The toxic dinoflagellate Alexandrium minutum impairs the performance of oyster embryos and larvae

Multi-stressor dynamics: Effects of marine heatwave stress and harmful algal blooms on juvenile mussel (Perna canaliculus) survival and physiology

In New Zealand, the frequency and intensity of marine heatwaves (MHWs) and blooms of the harmful algal species, Alexandrium pacificum, are increasing in areas where there are natural reefs and commercial farms of the mussel, Perna canaliculus. In this study, we assessed the whole organism, tissue and molecular-level response of juvenile (spat) P. canaliculus exposed to these abiotic and biotic stressors, alone and together. Spat of P. canaliculus were held at a control temperature of 17 °C or a MHW temperature of 22 °C for the duration of the experiment and exposed to two environmentally relevant concentrations of the dinoflagellate A. pacificum for 4 days, followed by a recovery period of 4 days. The MHW temperature alone had no detrimental effect on mussel survival; however, exposure to A. pacificum at both temperatures led to reduced spat survival, especially at higher microalgal concentrations (< 90 % survival vs 100 % at the control temperature, no A. pacificum). The combination of both MHW temperature and A. pacificum exposure resulted in higher mortality than either stressor alone (as low as 59 % survival compared to 99 % at the MHW temperature alone). Spat exposed to A. pacificum alone showed up to a 4-fold reduction in byssal plaque production, crucial for spat attachment and therefore survival. Growth rate was >7.5 times slower, and there were increased histological alterations as compared to mussels in the control treatment. Similarly, spat exposed to A. pacificum showed significant changes in gene expression, particularly in pathways related to lipid metabolism and detoxification. Spat exposed to MHW temperatures alone had fewer differentially expressed genes, most being heat shock proteins. These findings emphasise the importance of understanding multi-stressor impacts in marine environments, particularly in the context of climate change and harmful algal blooms (HABs).

Effects of the toxic dinoflagellate, Alexandrium pacificum, on the marine diatom, Chaetoceros muelleri, and mussel (Perna canaliculus) sperm and hemocytes

Harmful algal blooms (HABs) of Alexandrium pacificum have affected the Marlborough Sounds in New Zealand since 2010, posing a threat to green-lipped mussel (GLM, Perna canaliculus) farming. Previous studies have shown A. pacificum has negative effects GLM embryos and larvae. To further investigate these toxic mechanisms, in vitro bioassays were conducted on GLM spermatozoa, hemocytes, and the diatom, Chaetoceros muelleri. The three cell types were exposed to several treatments of A. pacificum for 2 h and responses were measured using flow cytometry and pulse amplitude-modulated fluorometry. Significant spermatozoa mortality was recorded in treatments containing A. pacificum cells or fragments, while hemocyte and C. muelleri mortality was recorded in cell-free treatments of A. pacificum which contained paralytic shellfish toxins (PSTs). Variation in sensitivity between cell types as well as the sublethal effects observed, emphasise the diverse toxic mechanisms of A. pacificum on co-occurring species in the environment.

Effects of the toxic dinoflagellate Protoceratium reticulatum and its yessotoxins on the survival and feed ingestion of Argopecten purpuratus veliger larvae

The effects of yessotoxins (YTXs) produced by the dinoflagellate Protoceratium reticulatum in the early stages of bivalves have not been studied in detail. The present study evaluates the effects of P. reticulatum and YTXs on the survival and feed ingestion of veliger larvae of Argopecten purpuratus. Larvae were 96 h-exposed to 500, 1000 and 2000 P. reticulatum cells mL-1, and their equivalent YTX extract was prepared in methanol. Results show a survival mean of 82 % at the highest density of dinoflagellate, and 38 % for larvae with the highest amount of YTX extract. Feed ingestion is reduced in the dinoflagellate exposure treatments as a function of cell density. Therefore, the effect of YTXs on A. purpuratus represents a new and important area of study for investigations into the deleterious effects of these toxins in the early stages of the life cycle of this and, potentially, other bivalves.

