ROS generation and NPQ activation on determining cell fates in Karenia under acute irradiance stress. This research also provides insights into potential survival strategies and mechanisms of cell loss under a changeable light environment.

The influence of Lake Okeechobee discharges on Karenia brevis blooms and the effects on wildlife along the central west coast of Florida

Blooms of the dinoflagellate Karenia brevis (K. brevis) are a common occurrence in the Gulf of Mexico, especially along Florida's coast. The blooms produce brevetoxins, potent neurotoxins that are associated with mortalities of marine wildlife. In recent years, K. brevis blooms seem to have become more frequent and intense. The cause of these suspected increases is highly debated, with one suggested explanation being anthropogenic eutrophication. Patient records from the Clinic for the Rehabilitation of Wildlife (CROW) on Sanibel Island, Florida, USA, and K. brevis cell count samples from the west coast of Florida were used to assess trends in red tides and affected wildlife. Flow data from the Okeechobee waterway was used to investigate if discharges from Lake Okeechobee and the Caloosahatchee Estuary, where eutrophication is present, influence red tides along Florida's central west coast. Overall, K. brevis blooms show trends of increasing intensity and duration along Florida's coast between 1954 and 2020 (latest data available). This means the amount of wildlife affected will likely increase in the future, as a linear relationship was found between the number of admissions to CROW and K. brevis densities. Furthermore, water discharges from the Okeechobee waterway (including Lake Okeechobee and the Caloosahatchee Estuary) into the Gulf of Mexico were significantly correlated with K. brevis densities, which suggests that anthropogenic pollution might play a role in the observed increases. Clear correlations were found between K. brevis densities and brevetoxicosis patient numbers admitted, and this was strongest with overall admissions lagging 23 days behind cell counts. This further confirms brevetoxins as the likely cause of their morbidity and supports previous research on brevetoxin retention in the environment. Different species groups had significant correlations with K. brevis cell counts, double-crested cormorants showing the strongest link, and there were significant differences between these groups in lag times. The differences are likely due to their distinct foraging behaviours or susceptibilities to brevetoxins. These findings can help predict future trends in red tides and can guide further research on the effects of discharges on K. brevis blooms.

First record of a Takayama bloom in Haizhou Bay in response to dissolved organic nitrogen and phosphorus

Since 1990s, harmful algal blooms (HABs) of Kareniaceae, primarily caused by species of Karenia and Karlodinium and rarely by Takayama species, have been substantially increasing in frequency and duration in the coastal waters of China. In this study, we recorded a bloom of high abundance of T. acrotrocha in the Haizhou Bay, the Yellow Sea in September 2020, which is the first record of a Takayama bloom in the temperate coastal waters of China. We found that high concentrations of DON and DOP accelerated the proliferation of T. acrotrocha in the Haizhou Bay. Intensive mariculture, and terrestrial nitrogen and phosphorus input may be responsible for the eutrophication in the Haizhou Bay featuring high concentrations of DON and DOP, and high DIN/DIP ratios. The results suggested that, under ocean warming, the HABs of Kareniaceae are becoming increasingly dominant in eutrophic temperate coasts with intensive mariculture activities.

Current Situation of Palytoxins and Cyclic Imines in Asia-Pacific Countries: Causative Phytoplankton Species and Seafood Poisoning

Among marine biotoxins, palytoxins (PlTXs) and cyclic imines (CIs), including spirolides, pinnatoxins, pteriatoxins, and gymnodimines, are not managed in many countries, such as the USA, European nations, and South Korea, because there are not enough poisoning cases or data for the limits on these biotoxins. In this article, we review unregulated marine biotoxins (e.g., PlTXs and CIs), their toxicity, causative phytoplankton species, and toxin extraction and detection protocols. Due to global warming, the habitat of the causative phytoplankton has expanded to the Asia-Pacific region. When ingested by humans, shellfish that accumulated toxins can cause various symptoms (muscle pain or diarrhea) and even death. There are no systematic reports on the occurrence of these toxins; however, it is important to continuously monitor causative phytoplankton and poisoning of accumulating shellfish by PlTXs and CI toxins because of the high risk of toxicity in human consumers.

Rapid and sensitive detection of Karenia mikimotoi by loop-mediated isothermal amplification combined with a lateral flow dipstick

Harmful algal blooms frequently occur in various coastal regions worldwide, deteriorating marine ecology and causing huge economic losses. Therefore, developing a potential method for rapid detection of harmful algae species is highly necessitated. In this study, a loop-mediated isothermal amplification (LAMP) method coupled with a lateral flow dipstick (LFD) was developed for detecting the harmful algae Karenia mikimotoi. In this method, the internal transcribed spacer (ITS) sequence of K. mikimotoi was used as the template, and the corresponding specific primers were designed by the online software PrimerExplorer V5. Biotin was labeled on the 5' end of forward inner primer (FIP), and the LAMP reaction was performed under the determined optimal conditions of 63℃ and 60 min. The lowest concentration of K. mikimotoi DNA tested using LAMP was 3.3 × 10^-1 pg/μL. Additionally, a 6-FAM-labeled probe was designed and displayed on the LFD after hybridization of the amplified product with the probe. The results demonstrated that LAMP-LFD could be a promising approach for detecting and monitoring harmful algae due to its high sensitivity and specificity.

