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

A comparison between the FlowCam 8100, microscopy, and sandwich hybridization assay for quantifying abundances of the saxitoxin-producing dinoflagellate, Alexandrium catenella

Light microscopy, FlowCam, and sandwich hybridization assay (SHA) are three approaches that facilitate the monitoring of harmful algal bloom (HAB) forming phytoplankton. Yet, cross-comparisons among these techniques have not been conducted. This study addressed that gap using the saxitoxin-producing 'red tide' dinoflagellate Alexandrium catenella, a species responsible for blooms and paralytic shellfish poisoning worldwide. To achieve this goal, the dynamic ranges of each technique were compared using A. catenella cultures spanning low (pre-bloom), moderate (bloom), and high (dense bloom) levels. To assess field detection, water samples containing very low (<3 cells mL^-1) A. catenella levels were collected from Long Island Sound, USA (Jun-Aug 2021) and evaluated using each method. Field samples were also spiked with A. catenella to high (160 cells mL^-1) or low (40 cells mL^-1) concentrations. In general, microscopy, FlowCam, and SHA returned comparable A. catenella cell concentrations for all tests. Mean cell concentrations from laboratory intercalibration experiments were not significantly different for any method or concentration (ANOVA, p > 0.05). However, relative to microscopy at times SHA produced non-detect signals <2 cells mL^-1 in field samples and the FlowCam slightly underestimated cell concentrations when A. catenella abundances were high in laboratory and field samples. Mean cell concentrations of spike experiments were not significantly different for any test date, sampling location, or method, despite variability among methods within the high concentration treatment (ANOVA, p > 0.05 for all treatments). Findings are relevant to HAB researchers, managers, and public health officials because they help reconcile disparate cell abundance datasets that inform numerical models and enhance HAB monitoring and prediction. Results are also likely broadly applicable to several HAB species.

Bloom development of toxic dinoflagellate Alexandrium catenella (Group I) in Jinhae-Masan Bay, Korea: Germination strategy of resting cysts in relation to temperature and salinity

To better understand the role of resting cysts in the outbreak of paralytic shellfish poisoning and bloom dynamics in Jinhae-Masan Bay, Korea, this study investigated the germination features of ellipsoidal Alexandrium cysts isolated from sediments collected in winter and summer under different combinations of temperature and salinity. Morphology and phylogeny of germling cells revealed that the ellipsoidal Alexandrium cysts belong to Alexandrium catenella (Group I). The cysts could germinate across a wide range of temperature (5-25 °C) with germination success within 5 days, indicating that continuous seeding for the maintenance of vegetative cells in the water column may occur through the year without an endogenous clock to regulate germination timing. In addition, the cyst germination of A. catenella (Group I) was not controlled by seasonal salinity changes. Based on the results, this study provides a schematic scenario of the bloom development of A. catenella (Group I) in Jinhae-Masan Bay, Korea.

SoundToxins: A Research and Monitoring Partnership for Harmful Phytoplankton in Washington State

The more frequent occurrence of marine harmful algal blooms (HABs) and recent problems with newly-described toxins in Puget Sound have increased the risk for illness and have negatively impacted sustainable access to shellfish in Washington State. Marine toxins that affect safe shellfish harvest because of their impact on human health are the saxitoxins that cause paralytic shellfish poisoning (PSP), domoic acid that causes amnesic shellfish poisoning (ASP), diarrhetic shellfish toxins that cause diarrhetic shellfish poisoning (DSP) and the recent measurement of azaspiracids, known to cause azaspiracid poisoning (AZP), at low concentrations in Puget Sound shellfish. The flagellate, Heterosigma akashiwo, impacts the health and harvestability of aquacultured and wild salmon in Puget Sound. The more recently described flagellates that cause the illness or death of cultivated and wild shellfish, include Protoceratium reticulatum, known to produce yessotoxins, Akashiwo sanguinea and Phaeocystis globosa. This increased incidence of HABs, especially dinoflagellate HABs that are expected in increase with enhanced stratification linked to climate change, has necessitated the partnership of state regulatory programs with SoundToxins, the research, monitoring and early warning program for HABs in Puget Sound, that allows shellfish growers, Native tribes, environmental learning centers and citizens, to be the “eyes on the coast”. This partnership enables safe harvest of wholesome seafood for consumption in the region and helps to describe unusual events that impact the health of oceans, wildlife and humans.