The toxic dinoflagellate Alexandrium catenella adversely affects early life stages of tehuelche scallop

The effects of the toxic dinoflagellate Alexandrium catenella were investigated on growth, survival, and histopathology in larvae and spat of the Tehuelche scallop Aequipecten tehuelchus from Patagonia, Argentina. The study consisted of laboratory incubations of scallop larvae/spat with A. catenella, using environmentally realistic abundances of the dinoflagellate. Survival, growth, and histopathological effects were documented for scallop larvae/spat before, during, and after 7-day-long exposure to A. catenella. The scallops were grouped in flasks containing 0 (control), 20, 200, and 2000 cells mL-1 of A. catenella. The presence of A. catenella induced reduced larvae survival after 24 h, whereas a clear effect was observed after 3 days (survival of control larvae 95%, 72, and 79% for 20 and 200 cells mL-1, respectively, and 43% for 2000 cells mL-1). The growth rates of the control larvae and those exposed to 20 mL-1 cells were significantly different from zero. Histopathological effects (melanization, loss of connective tissue, necrosis, and inflammatory responses) were observed in spat exposed to A. catenella. These effects were more pronounced at the highest dinoflagellate concentration. Blooms of A. catenella frequently coincide with the reproductive season of A. tehuelchus, thus there is a need to further study the relationship between harmful algal blooms and the effect on scallops' natural populations in the region.

Exposure to Alexandrium spp. impairs the development of Green-lipped mussel (Perna canaliculus) embryos and larvae

The green-lipped mussel (GLM) Perna canaliculus is an economically, ecologically, and culturally important species in Aotearoa New Zealand. Since 2011, harmful algal blooms (HABs) of Alexandrium spp. have occurred annually in the Marlborough Sounds, the largest GLM aquaculture region in New Zealand. Across a similar timeframe, there has been a severe reduction in wild spat (juvenile mussel) catch. This research investigated the effects of Alexandrium pacificum (which produces paralytic shellfish toxins; PSTs) and A. minutum (a non-producer of PSTs) on the development of four GLM larval life stages (gametes, embryos, D-stage and settlement). Early life stages of GLM were exposed to environmentally relevant concentrations of Alexandrium spp. as whole cell, lysate and filtrate treatments. A 48-h exposure of embryos to whole A. pacificum cells at 500 cells mL-1 caused lysis of embryos, severe abnormalities, and reduced development through to veliger (D-stage) larvae by 85%. GLM growth was impaired at cell concentrations as low as 250 cells mL-1 during a 4-day exposure of D-stage larvae to both Alexandrium spp. Exposure of GLM to both whole and lysed treatments of Alexandrium spp. at 500 cells mL-1 resulted in halved larval growth rates (2.00 µm day-1 vs 4.48 µm day-1 in the control) and growth remained impeded during a 4-day recovery period. Both A. pacificum and A. minutum were found to negatively impact D-larvae. Both whole-cell and lysed-cell treatments of A. pacificum had similar negative effects, suggesting that Alexandrium spp. toxicity to D-larvae is independent of PSTs. Additionally, cell membrane-free treatments of A. pacificum had no negative effects on embryo development, indicating that cell surface-associated bioactive compounds may be responsible for the observed negative effects during this early life stage. Conversely, non-PST-producing A. minutum was toxic to D-stage larvae but not to embryos; larval growth was reduced following a brief 1 h exposure of sperm to cell membrane-free treatments of A. pacificum. No effects were recorded in GLM larvae exposed during settlement, highlighting the potential for differences in susceptibility of early life stages to Alexandrium spp. exposure and the influence of exposure durations. In the wild, blooms of Alexandrium spp. can persist for several months, reaching cell densities higher than those investigated in the present study, and as such may be detrimental to the vulnerable early life stages of GLM.