Assessing the Effect of Modified Clay on the Toxicity of Karenia mikimotoi Using Marine Medaka (Oryzias melastigma) as a Model Organism

Blooms of the toxic dinoflagellate Karenia mikimotoi could threaten the survival of marine life, and modified clay (MC) is considered a promising method for the control of harmful algal blooms. Here, using marine medaka as the model organism, the toxicity of K. mikimotoi before and after MC disposal was investigated. The results showed that only a certain density of intact K. mikimotoi cells could cause obvious damage to fish gills and lead to rapid death. A systematic analysis of morphology, physiology, and molecular biology parameters revealed that the fish gills exhibited structural damage, oxidative damage, osmotic regulation impairment, immune response activation, and signal transduction enhancement. MC can flocculate K. mikimotoi rapidly in water and reduce its toxicity by reducing the density of intact algae cells and hemolytic toxicity. The results indicate that MC is an effective and safe method for controlling K. mikimotoi blooms.

Phylogenetic diversity and spatiotemporal dynamics of bacterial and microeukaryotic plankton communities in Gwangyang Bay of the Korean Peninsula

Nutrient dynamics function globally, flowing from rivers to the ocean (estuarine–coastal zone), and are vulnerable to climate change. Microbial habitats can be affected by marine nutrient dynamics and may provide a clue to predict microbial responses to environmental heterogeneity in estuarine–coastal zones. We surveyed surface seawater in Gwangyang Bay, a semi-enclosed estuary in Korea, from 2016 to 2018 using a metabarcoding approach with prokaryotic 16S and eukaryotic 18S rRNA genes. Bacterial and microeukaryotic communities in these waters showed distinct local communities in response to environmental heterogeneity and community transition at spatiotemporal scales in the estuarine–coastal zone. The relative abundance of prokaryotic and eukaryotic operational taxonomic units suggested a microbial trophic interaction in the Gwangyang Bay waters. We found that the community assembly process in prokaryotic communities was primarily influenced by biological interaction (immigration–emigration), whereas that in eukaryotic communities was more affected by environmental stress (habitat specificity) rather than by biotic factors. Our findings in the Gwangyang Bay waters may provide information on underlying (biotic or abiotic) factors of the assembly process in microbial communities in the estuarine–coastal zone.

Application of clay minerals to remove extracellular ichthyotoxins produced by the dinoflagellates Karlodinium veneficum and Karenia mikimotoi

Mitigation of fish-killing algal toxins by clay minerals offers great promise as an emergency strategy for fish farms threatened by harmful algal blooms, but its efficiency is highly clay and algal species (i.e. ichthyotoxin) specific. We here screened several different clay types (kaolin, zeolite, Korean loess and six bentonites) for their adsorptive capacity of extracellular Karlodinium veneficum and Karenia mikimotoi ichthyotoxins as quantified with the rainbow trout RTgill-W1 cell line assay. Treatment with Korean loess, zeolite (0-0.5 g L ^- 1), polyaluminium chloride (0-0.1 g L ^- 1) and clays modified with this flocculant (0-0.25 g L ^- 1) could not significantly improve gill cell viability compared to toxic controls. Kaolin only demonstrated effective removal in case of K. mikimotoi, but concentrations required for complete removal of cytotoxicity were at least 2 x those required for bentonite. Bentonites of high swelling capacity and ideally small particle size (<2 µm) proved best suited for ichthyotoxin removal against both algal species (100% removal at concentrations as low as 0.1 g L ^- 1). Complete elimination of K. veneficum and K. mikimotoi toxicity towards the rainbow trout gill cell line was achieved by bentonite clay, demonstrating the potential to control ichthyotoxicity in an aquaculture setting through targeted clay application.

Chemodiversity of Brevetoxins and Other Potentially Toxic Metabolites Produced by Karenia spp. and Their Metabolic Products in Marine Organisms

In recent decades, more than 130 potentially toxic metabolites originating from dinoflagellate species belonging to the genus Karenia or metabolized by marine organisms have been described. These metabolites include the well-known and large group of brevetoxins (BTXs), responsible for foodborne neurotoxic shellfish poisoning (NSP) and airborne respiratory symptoms in humans. Karenia spp. also produce brevenal, brevisamide and metabolites belonging to the hemi-brevetoxin, brevisin, tamulamide, gymnocin, gymnodimine, brevisulcenal and brevisulcatic acid groups. In this review, we summarize the available knowledge in the literature since 1977 on these various identified metabolites, whether they are produced directly by the producer organisms or biotransformed in marine organisms. Their structures and physicochemical properties are presented and discussed. Among future avenues of research, we highlight the need for more toxin occurrence data with analytical techniques, which can specifically determine the analogs present in samples. New metabolites have yet to be fully described, especially the groups of metabolites discovered in the last two decades (e.g tamulamides). Lastly, this work clarifies the different nomenclatures used in the literature and should help to harmonize practices in the future.

Development of an Efficient Extraction Method for Harvesting Gymnodimine-A from Large-Scale Cultures of Karenia selliformis

Gymnodimine-A (GYM-A) is a fast-acting microalgal toxin and its production of certified materials requires an efficient harvesting technology from the large-scale cultures of toxigenic microalgae. In this study the recoveries of GYM-A were compared between several liquid-liquid extraction (LLE) treatments including solvents, ratios and stirring times to optimize the LLE technique for harvesting GYM-A from Karenia selliformis cultures, of which the dichloromethane was selected as the extractant and added to microalgal cultures at the ratio 55 mL L-1 (5.5%, v/v). The recovery of GYM-A obtained by the LLE technique was also compared with filtration and centrifugation methods. The stability of GYM-A in culture media were also tested under different pH conditions. Results showed that both the conventional filter filtration and centrifugation methods led to fragmentation of microalgal cells and loss of GYM-A in the harvesting processes. A total of 5.1 µg of GYM-A were obtained from 2 L of K. selliformis cultures with a satisfactory recovery of 88%. Interestingly, GYM-A obviously degraded in the culture media with the initial pH 8.2 and the adjusted pH of 7.0 after 7 days, but there was no obvious degradation in the acidic medium at pH 5.0. Therefore, the LLE method developed here permits the collection of large-volume cultures of K. selliformis and the high-efficiency extraction of GYM-A. This work provides a simple and valuable technique for harvesting toxins from large-scale cultures of GYM-producing microalgae.