Phycotoxins and marine annelids - A global review

Several species of microalgae can produce potent phycotoxins that negatively affect aquatic organisms and their consumers following different exposure routes, as well as toxicokinetic (TK) and toxicodynamic (TD) processes. Benthic organisms are especially vulnerable as they are exposed to both benthic and planktonic species causative of harmful algal blooms (HABs). While benthic algae can come into direct contact with annelids during substrate remobilization, planktonic cells can settle to the bottom mostly during senescence and/or encystment stages, and in shallow and calm waters. We performed a systematic, qualitative review of the literature on the phycotoxin TK and TD processes in marine annelids, summarizing the most relevant findings and general trends. Besides, by using innovative analytical/statistical approaches, we were able to detect patterns and gaps in the current literature, thus pointing to future research directions. We retrieved and analyzed studies involving diarrhetic shellfish toxins (DSTs), paralytic shellfish toxins (PSTs), brevetoxins (PbTXs), domoic acid (DA), as well as palytoxin and its congeners, the ovatoxins (treated together as PLTXs). It is worth mentioning that studies evaluating other phycotoxins (e.g., ciguatoxins, yessotoxins) were not found in the literature. The absence of data on PbTXs, PSTs and DA is the largest gap hampering TK assessment in annelids, although some relevant information on TD is already available. Whereas lethal effects from DSTs have not been reported, more potent toxins like PbTXs, PSTs, DA and those grouped as PLTX-like compounds can cause mortality and/or marked decrease in annelid abundance. In addition, phycotoxins have been linked to sublethal effects on annelid cells. Although very sparse, field and laboratory studies offer strong evidence that annelids may be reliable indicators of toxin exposure and their negative effects during both early and later stages of HABs in marine environments. Besides quickly responding to these compounds at both organismic and suborganismic levels, annelids are easily found in areas affected by HABs. The use of annelids in future investigations evaluating the action mechanisms of toxic microalgae on marine invertebrates should be thus encouraged. In this case, the choice for widely dispersed and numerically dominant species of annelids would strengthen the validation and extrapolation of results from risk assessments in areas affected by HABs worldwide.

Energy metabolism and genetic information processing mark major transitions in the life history of Scrippsiella acuminata (Dinophyceae)

Many dinoflagellates perform sexual reproduction and form cysts as a life history strategy to survive adverse environmental conditions and seed annual harmful algal blooms (HABs). The molecular mechanisms underpinning the life stage transitions can provide clues about how key environmental factors induce encystment and initiation of a HAB but are still poorly understood. Here, we conducted an integrated physiological and transcriptomic study to unravel the mechanisms in Scrippsiella acuminata. We established a culture from a bloom, induced cyst formation, and divided the process into four life stages. Transcriptomic analysis of these stages revealed 19,900 differentially expressed genes (DEGs). The expression of genes related to photosynthesis was significantly up-regulated from vegetative stage to immature cyst stage, consistent with the marked increase in cell contents of energy-storing macromolecules (carbohydrates and lipids). When proceeding to resting cysts, most photosynthesis genes were down-regulated while "genetic information processing" related genes were up-regulated. Comparing germinating cysts with resting cysts revealed 100 DEGs involved in energy metabolism, indicating a high energy requirement of germination. In addition, the transition from germinating cysts to vegetative cells featured up-regulation of photosynthesis. Our results demonstrate that energy storage and consumption play a pivotal role in cyst formation and germination respectively and genetic information processing is crucial in cyst dormancy.

Growing Degree-Day Measurement of Cyst Germination Rates in the Toxic Dinoflagellate Alexandrium catenella

Blooms of many dinoflagellates, including several harmful algal bloom (HAB) species, are seeded and revived through the germination of benthic resting cysts. Temperature is a key determinant of cysts’ germination rate, and temperature–germination rate relationships are therefore fundamental to understanding species’ germling cell production, cyst bed persistence, and resilience to climate warming. This study measured germination by cysts of the HAB dinoflagellate Alexandrium catenella using a growing degree-day (DD) approach that accounts for the time and intensity of warming above a critical temperature. Time courses of germination at different temperatures were fit to lognormal cumulative distribution functions for the estimation of the median days to germination. As temperature increased, germination times decreased hyperbolically. DD scaling collapsed variability in germination times between temperatures after cysts were oxygenated. A parallel experiment demonstrated stable temperature–rate relationships in cysts collected during different phases of seasonal temperature cycles in situ over three years. DD scaling of the results from prior A. catenella germination studies showed consistent differences between populations across a wide range of temperatures and suggests selective pressure for different germination rates. The DD model provides an elegant approach to quantify and compare the temperature dependency of germination among populations, between species, and in response to changing environmental conditions.