Rising CO2 will increase toxicity of marine dinoflagellate Alexandrium minutum

Ocean acidification caused by increasing emission of carbon dioxide (CO₂) is expected to have profound impacts on marine ecological processes, including the formation and evolution of harmful algal blooms (HABs). We designed a set of experiments in the laboratory to examine the effects of increasing CO₂ on the growth and toxicity of a toxic dinoflagellate Alexandrium minutum producing paralytic shellfish toxins (PSTs). It was found that high levels of CO₂ (800 and 1200 ppm) significantly promoted the growth of A. minutum compared to the group (400 ppm) representing the current CO₂ level. The total yields of PSTs by A. minutum, including both intracellular and extracellular toxins, were significantly enhanced, probably due to the induction of core enzyme activity and key amino acids synthesis for PST production. More interestingly, high level of CO₂ promoted the transformation from gonyautoxin2&3 to gonyautoxin1&4 and depressed the release of PSTs from inside to outside of the cells. All these processes collectively led to an apparent increase of A. minutum toxicity. Our study demonstrated that rising CO₂ would increase the risk of toxic A. minutum based on the comprehensive analyses of different processes including algal growth and toxin synthesis, transformation and release.

Effects of Harmful Algal Blooms on Fish and Shellfish Species: A Case Study of New Zealand in a Changing Environment

Harmful algal blooms (HABs) have wide-ranging environmental impacts, including on aquatic species of social and commercial importance. In New Zealand (NZ), strategic growth of the aquaculture industry could be adversely affected by the occurrence of HABs. This review examines HAB species which are known to bloom both globally and in NZ and their effects on commercially important shellfish and fish species. Blooms of Karenia spp. have frequently been associated with mortalities of both fish and shellfish in NZ and the sub-lethal effects of other genera, notably Alexandrium spp., on shellfish (which includes paralysis, a lack of byssus production, and reduced growth) are also of concern. Climate change and anthropogenic impacts may alter HAB population structure and dynamics, as well as the physiological responses of fish and shellfish, potentially further compromising aquatic species. Those HAB species which have been detected in NZ and have the potential to bloom and harm marine life in the future are also discussed. The use of environmental DNA (eDNA) and relevant bioassays are practical tools which enable early detection of novel, problem HAB species and rapid toxin/HAB screening, and new data from HAB monitoring of aquaculture production sites using eDNA are presented. As aquaculture grows to supply a sizable proportion of the world’s protein, the effects of HABs in reducing productivity is of increasing significance. Research into the multiple stressor effects of climate change and HABs on cultured species and using local, recent, HAB strains is needed to accurately assess effects and inform stock management strategies.

Effects of Two Toxin-Producing Harmful Algae, Alexandrium catenella and Dinophysis acuminata (Dinophyceae), on Activity and Mortality of Larval Shellfish

Harmful algal bloom (HAB) species Alexandrium catenella and Dinophysis acuminata are associated with paralytic shellfish poisoning (PSP) and diarrhetic shellfish poisoning (DSP) in humans, respectively. While PSP and DSP have been studied extensively, less is known about the effects of these HAB species or their associated toxins on shellfish. This study investigated A. catenella and D. acuminata toxicity in a larval oyster (Crassostrea virginica) bioassay. Larval activity and mortality were examined through 96-h laboratory exposures to live HAB cells (10−1000 cells/mL), cell lysates (1000 cells/mL equivalents), and purified toxins (10,000 cells/mL equivalents). Exposure to 1000 cells/mL live or lysed D. acuminata caused larval mortality (21.9 ± 7.0%, 10.2 ± 4.0%, respectively) while exposure to any tested cell concentration of live A. catenella, but not lysate, caused swimming arrest and/or mortality in >50% of larvae. Exposure to high concentrations of saxitoxin (STX) or okadaic acid (OA), toxins traditionally associated with PSP and DSP, respectively, had no effect on larval activity or mortality. In contrast, pectenotoxin-2 (PTX2) caused rapid larval mortality (49.6 ± 5.8% by 48 h) and completely immobilized larval oysters. The results indicate that the toxic effects of A. catenella and D. acuminata on shellfish are not linked to the primary toxins associated with PSP and DSP in humans, and that PTX2 is acutely toxic to larval oysters.