q-PCR-based assay for the toxic dinoflagellate Karenia selliformis monitoring along the Tunisian coasts

Karenia selliformis is a marine dinoflagellate responsible for fish-kill events. Its presence has been reported along the Tunisian coasts (south-eastern Mediterranean Sea) since the 1990s. In the present study, a quantitative-PCR assay, based on the internal transcribed spacer (ITS) molecular marker, was developed to detect and quantify K. selliformis in environmental bivalve mollusk samples and in seawater samples. The assay was optimized, and its specificity was confirmed using cross-reactivity experiments against microalgal species commonly found on the Tunisian coasts and/or closely related to K. selliformis. Calibration curves were performed by tenfold dilutions of plasmid DNA harboring target sequence and genomic DNA, attaining a limit of detection of around 5 copies of target DNA per reaction, far below one K. selliformis cell per reaction. The field application of the developed assay showed a powerful detection capability. Thus, the designed assay could contribute to the deployment of in-field diagnostic tools for K. selliformis blooms monitoring.

Geographic distribution and historical presence of the resting cysts of Karenia mikimotoi in the seas of China

The toxic dinoflagellate Karenia mikimotoi frequently forms harmful algal blooms (HABs) and thus causes massive kills of fish and shellfish in worldwide coastal waters, which has led to intensive investigations on multiple facets of the species. Following our recent discovery of K. mikimotoi forming resting cyst, a very possible mechanism for the inoculation of blooms and geographic expansion for this and many other HABs-causing species, here we report our detection of K. mikimotoi resting cysts in 125 surface sediment samples collected from the coastal waters (covering a latitude range from 18.29°N to 39.85°N) and 3 sediment cores (accumulated in 70‒100 years) collected from the East China Sea where are adjacent to the frequent blooming areas of K. mikimotoi. Via applications of quantitative real-time PCR (LSU rDNA-targeted), species-specific fluorescence in situ hybridization (FISH), and nested-PCR-and-sequencing to both types of the sediment samples that were pretreated with sodium polytungstate solution (SPT), we demonstrated that 1) K. mikimotoi cysts are widely present in surface sediments of the China seas (Bohai Sea (BS), Yellow Sea (YS), East China Sea (ECS), and South China Sea (SCS)), 2) the abundance of cysts is generally low (0 to 33 cysts in 32 g wet sediment), with that in the ECS and the SCS being higher than that in the YS and the BS, and the highest abundance was observed in sites of the ECS (e.g., Ningde, Fujian province) where the blooms of the species occurred frequently, as quantified by both methods, and 3) the cysts of K. mikimotoi have been present in the sediments of the ECS since 1970s, a short time prior to the first recorded bloom of K. mikimotoi in the SCS at 1980s. Our results not only demonstrated the wide geographic distribution of resting cyst of K. mikimotoi along the coast of China, but also proved a 50 years preservation of the cysts in the sediments of coastal area prone to forming frequent blooms. We consider our results have provided critical insights into the mechanisms of frequent bloom outbreaks and global distribution of K. mikimotoi in general, and particularly into the historical origin of K. mikimotoi in China. Further investigations are suggested to focus on on-site surveys for the cyst production and germination rates.

Proteome Analysis of Whole-Body Responses in Medaka Experimentally Exposed to Fish-Killing Dinoflagellate Karenia mikimotoi

Karenia mikimotoi is a well-known harmful algal bloom species. Blooms of this dinoflagellate have become a serious threat to marine life, including fish, shellfish, and zooplanktons and are usually associated with massive fish death. Despite the discovery of several toxins such as gymnocins and gymnodimines in K. mikimotoi, the mechanisms underlying the ichthyotoxicity of this species remain unclear, and molecular studies on this topic have never been reported. The present study investigates the fish-killing mechanisms of K. mikimotoi through comparative proteomic analysis. Marine medaka, a model fish organism, was exposed to K. mikimotoi for a three-part time period (LT25, LT50 and LT90). Proteins extracted from the whole fish were separated by using two-dimensional gel electrophoresis, and differentially expressed proteins were identified with reference to an untreated control. The change in fish proteomes over the time-course of exposure were analyzed. A total of 35 differential protein spots covering 19 different proteins were identified, of which most began to show significant change in expression levels at the earliest stage of intoxication. Among the 19 identified proteins, some are closely related to the oxidative stress responses, energy metabolism, and muscle contraction. We propose that oxidative stress-mediated muscle damage might explain the symptoms developed during the ichthyotoxicity test, such as gasping for breath, loss of balance, and body twitching. Our findings lay the foundations for more in-depth studies of the mechanisms of K. mikimotoi's ichthyotoxicity.

Nutrient Alteration Drives the Impacts of Seawater Acidification on the Bloom-Forming Dinoflagellate Karenia mikimotoi