IMPORTANCE Germination by benthic life history stages is the first step of bloom initiation in many, diverse phytoplankton species. This study outlines a growing degree-day (DD) approach for comparing the temperature dependence of germination rates measured in different populations. Germination by cysts of Alexandrium catenella, a harmful algal bloom dinoflagellate that causes paralytic shellfish poisoning, is shown to require a defined amount of warming, measured in DD after cysts are aerated. Scaling by DD, the time integral of temperature difference from a critical threshold, enabled direct comparison of rates measured at different temperatures and in different studies.

The benthic toxic dinoflagellate Ostreopsis cf. ovata in the NW Mediterranean Sea: Relationship between sea surface temperature and bloom phenology

Blooms of the toxic benthic dinoflagellate Ostreopsis cf. ovata can induce ecological and human health issues in certain temperate areas. In order to prevent these negative effects, long-term monitoring studies of O. cf. ovata blooms have been conducted in several impacted areas to have a comprehensive understanding of bloom dynamics and efficient tools for risk management. O. cf. ovata blooms were monitored every summer (from mid-June to the end of August) on five identified sites in Larvotto beach (Monaco, NW Mediterranean Sea), between 2007 and 2019. This time-series represents one of the largest time-series in the world describing blooms of this species. Bloom phenological features (timing, duration, maximum cell abundance and growth rate), were found to be highly variable throughout the studied period, and were analyzed as a function of different hydroclimatic parameters, including sea surface temperature (SST). The highest net growth rates were related to temperatures ranging between 21°C and 25°C, and did not coincide with maximal temperature records (27.5°C). Such results suggest that, although global warming possibly influences the expansion of O. cf. ovata from tropical to temperate waters, the definite impact of temperature on bloom dynamics might be more complex than a simple facilitation factor for algal growth, at least in NW Mediterranean waters. Furthermore, monthly SST anomalies calculated over this 13-year survey showed a strong positive correlation between spring SST positive anomalies and the bloom starting date, indicating that blooms occurred earlier in the season when spring SSTs were warmer than usual. Overall results provide tools to modelers and managers who are facing crucial challenges to predict the distribution and phenology of O. cf. ovata blooms in European coastal waters, moreover in a context of global warming.

Temporal and Spatial Signaling Mediating the Balance of the Plankton Microbiome

The annual patterns of plankton succession in the ocean determine ecological and biogeochemical cycles. The temporally fluctuating interplay between photosynthetic eukaryotes and the associated microbiota balances the composition of aquatic planktonic ecosystems. In addition to nutrients and abiotic factors, chemical signaling determines the outcome of interactions between phytoplankton and their associated microbiomes. Chemical mediators control essential processes, such as the development of key morphological, physiological, behavioral, and life-history traits during algal growth. These molecules thus impact species succession and community composition across time and space in processes that are highlighted in this review. We focus on spatial, seasonal, and physiological dynamics that occur during the early association of algae with bacteria, the exponential growth of a bloom, and its decline and recycling. We also discuss how patterns from field data and global surveys might be linked to the actions of metabolic markers in natural phytoplankton assemblages.

Evidence for massive and recurrent toxic blooms of Alexandrium catenella in the Alaskan Arctic

Among the organisms that spread into and flourish in Arctic waters with rising temperatures and sea ice loss are toxic algae, a group of harmful algal bloom species that produce potent biotoxins. Alexandrium catenella, a cyst-forming dinoflagellate that causes paralytic shellfish poisoning worldwide, has been a significant threat to human health in southeastern Alaska for centuries. It is known to be transported into Arctic regions in waters transiting northward through the Bering Strait, yet there is little recognition of this organism as a human health concern north of the Strait. Here, we describe an exceptionally large A. catenella benthic cyst bed and hydrographic conditions across the Chukchi Sea that support germination and development of recurrent, locally originating and self-seeding blooms. Two prominent cyst accumulation zones result from deposition promoted by weak circulation. Cyst concentrations are among the highest reported globally for this species, and the cyst bed is at least 6× larger in area than any other. These extraordinary accumulations are attributed to repeated inputs from advected southern blooms and to localized cyst formation and deposition. Over the past two decades, warming has likely increased the magnitude of the germination flux twofold and advanced the timing of cell inoculation into the euphotic zone by 20 d. Conditions are also now favorable for bloom development in surface waters. The region is poised to support annually recurrent A. catenella blooms that are massive in scale, posing a significant and worrisome threat to public and ecosystem health in Alaskan Arctic communities where economies are subsistence based.