Unknown Extracellular and Bioactive Metabolites of the Genus Alexandrium: A Review of Overlooked Toxins

Various species of Alexandrium can produce a number of bioactive compounds, e.g., paralytic shellfish toxins (PSTs), spirolides, gymnodimines, goniodomins, and also uncharacterised bioactive extracellular compounds (BECs). The latter metabolites are released into the environment and affect a large range of organisms (from protists to fishes and mammalian cell lines). These compounds mediate allelochemical interactions, have anti-grazing and anti-parasitic activities, and have a potentially strong structuring role for the dynamic of Alexandrium blooms. In many studies evaluating the effects of Alexandrium on marine organisms, only the classical toxins were reported and the involvement of BECs was not considered. A lack of information on the presence/absence of BECs in experimental strains is likely the cause of contrasting results in the literature that render impossible a distinction between PSTs and BECs effects. We review the knowledge on Alexandrium BEC, (i.e., producing species, target cells, physiological effects, detection methods and molecular candidates). Overall, we highlight the need to identify the nature of Alexandrium BECs and urge further research on the chemical interactions according to their ecological importance in the planktonic chemical warfare and due to their potential collateral damage to a wide range of organisms.

The toxic dinoflagellate Alexandrium minutum affects oyster gamete health and fertilization potential

Dinoflagellates from the globally distributed genus Alexandrium are known to produce both paralytic shellfish toxins (PST) and uncharacterized bioactive extracellular compounds (BEC) with allelopathic, ichthyotoxic, hemolytic and cytotoxic activities. In France, blooms of Alexandrium minutum appear generally during the spawning period of most bivalves. These blooms could therefore alter gametes and/or larval development of bivalves, causing severe issues for ecologically and economically important species, such as the Pacific oyster Crassostrea (=Magallana) gigas. The aim of this work was to test the effects of three strains of A. minutum producing either only PST, only BEC, or both PST and BEC upon oyster gametes, and potential consequences on fertilization success. Oocytes and spermatozoa were exposed in vitro for 2 h to a range of environmentally realistic A. minutum concentrations (10-2.5 × 10⁴ cells mL^-1). Following exposure, gamete viability and reactive oxygen species (ROS) production were assessed by flow cytometry, spermatozoa motility and fertilization capacities of both spermatozoa and oocytes were analysed by microscopy. Viability and fertilization capacity of spermatozoa and oocytes were drastically reduced following exposure to 2.5 × 10⁴ cells mL^-1 of A. minutum. The BEC-producing strain was the most potent strain decreasing spermatozoa motility, increasing ROS production of oocytes, and decreasing fertilization, from the concentration of 2.5 × 10³ cells mL^-1. This study highlights the significant cellular toxicity of the BEC produced by A. minutum on oyster gametes. Physical contact between gametes and motile thecate A. minutum cells may also contribute to alter oyster gamete integrity. These results suggest that oyster gametes exposure to A. minutum blooms could affect oyster fertility and reproduction success.