Seawater acidification and nutrient alteration are two dominant environmental factors in coastal environments that influence the dynamics and succession of marine microalgae. However, the impacts of their combination have seldom been recorded. A simulated experimental system was set up to mimic the effects of elevated acidification on a bloom-forming dinoflagellate, Karenia mikimotoi, exposed to different nutrient conditions, and the possible mechanism was discussed. The results showed that acidification at different pH levels of 7.6 or 7.4 significantly influenced microalgal growth (p<0.05) compared with the control at pH 8.0. Mitochondria, the key sites of aerobic respiration and energy production, were impaired in a pH-dependent manner, and a simultaneous alteration of reactive oxygen species (ROS) production occurred. Cytochrome c oxidase (COX) and citrate synthase (CS), two mitochondrial metabolism-related enzymes, were actively induced with acidification exposure, suggesting the involvement of the mitochondrial pathway in coping with acidification. Moreover, different nutrient statuses indicated by various N:P ratios of 7:1 (N limitation) and 52:1 (P limitation) dramatically altered the impacts of acidification compared with those exposed to an N:P ratio of 17:1 (control), microalgal growth at pH 7.4 was obviously accelerated with the elevation of the nutrient ratio compared to that at pH 8.1 (p<0.05), and nutrient limitations seemed beneficial for growth in acidifying conditions. The production of alkaline phosphatase (AP) and acid phosphatase (AcP), an effective index indicating the microalgal growth status, significantly increased at the same time (p<0.05), which further supported this speculation. However, nitrate reductase (NR) was slightly inhibited. Hemolytic toxin production showed an obvious increase as the N:P ratio increased when exposed to acidification. Taken together, mitochondrial metabolism was suspected to be involved in the process of coping with acidification, and nutrient alterations, especially P limitation, could effectively alleviate the negative impacts induced by acidification. The obtained results might be a possible explanation for the competitive fitness of K. mikimotoi during bloom development.

Bioavailability of Organic Phosphorus Compounds to the Harmful Dinoflagellate Karenia mikimotoi

Karenia mikimotoi is one of the most well-known harmful bloom species in temperate coastal waters. The present study investigated the characteristics of alkaline phosphatase (APase) and phosphodiesterase (PDEase) activities in hydrolysis of two phosphomonoesters (adenosine triphosphate (ATP) and ribulose 5-phosphate (R5P)) and a phosphodiester (cyclic adenosine monophosphate (cAMP)) in K. mikimotoi and compared its growth and physiological responses to the different forms of phosphorus substrates. K. mikimotoi produced comparable quantities of APase and PDEase to hydrolyze the organic phosphorus substrates, although hydrolysis of the phosphomonoesters was much faster than that of the phosphodiester. The growth of K. mikimotoi on organic phosphorus substrates was comparable to or better than that on inorganic phosphate. The difference in particulate organic nutrients (carbon, nitrogen, and phosphorus) and hemolytic activity supported different rates of hydrolysis-assimilation of the various organic phosphorus substrates by K. mikimotoi. The hemolytic activities of K. mikimotoi in the presence of organic phosphorus substrates were several times those in the presence of inorganic phosphate during the exponential phase. This suggested the potential important role of organic phosphorus in K. mikimotoi blooms.

Method performance verification for the combined detection and quantitation of the marine neurotoxins cyclic imines and brevetoxin shellfish metabolites in mussels (Mytilus edulis) and oysters (Crassostrea gigas) by UHPLC-MS/MS

A single laboratory method performance verification is reported for a rapid sensitive UHPLC-MS/MS method for the quantification of eight cyclic imine and two brevetoxin analogues in two bivalve shellfish matrices: mussel (Mytilus edulis) and Pacific oyster (Crassostrea gigas). Targeted cyclic imine analogues were from the spirolide, gymnodimine and pinnatoxin groups, namely 20-Me-SPX-C, 13-desMe-SPX-C, 13,19-didesMe-SPX-C, GYM-A, 12-Me-GYM, PnTx-E, PnTx-F and PnTx-G. Brevetoxin analogues consisted of the shellfish metabolites BTX-B5 and S-desoxy-BTX-B2. A rapid dispersive extraction was used as well as a fast six-minute UHPLC-MS/MS analysis. Mobile phase prepared using ammonium fluoride and methanol was optimised for both chromatographic separation and MS/MS response to suit all analytes. Method performance verification checks for both matrices were carried out. Matrix influence was acceptable for the majority of analogues with the MS response for all analogues being linear across an appropriate range of concentrations. In terms of limits of detection and quantitation the method was shown to be highly sensitive when compared with other methods. Acceptable recoveries were found with most analogues, with laboratory precision in terms of intra- and inter-batch precision deemed appropriate. The method was applied to environmental shellfish samples with results showing low concentrations of cyclic imines to be present. The method is fast and highly sensitive for the detection and quantification of all targeted analogues, in both mussel and oyster matrices. Consequently, the method has been shown to provide a useful tool for simultaneous monitoring for the presence or future emergence of these two toxin groups in shellfish.

Toxic effects of the dinoflagellate Karenia mikimotoi on zebrafish (Danio rerio) larval behavior

Karenia mikimotoi is a toxic dinoflagellate that forms harmful blooms in coastal waters, threatening aquaculture worldwide. However, we do not know whether K. mikimotoi has a neurotoxic effect on aquatic animal behavior. Thus, this study investigated potential K. mikimotoi neurotoxicity in zebrafish larvae. Cells of K. mikimotoi were collected at the mid-exponential phase from a batch culture to prepare ruptured cell solutions (RCS). At 6 h post-fertilization (hpf), zebrafish embryos were exposed to different RCS concentrations (0, 10², 10³, 10⁴, and 2.5 × 10⁴ cells mL^-1). After 120 hpf, treated larvae were collected to analyze locomotor behavior; activities of acetylcholinesterase (AChE), superoxide dismutase (SOD), catalase (CAT); and expression of genes related to neurodevelopment. We found that RCS did not affect survival rate, but significantly decreased larval locomotion, as well as their AChE, SOD, and CAT activity. Additionally, the examination of the day-night behavioral experiment revealed RCS decreased locomotion only at night. Zebrafish larvae were also significantly hypoactive in response to light and sound stimulations. Of the neurodevelopment genes, three (th, neurog1, and neurod1) were downregulated, while two (bdnf and manf) were upregulated. Our study suggests that K. mikimotoi neurotoxicity occurs through causing oxidative damage, as well as disorders in the cholinergic system and nervous system development. The results provide new insight that K. mikimotoi in low abundance did not cause significant lethal effect but still exhibited significant neurotoxicity on aquatic animals.