Distribution of Alexandrium pacificum cysts in the area adjacent to the Changjiang River estuary, China

The coastal waters adjacent to the Changjiang River estuary (CRE) are characterized by nutrient pollution and recurrent harmful algal blooms. In this study, resting cysts of Alexandrium pacificum Litaker and A. catenella (Whedon & Kof.) Balech, two major species within the A. tamarense species complex in Chinese coastal waters, were studied using sediment samples collected from the area adjacent to the CRE in May 2014 and December 2015. Cysts were detected with two real-time quantitative PCR assays, as well as the primuline-staining method. Only cysts of A. pacificum were found in the study area, which mainly distributed in the mud depositional zone near the CRE. A low-abundance region of the cysts present in spring is in accordance with the intrusive pathway of the Nearshore Kuroshio Branch Current (NKBC), suggesting that A. pacificum blooms could be regulated by seasonal intrusion of NKBC.

Cyst-forming dinoflagellates in a warming climate

Many phytoplankton species, including many harmful algal bloom (HAB) species, survive long periods between blooms through formation of benthic resting stages. Because they are crucial to the persistence of these species and the initiation of new blooms, the physiology of benthic stages must be considered to accurately predict responses to climate warming and associated environmental changes. The benthic stages of dinoflagellates, called resting cysts, germinate in response to the combination of favorable temperature, oxygen-availability, and release from dormancy. The latter is a mechanism that prevents germination even when oxygen and temperature conditions are favorable. Here, evidence of temperature-mediated control of dormancy duration from the dinoflagellates Alexandrium catenella and Pyrodinium bahamense-two HAB species that cause paralytic shellfish poisoning (PSP)-is reviewed and presented alongside new evidence of complementary, temperature-based control of cyst quiescence (the state in which cysts germinate on exposure to favorable conditions). Interaction of the two temperature-based mechanisms with climate is explored through a simple model parameterized using results from recent experiments with A. catenella. Simulations demonstrate the importance of seasonal temperature cycles for the synchronization of cysts' release from dormancy and are consistent with biogeography-based inferences that A. catenella is more tolerant of warming in habitats that experience a larger range of seasonal temperature variation (i.e., have higher temperature seasonality). Temperature seasonality is much greater in shallow, long-residence time habitats than in deep, open-water ones. As warming shifts species' ranges, cyst beds may persist longer in more seasonally variable, shallow inshore habitats than in deep offshore ones, promoting HABs that are more localized and commence earlier each year. Recent field investigations of A. catenella also point to the importance of new cyst formation as a factor triggering bloom termination through mass sexual induction. In areas where temperature seasonality restricts the flux of new swimming cells (germlings) to narrow temporal windows, warming is unlikely to promote longer and more intense HAB impacts-even when water column conditions would otherwise promote prolonged bloom development. Many species likely have a strong drive to sexually differentiate and produce new cysts once concentrations reach levels that are conducive to new cyst formation. This phenomenon can impose a limit to bloom intensification and suggests an important role for cyst bed quiescence in determining the duration of HAB risk periods.

Environmental drivers of paralytic shellfish toxin producing Alexandrium catenella blooms in a fjord system of northern Southeast Alaska