Allelochemicals of Alexandrium minutum: Kinetics of membrane disruption and photosynthesis inhibition in a co-occurring diatom

Allelopathy is an efficient strategy by which some microalgae can outcompete other species. Allelochemicals from the toxic dinoflagellate Alexandrium minutum have deleterious effects on diatoms, inhibiting metabolism and photosynthesis and therefore give a competitive advantage to the dinoflagellate. The precise mechanisms of allelochemical interactions and the molecular target of allelochemicals remain however unknown. To understand the mechanisms, the short-term effects of A. minutum allelochemicals on the physiology of the diatom Chaetoceros muelleri were investigated. The effects of a culture filtrate were measured on the diatom cytoplasmic membrane integrity (polarity and permeability) using flow-cytometry and on the photosynthetic performance using fluorescence and absorption spectroscopy. Within 10 min, the unknown allelochemicals induced a depolarization of the cytoplasmic membranes and an impairment of photosynthesis through the inhibition of the plastoquinone-mediated electron transfer between photosystem II and cytochrome b₆f. At longer time of exposure, the cytoplasmic membranes were permeable and the integrity of photosystems I, II and cytochrome b₆f was compromised. Our demonstration of the essential role of membranes in this allelochemical interaction provides new insights for the elucidation of the nature of the allelochemicals. The relationship between cytoplasmic membranes and the inhibition of the photosynthetic electron transfer remains however unclear and warrants further investigation.

Marine invertebrate interactions with Harmful Algal Blooms - Implications for One Health

Harmful Algal Blooms (HAB) are natural atypical proliferations of micro or macro algae in either marine or freshwater environments which have significant impacts on human, animal and ecosystem health. The causative HAB organisms are primarily dinoflagellates and diatoms in marine and cyanobacteria within freshwater ecosystems. Several hundred species of HABs, most commonly marine dinoflagellates affect animal and ecosystem health either directly through physical, chemical or biological impacts on surrounding organisms or indirectly through production of algal toxins which transfer through lower-level trophic organisms to higher level predators. Traditionally, a major focus of HABs has concerned their natural production of toxins which bioaccumulate in filter-feeding invertebrates, which with subsequent trophic transfer and biomagnification cause issues throughout the food web, including the human health of seafood consumers. Whilst in many regions of the world, regulations, monitoring and risk management strategies help mitigate against the impacts from HAB/invertebrate toxins upon human health, there is ever-expanding evidence describing enormous impacts upon invertebrate health, as well as the health of higher trophic level organisms and marine ecosystems. This paper provides an overview of HABs and their relationships with aquatic invertebrates, together with a review of their combined impacts on animal, human and ecosystem health. With HAB/invertebrate outbreaks expected in some regions at higher frequency and intensity in the coming decades, we discuss the needs for new science, multi-disciplinary assessment and communication which will be essential for ensuring a continued increasing supply of aquaculture foodstuffs for further generations.

Establish axenic cultures of armored and unarmored marine dinoflagellate species using density separation, antibacterial treatments and stepwise dilution selection

Academic research on dinoflagellate, the primary causative agent of harmful algal blooms (HABs), is often hindered by the coexistence with bacteria in laboratory cultures. The development of axenic dinoflagellate cultures is challenging and no universally accepted method suit for different algal species. In this study, we demonstrated a promising approach combined density gradient centrifugation, antibiotic treatment, and serial dilution to generate axenic cultures of Karenia mikimotoi (KMHK). Density gradient centrifugation and antibiotic treatments reduced the bacterial population from 5.79 ± 0.22 log₁₀ CFU/mL to 1.13 ± 0.07 log₁₀ CFU/mL. The treated KMHK cells were rendered axenic through serial dilution, and algal cells in different dilutions with the absence of unculturable bacteria were isolated. Axenicity was verified through bacterial (16S) and fungal internal transcribed spacer (ITS) sequencing and DAPI epifluorescence microscopy. Axenic KMHK culture regrew from 1000 to 9408 cells/mL in 7 days, comparable with a normal culture. The established methodology was validated with other dinoflagellate, Alexandrium tamarense (AT6) and successfully obtained the axenic culture. The axenic status of both cultures was maintained more than 30 generations without antibiotics. This efficient, straightforward and inexpensive approach suits for both armored and unarmored dinoflagellate species.