Responses of Primary Productivity and Phytoplankton Community to the Atmospheric Nutrient Deposition in the East China Sea

Atmospheric deposition of nutrients to the surface seawater may significantly affect marine phytoplankton growth. Two in situ bioassay experiments were performed in the East China Sea (ECS) by adding nutrients (N, P, and Si) and atmospheric aerosols into the surface seawater. Chlorophyll a (Chl a) concentrations were largely enhanced by simultaneous input of N and P with the maximal increase of 0.68–0.78 μg Chl a per μmol N addition. This Chl a increment was significantly lower (0.19–0.47 μg) in aerosol treatments as a result of initial N-replete condition (N/P ratio ~50) and extremely high N/P ratio in aerosols (>300). Among the multiple influencing factors, atmospheric dry flux of NH4+ + NO3− (AN) was found to be an effective predictor for springtime Chl a in the ECS with a time lag of three days and were strongly correlated with Chl a concentrations on day 3 (r = 0.81, p < 0.001), which might be partly explained by the asynchronous supplies of N (atmospheric deposition) and P (subsurface water). Although dinoflagellates dominated the phytoplankton community in both initial seawaters, additions of P and N + P + Si profoundly enhanced the cell densities and dominance of diatom species Thalassiosira sp. and Nitzschia closterium in the 2012 and 2014 bioassay experiments, respectively. Moreover, the percentage of dinoflagellates were promoted by adding higher NH4+/NO3− ratio (6/4 vs. 1/9) when silicate was at a low concentration (~2 μmol L−1). Atmospheric deposition is likely to be an important N source supporting the high primary production in the ECS and its supply of excess N relative to P may influence dominant phytoplankton groups.

Quantitative PCR assay for the simultaneous identification and enumeration of multiple Karenia species

Quantitative PCR (qPCR) is the method of choice for specific detection and quantification of harmful algal bloom (HAB) species. Development of qPCR assay for simultaneous enumeration of species that frequently co-exist in HABs is required. A high sensitivity TaqMan qPCR assay, using probe and primers, located at ITS1-5.8S-ITS2 rDNA region, detecting, specifically, Karenia selliformis, K. bidigitata, and K. mikimotoi, was designed. ITS1-5.8S-ITS2 rDNA region copy numbers per Karenia cell genome were estimated to 217.697 ± 67.904, allowing cell quantification. An application of the designed methodology in field samples has been conducted, and it showed high sensitivity (detection of around 10^-1 cell/100 mg of bivalve mollusk tissue, equivalent to about 20 copies of the target sequence). We suggest that the optimized method could contribute to early detection of three closely related Karenia species in seafood cultivating areas to promote control quality, guarantee a fast and effective intervention, and improve public health prevention.

The Harmful Unarmored Dinoflagellate Karlodinium in Japan and Philippines, with Reference to Ultrastructure and Micropredation of Karlodinium azanzae sp. nov. (Kareniaceae, Dinophyceae)1

In all, 26 cultures of the harmful marine dinoflagellate Karlodinium, isolated from Japanese and Philippine coastal waters, were examined using LM, SEM, and molecular phylogeny inferred from ITS and LSU rDNA. Seven Karlodinium species (six from Japan and four from Philippines), K. australe, K. ballantinum, K. decipiens, K. gentienii, K. veneficum, K. zhouanum, and a novel species Karlodinium azanzae sp. nov., were identified based on their morphology and phylogenetic positions. Karlodinium azanzae from Manila Bay, Philippines was further characterized by TEM, HPLC (chloroplast pigment), and bioassay on brine shrimp and other marine zooplankton. Cells of K. azanzae were the largest (mean 25.3 µm long) in Karlodinium, possessed numerous tiny reflective particles, starch grains, and lipid granules, and usually swam at the bottom of the culture vessel. The straight apical structure complex and a ventral pore were common to the genus. The longitudinally elongated nucleus was located at the center, and the yellowish chloroplasts contained an embedded pyrenoid and carotenoid pigments typical of the genus (i.e., fucoxanthin as major carotenoid with its derivatives). TEM revealed a part of the flagellar apparatus, of which the long striated ventral connective is the first report in the Kareniaceae. Phylogenetic trees showed closest affinity of K. azanzae with K. australe and K. armiger. The new species could be differentiated from related species by cell size, position of the nucleus, and characteristic swimming behavior. Lethality of K. azanzae to large zooplankton and micropredation using a developed peduncle was also observed.

An assessment of trends in the frequency and duration of Karenia brevis red tide blooms on the South Texas coast (western Gulf of Mexico)

Limited data coverage on harmful algal blooms (HABs) in some regions makes assessment of long-term trends difficult, and also impedes understanding of bloom ecology. Here, observations reported in a local newspaper were combined with cell count and environmental data from resource management agencies to assess trends in Karenia brevis "red tide" frequency and duration in the Nueces Estuary (Texas) and adjacent coastal waters, and to determine relationships with environmental factors. Based on these analyses, the Coastal Bend region of the Texas coast has experienced a significant increase in the frequency of red tide blooms since the mid-1990s. Salinity was positively correlated with red tide occurrence in the Nueces Estuary, and a documented long-term increase in salinity of the Nueces Estuary may be a major factor in the long-term increase in bloom frequency. This suggests that freshwater inflow management efforts in Texas should consider impacts on red tide habitat suitability (i.e., salinity regime) in downstream estuaries. Natural climate variability such as the El Niño-Southern Oscillation, which is strongly related to rainfall and salinity in Central and South Texas, was also an influential predictor of red tide presence/absence. Though no significant change in the duration of blooms was detected, there was a negative correlation between duration and temperature. Specifically, summer-like temperatures were not favorable to K. brevis bloom development. The relationships found here between red tide frequency/duration and environmental drivers present a new avenue of research that will aid in refining monitoring and forecasting efforts for red tides on the Texas coast and elsewhere. Findings also highlight the importance of factors (i.e., salinity, temperature) that are likely to be altered in the future due to both population growth in coastal watersheds and anthropogenic climate change.