Paralytic shellfish poisoning (PSP) is a persistent problem that threatens human health and the availability of shellfish resources in Alaska. Regular outbreaks of marine dinoflagellates in the genus Alexandrium produce paralytic shellfish toxins (PSTs) that make shellfish consumption unsafe, and impose economic hardships on Alaska's shellfish industry. Phytoplankton and environmental monitoring spanning 2008-2016, and a pilot benthic cyst survey in 2016, were focused in the Juneau region of Southeast Alaska to investigate Alexandrium catenella distributions and conditions favorable to bloom development. Overwintering Alexandrium cysts were found in near-shore sediments throughout the study region. Alexandrium catenella cells were present in the water column across a range of sea surface temperatures (7-15 °C) and surface salinities (S = 4-30); however, an optimal temperature/salinity window (10-13 °C, 18-23) supported highest cell concentrations. Measurable levels of PSTs were associated with lower concentrations (100 cells L^-1) of A. catenella, indicating high cell densities may not be required for shellfish toxicity to occur. Several interacting local factors were identified to support A. catenella blooms: 1) sea surface temperatures ≥7 °C; 2) increasing air temperature; 3) low to moderate freshwater discharge; and 4) several consecutive days of dry and calm weather. In combination, these bloom favorable conditions coincide with toxic bloom events during May and June in northern Southeast Alaska. These findings highlight how integrated environmental and phytoplankton monitoring can be used to enhance early warning capacity of toxic bloom events, providing more informed guidance to shellfish harvesters and resource managers in Alaska.

Temperature mediates secondary dormancy in resting cysts of Pyrodinium bahamense (Dinophyceae)

High-biomass blooms of the toxic dinoflagellate Pyrodinium bahamense occur most summers in Tampa Bay, Florida, USA, posing a recurring threat to ecosystem health. Like many dinoflagellates, P. bahamense forms immobile resting cysts that can be deposited on the seafloor-creating a seed bank that can retain the organism within the ecosystem and initiate future blooms when cysts germinate. In this study, we examined changes in the dormancy status of cysts collected from Tampa Bay and applied lessons from plant ecology to explore dormancy controls. Pyrodinium bahamense cysts incubated immediately after field collection displayed a seasonal pattern in dormancy and germination that matched the pattern of cell abundance in the water column. Newly deposited (surface) cysts and older (buried) cysts exhibited similar germination patterns, suggesting that a common mechanism regulates dormancy expression in new and mature cysts. Extended cool- and warm-temperature conditioning of field-collected cysts altered the cycle of dormancy compared with that of cysts in nature, with the duration of cool temperature exposure being the best predictor of when cysts emerged from dormancy. Extended warm conditioning, on the other hand, elicited a return to dormancy, or secondary dormancy, in nondormant cysts. These results directly demonstrate environmental induction of secondary dormancy in dinoflagellates-a mechanism common and thoroughly documented in higher plants with seasonal growth cycles. Our findings support the hypothesis that a seasonal cycle in cyst germination drives P. bahamense bloom periodicity in Tampa Bay and point to environmentally induced secondary dormancy as an important regulatory factor of that cycle.

Future temperature and salinity do not exert selection pressure on cyst germination of a toxic phytoplankton species

Environmental conditions regulate the germination of phytoplankton resting stages. While some factors lead to synchronous germination, others stimulate germination of only a small fraction of the resting stages. This suggests that habitat filters may act on the germination level and thus affect selection of blooming strains. Benthic "seed banks" of the toxic dinoflagellate Alexandrium ostenfeldii from the Baltic Sea are genetically and phenotypically diverse, indicating a high potential for adaptation by selection on standing genetic variation. Here, we experimentally tested the role of climate-related salinity and temperature as selection filters during germination and subsequent establishment of A. ostenfeldii strains. A representative resting cyst population was isolated from sediment samples, and germination and reciprocal transplantation experiments were carried out, including four treatments: Average present day germination conditions and three potential future conditions: high temperature, low salinity, and high temperature in combination with low salinity. We found that the final germination success of A. ostenfeldii resting cysts was unaffected by temperature and salinity in the range tested. A high germination success of more than 80% in all treatments indicates that strains are not selected by temperature and salinity during germination, but selection becomes more important shortly after germination, in the vegetative stage of the life cycle. Moreover, strains were not adapted to germination conditions. Instead, highly plastic responses occurred after transplantation and significantly higher growth rates were observed at higher temperature. High variability of strain-specific responses has probably masked the overall effect of the treatments, highlighting the importance of testing the effect of environmental factors on many strains. It is likely that A. ostenfeldii populations can persist in the future, because suitable strains, which are able to germinate and grow well at potential future climate conditions, are part of the highly diverse cyst population.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.c8c83nr.

qPCR Assays for the Detection and Quantification of Multiple Paralytic Shellfish Toxin-Producing Species of Alexandrium