Cultures of Dinophysis sacculus, D. acuminata and pectenotoxin 2 affect gametes and fertilization success of the Pacific oyster, Crassostrea gigas

Harmful algal blooms (HABs) of toxic species of the dinoflagellate genus Dinophysis are a threat to human health as they are mainly responsible for diarrheic shellfish poisoning (DSP) in the consumers of contaminated shellfish. Such contamination leads to shellfish farm closures causing major economic and social issues. The direct effects of numerous HAB species have been demonstrated on adult bivalves, whereas the effects on critical early life stages remain relatively unexplored. The present study aimed to determine the in vitro effects of either cultivated strains of D. sacculus and D. acuminata isolated from France or their associated toxins (i.e. okadaic acid (OA) and pectenotoxin 2 (PTX2)) on the quality of the gametes of the Pacific oyster Crassostrea gigas. This was performed by assessing the ROS production and viability of the gametes using flow cytometry, and fertilization success using microscopic counts. Oocytes were more affected than spermatozoa and their mortality and ROS production increased in the presence of D. sacculus and PTX2, respectively. A decrease in fertilization success was observed at concentrations as low as 0.5 cell mL^-1 of Dinophysis spp. and 5 nM of PTX2, whereas no effect of OA could be observed. The effect on fertilization success was higher when both gamete types were concomitantly exposed compared to separate exposures, suggesting a synergistic effect. Our results also suggest that the effects could be due to cell-to-cell contact. These results highlight a potential effect of Dinophysis spp. and PTX2 on reproduction and recruitment of the Pacific oyster.

Effects of marine harmful algal blooms on bivalve cellular immunity and infectious diseases: A review

Bivalves were long thought to be "symptomless carriers" of marine microalgal toxins to human seafood consumers. In the past three decades, science has come to recognize that harmful algae and their toxins can be harmful to grazers, including bivalves. Indeed, studies have shown conclusively that some microalgal toxins function as active grazing deterrents. When responding to marine Harmful Algal Bloom (HAB) events, bivalves can reject toxic cells to minimize toxin and bioactive extracellular compound (BEC) exposure, or ingest and digest cells, incorporating nutritional components and toxins. Several studies have reported modulation of bivalve hemocyte variables in response to HAB exposure. Hemocytes are specialized cells involved in many functions in bivalves, particularly in immunological defense mechanisms. Hemocytes protect tissues by engulfing or encapsulating living pathogens and repair tissue damage caused by injury, poisoning, and infections through inflammatory processes. The effects of HAB exposure observed on bivalve cellular immune variables have raised the question of possible effects on susceptibility to infectious disease. As science has described a previously unrecognized diversity in microalgal bioactive substances, and also found a growing list of infectious diseases in bivalves, episodic reports of interactions between harmful algae and disease in bivalves have been published. Only recently, studies directed to understand the physiological and metabolic bases of these interactions have been undertaken. This review compiles evidence from studies of harmful algal effects upon bivalve shellfish that establishes a framework for recent efforts to understand how harmful algae can alter infectious disease, and particularly the fundamental role of cellular immunity, in modulating these interactions. Experimental studies reviewed here indicate that HABs can modulate bivalve-pathogen interactions in various ways, either by increasing bivalve susceptibility to disease or conversely by lessening infection proliferation or transmission. Alteration of immune defense and global physiological distress caused by HAB exposure have been the most frequent reasons identified for these effects on disease. Only few studies, however, have addressed these effects so far and a general pattern cannot be established. Other mechanisms are likely involved but are under-studied thus far and will need more attention in the future. In particular, the inhibition of bivalve filtration by HABs and direct interaction between HABs and infectious agents in the seawater likely interfere with pathogen transmission. The study of these interactions in the field and at the population level also are needed to establish the ecological and economical significance of the effects of HABs upon bivalve diseases. A more thorough understanding of these interactions will assist in development of more effective management of bivalve shellfisheries and aquaculture in oceans subjected to increasing HAB and disease pressures.