Bioluminescence and toxicity as driving factors in harmful algal blooms: Ecological functions and genetic variability

Dinoflagellates are an ecologically important group of marine microbial eukaryotes with a remarkable array of adaptive strategies. It is ironic that two of the traits for which dinoflagellates are best known, toxin production and bioluminescence, are rarely linked when considering the ecological significance of either. Although dinoflagellate species that form some of the most widespread and frequent harmful algal blooms (HABs) are bioluminescent, the molecular and eco-evolutionary associations between these two traits has received little attention. Here, the major themes of biochemistry and genetics, ecological functions, signaling mechanisms, and evolution are addressed, with parallels and connections drawn between the two. Of the 17 major classes of dinoflagellate toxins, only two are produced by bioluminescent species: saxitoxin (STX) and yessotoxin. Of these, STX has been extensively studied, including the identification of the STX biosynthetic genes. While numerous theories have been put forward as to the eco-evolutionary roles of both bioluminescence and toxicity, a general consensus is that both function as grazing deterrents. Thus, both bioluminescence and toxicity may aid in HAB initiation as they alleviate grazing pressure on the HAB species. A large gap in our understanding is the genetic variability among natural bloom populations, as both toxic and non-toxic strains have been isolated from the same geographic location. The same applies to bioluminescence, as there exist both bioluminescent and non-bioluminescent strains of the same species. Recent evidence demonstrating that blooms are not monoclonal events necessitates a greater level of understanding as to the genetic variability of these traits among sub-populations as well as the mechanisms by which cells acquire or lose the trait, as sequence analysis of STX+ and STX- species indicate the key gene required for toxicity is lost rather than gained. While the extent of genetic variability for both bioluminescence and toxicity among natural HAB sub-populations remains unknown, it is an area that needs to be explored in order to gain greater insights into the molecular mechanisms and environmental parameters driving HAB evolution.

Unraveling the Karenia selliformis complex with the description of a non-gymnodimine producing Patagonian phylotype

Karenia selliformis is a bloom-forming toxic dinoflagellate known for production of gymnodimines (GYMs) and causing mass mortalities of marine fauna. Blooms have been reported from coastal waters of New Zealand, Mexico, Tunisia, Kuwait, Iran, China and Chile. Based on molecular phylogeny, morphology, toxin production, pigment composition and cell growth of Chilean K. selliformis isolated in 2018 (CREAN_KS01 and CREAN_KS02), this study revealed a more complex diversity within this species than previously thought. A phylogenetic reconstruction based on the large sub-unit ribosomal nucleotide (LSU rDNA) and Internal Transcriber Spacer (ITS) sequences of 12 worldwide isolates showed that within the K. selliformis clade there are at least two different phylotypes with clear phenotypic differences. Morphological differences related to the dorsal-ventral compression, shape of the hyposome and the presence of pores on the left lobe of the hyposome. A comparison of pigment signatures among worldwide isolates revealed the existence of both acyl-oxyfucoxanthin and fucoxanthin-rich strains within the phylotypes. A LC-MS/MS screening on both Chilean 2018 K. selliformis strains showed for first time no GYMs production among cultured clones of this species. However, both CREAN_KS01 and CREAN_KS02 contained two compounds with the same mass transition as brevenal, a brevetoxin related compound. A fish gill cell-based assay showed that the CREAN_KS02 strain was highly cytotoxic but pure GYM standard did not exhibit loss of cell viability, even at cell concentrations equivalent or exceeding those reported in nature. The fatty acid profile of CREAN_KS02 included high levels of saturated (14:0; 16:0) and polyunsaturated (18:3ω6+18:5ω3; 22:6ω3) fatty acids but superoxide production in this strain was low (0.86±0.53 pmol O₂^- cell^-1 h^-1). A factorial T-S growth experiment using the CREAN_KS02 strain showed a μ_max of 0.41±0.03 d^-1 at high salinity and temperature, which points to its optimal environmental niche in offshore waters during the summer season. In conclusion, the present study provides evidence for significant genetic and phenotypic variability among worldwide isolates, which points to the existence of a K. selliformis "species complex". The massive fauna mortality during K. selliformis bloom events in the Chilean coast cannot be explained by GYMs nor brevetoxins, but can to a large extent be accounted for by the high production of long-chain PUFAs and/or still uncharacterized highly toxic compounds.

Documenting the duration and chlorophyll pigments of an allochthonous Karenia brevis bloom in the Loxahatchee River Estuary (LRE), Florida