Paralytic shellfish toxin producing dinoflagellates have negatively impacted the shellfish aquaculture industry worldwide, including in Australia and New Zealand. Morphologically identical cryptic species of dinoflagellates that may differ in toxicity, in particular, species of the former Alexandrium tamarense species complex, co-occur in Australia, as they do in multiple regions in Asia and Europe. To understand the dynamics and the ecological drivers of the growth of each species in the field, accurate quantification at the species level is crucial. We have developed the first quantitative polymerase chain reaction (qPCR) primers for A. australiense, and new primers targeting A. ostenfeldii, A. catenella, and A. pacificum. We showed that our new primers for A. pacificum are more specific than previously published primer pairs. These assays can be used to quantify planktonic cells and cysts in the water column and in sediment samples with limits of detection of 2 cells/L for the A. catenella and A. australiense assays, 2 cells/L and 1 cyst/mg sediment for the A. pacificum assay, and 1 cells/L for the A. ostenfeldii assay, and efficiencies of >90%. We utilized these assays to discriminate and quantify co-occurring A. catenella, A. pacificum, and A. australiense in samples from the east coast of Tasmania, Australia.

Environmental factors influencing the distribution and abundance of Alexandrium catenella in Kachemak bay and lower cook inlet, Alaska

Despite the long history of paralytic shellfish poisoning (PSP) events in Alaska, little is known about the seasonal distribution and abundance of the causative organism, Alexandrium, or the environmental factors that govern toxic bloom development. To address this issue, a five year study (2012-2017) was undertaken in Kachemak Bay and lower Cook Inlet Alaska to determine how the occurrence of Alexandrium catenella, the dominant PSP-causing Alexandrium species, was influenced by temperature, salinity, nutrient concentrations, and other environmental factors. Cell concentrations from 572 surface water samples were estimated using quantitative PCR. Monthly sampling revealed a seasonal pattern of A. catenella bloom development that was positively correlated with water temperature. Prevailing salinity conditions did not significantly affect abundance, nor was nutrient limitation a direct factor. Elevated cell concentrations were detected in 35 samples from Kachemak Bay (100-3050 cell eq. L^-1) while a maximum abundance of 67 cell eq. L^-1 was detected in samples from lower Cook Inlet sites. Monitoring data showed average water temperatures in Kachemak Bay increased by ∼2 °C over the course of the study and were accompanied by an increase in Alexandrium abundance. Based on these findings, 7-8 °C appears to represent a temperature threshold for significant bloom development in Kachemak Bay, with the greatest risk of shellfish toxicity occurring when temperatures exceed 10-12 °C. The role of temperature is further supported by time series data from the Alaska Coastal Current (station GAK1), which showed that summertime shellfish toxicity events in Kachemak Bay generally followed periods of anomalously high winter water temperatures. These data indicate monitoring changes in water temperatures may be used as an early warning signal for subsequent development of shellfish toxicity in Kachemak Bay.

What are the main environmental factors driving the development of the neurotoxic dinoflagellate Vulcanodinium rugosum in a Mediterranean ecosystem (Ingril lagoon, France)?

Vulcanodinium rugosum, a dinoflagellate developing in Ingril Lagoon (Mediterranean, France) is responsible for shellfish intoxications due to the neurotoxin pinnatoxin G. A one year survey (March 2012-April 2013) was conducted in this oligotrophic shallow lagoon and key environmental parameters were recorded (temperature, salinity and nutrients). The spatio-temporal distribution of V. rugosum in water column and on macrophytes was also determined. Planktonic cells of V. rugosum were observed at all sampling stations, but in relatively low concentrations (maximum of 1000 cell/L). The highest abundances were observed from June to September 2012. There was a positive correlation between cell densities and both temperature and salinity. Non-motile cells were detected on macrophytes, with a maximum concentration of 6300 cells/g wet weight. Nitrite and ammonium were negatively related to V. rugosum abundance whereas total nitrogen, total phosphorus and phosphates showed a positive correlation. Altogether, in situ results suggest that V. rugosum is rather thermophilic and that organic nutrients should be considered when studying the nutrition requirements for this noxious expanding dinoflagellate.

Life histories of microalgal species causing harmful blooms: Haploids, diploids and the relevance of benthic stages

In coastal and offshore waters, Harmful Algal Blooms (HABs) currently threaten the well-being of coastal countries. These events, which can be localized or involve wide-ranging areas, pose risks to human health, marine ecosystems, and economic resources, such as tourism, fisheries, and aquaculture. Dynamics of HABs vary from one site to another, depending on the hydrographic and ecological conditions. The challenge in investigating HABs is that they are caused by organisms from multiple algal classes, each with its own unique features, including different life histories. The complete algal life cycle has been determined in <1% of the described species, although elucidation of the life cycles of bloom-forming species is essential in developing preventative measures. The knowledge obtained thus far has confirmed the complexity of the algal life cycle, which is composed of discrete life stages whose morphology, ecological niche (plankton/benthos), function, and lifespan vary. The factors that trigger transitions between the different stages in nature are mostly unknown, but it is clear that an understanding of this process provides the key to effectively forecasting bloom recurrence, maintenance, and decline. Planktonic stages constitute an ephemeral phase of the life cycle of most species whereas resistant, benthic stages enable a species to withstand adverse conditions for prolonged periods, thus providing dormant reservoirs for eventual blooms and facilitating organismal dispersal. Here we review current knowledge of the life cycle strategies of major groups of HAB producers in marine and brackish waters. Rather than providing a comprehensive discussion, the objective was to highlight several of the research milestones that have changed our understanding of the plasticity and frequency of the different life cycle stages as well as the transitions between them. We also discuss the relevance of benthic and planktonic forms and their implications for HAB dynamics.

Life-history stages of natural bloom populations and the bloom dynamics of a tropical Asian ribotype of Alexandrium minutum

In 2015, a remarkably high density bloom of Alexandrium minutum occurred in Sungai Geting, a semi-enclosed lagoon situated in the northeast of Peninsular Malaysia, causing severe discoloration and contaminated the benthic clams (Polymesoda). Plankton and water samples were collected to investigate the mechanisms of bloom development of this toxic species. Analysis of bloom samples using flow cytometry indicated that the bloom was initiated by the process of active excystment, as planomycetes (>4C cells) were observed in the early stage of the bloom. Increase in planozygotes (2C cells) was evident during the middle stage of the bloom, coinciding with an abrupt decrease in salinity and increase of temperature. The bloom was sustained through the combination of binary division of vegetative cells, division of planozygotes, and cyst germination through continuous excystment. Nutrient depletion followed by precipitation subsequently caused the bloom to terminate. This study provides the first continuous record of in situ life-cycle stages of a natural bloom population of A. minutum through a complete bloom cycle. The event has provided a fundamental understanding of the pelagic life-cycle stages of this tropical dinoflagellate, and demonstrated a unique bloom development characteristic shared among toxic Alexandrium species in coastal embayments.

Role of resting cysts in Chilean Alexandrium catenella dinoflagellate blooms revisited

The detection of sparse Alexandrium catenella-resting cysts in sediments of southern Chilean fjords has cast doubts on their importance in the recurrence of massive toxic dinoflagellate blooms in the region. The role of resting cysts and the existence of different regional Chilean populations was studied by culturing and genetic approaches to define: (1) cyst production; (2) dormancy period; (3) excystment success; (4) offspring viability and (5) strain mating compatibility. This study newly revealed a short cyst dormancy (minimum 69 days), the role of key abiotic factors (in decreasing order salinity, irradiance, temperature and nutrients) controlling cyst germination (max. 60%) and germling growth rates (up to 0.36-0.52div.day^-1). Amplified fragment length polymorphism (AFLP) characterization showed significant differences in genetic distances (GD) among A. catenella populations that were primarily determined by the geographical origin of isolates and most likely driven by oceanographic dispersal barriers. A complex heterothallic mating system pointed to variable reproductive compatibility (RCs) among Chilean strains that was high among northern (Los Lagos/North Aysén) and southern populations (Magallanes), but limited among the genetically differentiated central (South Aysén) populations. Field cyst surveys after a massive 2009 bloom event revealed the existence of exceptional high cyst densities in particular areas of the fjords (max. 14.627cystscm^-3), which contrast with low cyst concentrations (<221.3cystscm^-3) detected by previous oceanographic campaigns. In conclusion, the present study suggests that A. catenella resting cysts play a more important role in the success of this species in Chilean fjords than previously thought. Results from in vitro experiments suggest that pelagic-benthic processes can maintain year-round low vegetative cell concentrations in the water column, but also can explain the detection of high cysts aggregations after the 2009-bloom event. Regional drivers that lead to massive outbreaks, however, are still unknown but potential scenarios are discussed.