In Fall 2017 a large bloom of the toxic dinoflagellate Karenia brevis developed in the Gulf of Mexico. After persisting for months, in Fall 2018 wind and water circulation patterns drove K. brevis towards the east coast of Florida. On September 29, 2018 Palm Beach County, FL beaches were closed due to respiratory and gastrointestinal issues associated with brevotoxins, and effects of brevotoxins were reported from within estuarine segments of the Loxahatchee River Estuary (LRE). This was the first apparent report of a K. brevis bloom impacting inshore portions of the LRE prompting us to question the longevity of K. brevis within a relatively shallow, well-flushed coastal-estuarine system. Within 3 days (October 1, 2018) of the first reported effects of toxins, K. brevis reached over one million cells/L and chlorophyll-a concentrations peaked at 13 µg L^-1. Within 11 days (October 9, 2018) both K. brevis and chlorophyll pigment concentrations significantly (p-perm ≤ 0.05) dropped to an average of ≤ 30,000 cells L^-1 and < 4 µg L^-1 chlorophyll-a, indicating that the bloom had diminished. Using distance-based linear modeling (DistLM) K. brevis abundance alone explained 66% of the variation in a multivariate measure of chlorophylls (driven by carotenoids and chlorophyll-c pigment concentrations), supporting a K. brevis dominated bloom. Following the K. brevis bloom, additional HAB species K. mikimotoi and Pseudo-nitzschia spp singularly explained 6% of the variations in the multivariate measure of chlorophylls. The low explanatory power of individual HAB species, including K. brevis (≤ 0%), signifies the recovery of the phytoplankton population, where non-HAB species likely contributed to the variability in the multivariate measure of chlorophylls and overall chlorophyll-a concentrations (average of 2 µg L^-1 during non-bloom conditions). Finally, we evaluated ambient and historical water quality data to assess how these parameters changed before, during, and after the 2018 K. brevis bloom. Temperature, salinity, and nutrients in the LRE were comparable to reports of other K. brevis bloom events along the west coast of Florida. Reduced ammonia-nitrogen (NH₃-N) concentrations and increased tidal amplitude coincided with the end of the bloom in 2018. More work is needed to understand the specific mechanisms constraining K. brevis blooms in tidal estuaries. We suggest that future research focus on water residence times along with nutrient availability in controlling allochthonous HABs in lotic and tidally flushed estuaries. Also, we anticipate this work may stimulate additional efforts to characterize HABs using in situ observations coupled with multivariate measures of chlorophylls, though we recognize much work remains to fully define the value of this approach.

Comparative uptake and assimilation of nitrate, ammonium, and urea by dinoflagellate Karenia mikimotoi and diatom Skeletonema costatum s.l. in the coastal waters of the East China Sea

The nitrogen uptake kinetics and physiological growth of Karenia mikimotoi and Skeletonema costatum sensu lato grown on different N substrates and concentrations were compared in the laboratory. In the presence of three N substrates, both species preferred to take up NH₄⁺. K. mikimotoi and S. costatum s.l. showed the highest substrate affinities for urea and NO₃^-, respectively. Both species grew well on three N substrates, and the growth parameters were comparable among the different N substrates. However, K. mikimotoi assimilated urea more efficiently than it assimilated either NO₃^- or NH₄⁺. Different with S. costatum s.l., K. mikimotoi grew slowly and steady and the physiological and growth activities in N-depleted conditions were higher than those in N-replete conditions. Our results suggested that K. mikimotoi shows a greater readiness for uptake and assimilation of urea, and that this species is more competitive in an N-depleted environment when compared with S. costatum s.l.

Cell density-dependent suppression on the development and photosynthetic activities of Sargassum fusiformis embryos by dinoflagellate Karenia mikimotoi

Lots of research has demonstrated that macroalgae can strongly inhibit the growth of harmful algal bloom (HAB) species in general. However, the effects of HABs or HAB-forming species on macrophytes are still largely uncharacterized until now. In the present study, the effects of the dinoflagellate Karenia mikimotoi cell density gradient, live cell suspension (LC), ruptured cell suspension (RC) as well as the cell-free supernatant (FC) of K. mikimotoi at 1000 μg Chla l-1 (~1.0 × 105 cells ml-1) on the development and photosynthesis of Sargassum fusiforme embryos were investigated in a series of laboratory experiments. The results showed that co-cultivation with K. mikimotoi at 500 μg Chla l-1(~5.0 × 104 cells ml-1) and higher cell densities significantly (P<0.05) inhibited the development, pigment content and photosynthetic activities of the embryos. In addition, the inhibitory effects increased with increased cell densities and prolonged exposure time. Compared to the embryos cultured with the F/2 medium (Control), exposure to LC, RC and FC of K. mikimotoi at 1000 μg Chla l-1for 2 weeks all led to decreased relative growth rate (RGR), chlorophyll (Chl) a content, carotenoids (Car) content and photosynthetic activities of the embryos, with LC and RC exhibiting the maximal and the minimal suppression. The dominant inhibitory effects of FC on the embryos indicated that the suppression was mainly caused by the allelochemicals, while the slightest inhibitory effects of RC on the embryos suggested that some intracellular growth-promoting substances were synchronously released when K. mikimotoi cells lyzed. In addition, the most severe growth suppression of embryos by LC indicated that intact cell contact by K. mikimotoi probably also contributed to the inhibitory effects. These results indicated that a dense HAB formed by K. mikimotoi could seriously suppress the development and photosynthesis of S. fusiforme embryos and eventually reduce the seedlings stock.

Inhibition to crucial enzymes in the lethal effects of the dinoflagellate Karenia mikimotoi on the rotifer Brachionus plicatilis

Blooms of the dinoflagellate Karenia mikimotoi have cause great financial losses to the marine aquaculture industry. However, the toxicity mechanism of this species is still not fully known. In this study, we evaluated the short-term effects of K. mikimotoi on the rotifer Brachionus plicatilis by micro and sub micro observing and by measuring inhibition of crucial enzymes. Behaviour disorder, mucus production, corona and cilium damage, vesical production, and body shrinkage occurred within 1 h after rotifers were treated with K. mikimotoi at a density of 3 × 10⁴ cells/mL. Enzyme activity assays showed that K. mikimotoi at low densities significantly inhibited multiple enzymes within 3 h, and obvious density-effect trends were also observed. For instance, activity of esterase and acetylcholinesterase of rotifers significantly decreased to 94.3/83.3% and 82.8/66.9% of control treatment values in 30 and 1000 cells/mL algal treatment, respectively. Total ATPase and Na⁺-K⁺-ATPase activities of rotifers also decreased to 82.3% and 68.6% of control values in 1000 cells/mL treatment. The LDH releasement test and MDA tests showed no significant difference between algae treatment and control. It suggested that K. mikimotoi might not cause significant cytolysis and oxidative damage to rotifers, but may cause mortality by inhibiting the activity of crucial enzymes, which may lead to cell permeability disorder and body shrinkage.

Evidence for Production of Sexual Resting Cysts by the Toxic Dinoflagellate Karenia mikimotoi in Clonal Cultures and Marine Sediments

The toxic dinoflagellate Karenia mikimotoi has been well-known for causing large-scale and dense harmful algal blooms (HABs) in coastal waters worldwide and serious economic loss in aquaculture and fisheries and other adverse effects on marine ecosystems. Whether K. mikimotoi forms resting cysts has been a puzzling issue regarding to the mechanisms of bloom initiation and geographic expansion of this species. We provide morphological and molecular confirmation of sexually produced thin-walled resting cysts by K. mikimotoi based on observations of laboratory cultures and their direct detection in marine sediments. Light and scanning electron microscopy evidences for sexual reproduction include attraction and pairing of gametes, gamete fusion, formation of planozygote and thin-walled cyst, and the documentation of the thin-walled cyst germination processes. Evidence for cysts in marine sediments was in three aspects: positive PCR detection of cysts using species-specific primers in the DNA extracted from whole sediments; fluorescence in situ hybridization detection of cysts using FISH probes; and single-cell PCR sequencing for cysts positively labeled with FISH probes. The existence of sexually produced, thin-walled resting cysts by K. mikimotoi provides a possible mechanism accounting for the initiation of annually recurring blooms at certain regions and global expansion of the species during the past decades.

Molecular method for rapid detection of the red tide dinoflagellate Karenia mikimotoi in the coastal region of Xiangshan Bay, China

The species Karenia mikimotoi is a common nearshore red tide alga that can secrete hemolytic exotoxin and ichthyotoxin, which can induce the death of fish and shellfish, causing severe economic losses. In this study, loop-mediated isothermal amplification (LAMP) was employed in combination with the lateral flow dipstick (LFD) visual detection method to establish the LAMP-LFD rapid detection method for K. mikimotoi. The internal transcribed spacer ITS1-5.8S-ITS2 of K. mikimotoi was used as the target sequence and was amplified with specific primers designed in this study. The results indicated that the amplification optimal reaction conditions for LAMP in this paper were for 20 min at 65 °C. Moreover, LAMP had excellent specificity, showing negative results for other common red tide causing algal species. In field samples, we successfully reduced the total time, with only 23 min needed from LAMP amplification to LFD result display, which was shorter than that of conventional PCR. Consequently, LAMP-LFD should be useful for rapid field detection of low-density K. mikimotoi and for the early prevention of red tide induced by such algae.

Is Karenia brevis really a low-light-adapted species?

Despite nearly annual blooms of the neurotoxic dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup in the Gulf of Mexico, defining the suite of biological traits that explain its proliferation has remained challenging. Studies have described K. brevis as a low-light-adapted species, incapable of sustaining growth under high light, which is at odds with observed surface aggregations sometimes within centimeters of the sea surface and also with short-term experiments showing photosynthetic machinery accommodating high irradiances. Here, growth and photophysiology of three K. brevis isolates were evaluated under a range of environmentally relevant irradiances (10-1500 μmol photons m^-2 s^-1) in the laboratory. No differences in growth-irradiance curves were observed among isolates; all sustained maximum growth rates at the highest irradiances examined, even in exposures as long as three weeks. The growth efficiency α of K. brevis under light-limiting conditions appeared mediocre among dinoflagellates, and poorer than that of other phytoplankton (e.g., diatoms, cyanobacteria), implying that K. brevis is not a low-light specialist. This finding substantially alters earlier parameterizations of K. brevis growth-irradiance curves. Therefore, a model was developed to contextualize how these new growth-irradiance curves might affect bottom growth rates. This model was subsequently applied to a case study comparing seasonal light forcing offshore of Pinellas County, FL, USA, with a single empirical value for light attenuation, and seasonal bottom water temperatures. Predictions suggested that light may limit bottom growth as close as 1 km from shore in winter, but would only begin limiting growth 20 km from shore in summer. Population maintenance (no net growth) was possible as far offshore as 90 km in summer and 68 km in winter. These ranges intercept areas thought to be important for bloom initiation.

A review of karenia mikimotoi: Bloom events, physiology, toxicity and toxic mechanism

Karenia mikimotoi is a worldwide bloom-forming dinoflagellate in the genus Karenia. Blooms of this alga have been observed since the 1930s and have caused mass mortalities of fish, shellfish, and other invertebrates in the coastal waters of many countries, including Japan, Norway, Ireland, and New Zealand. This species has frequently bloomed in China, causing great financial losses (more than 2 billion yuan, Fujian Province, 2012). K. mikimotoi can adapt to various light, temperature, salinity, and nutrient conditions, which together with its complex life history, strong motility, and density-dependent allelopathy, allows it to form blooms that are lethal to almost all marine organisms. However, its toxicity differs between subspecies and some target-species-specific toxicity has also been recorded. Significant gill disorder is observed in affected fish, to which the massive fish kills are attributed, rather than to the hypoxia that occurs in the fading stage of a bloom. However, although this species is haemolytic and cytotoxic, and generates reactive oxygen species, none of the isolated toxins or lipophilic extracts have toxic effects as extreme as those of the intact algal cells. The toxic effects of K. mikimotoi are strongly related to contact with intact cells. Several reasonable hypotheses of how and why this species blooms and causes mass mortalities have been proposed, but further research is required.