Ostreopsis cf. ovata dynamics in the NW Mediterranean Sea in relation to biotic and abiotic factors

Harmful Ostreopsis cf. ovata blooms could extend in time span with climate change in the Western Mediterranean Sea

Fast environmental changes and high coastal human pressures and impacts threaten the Mediterranean Sea. Over the last decade, recurrent blooms of the harmful dinoflagellate Ostreopsis cf. ovata have been recorded in many Mediterranean beaches. These microalgae produce toxins that affect marine organisms and human health. Understanding the environmental conditions that influence the appearance and magnitude of O. cf. ovata blooms, as well as how climate change will modify its future distribution and dynamics, is crucial for predicting and managing their effects. This study investigates whether the spatio-temporal distribution of this microalga and the frequency of its blooms could be altered in future climate change scenarios in the Mediterranean Western basin. For the first time, an ecological habitat model (EHM) is forced by physico-chemical climate change simulations at high-resolution, under the strong greenhouse gas emission trajectory (RCP8.5). It allows to characterize how O. cf. ovata may respond to projected conditions and how its distribution could shift over a wide spatial scale, in this plausible future. Before being applied to the EHM, future climate simulations are further refined by using a statistical adaptation method (Cumulative Distribution Function transform) to improve the predictions robustness. Temperature (optimum 23-26 °C), high salinity (>38 psu) and high inorganic nutrient concentrations (nitrate >0.25 mmol N·m-3 and phosphate >0.035 mmol P·m-3) drive O. cf. ovata abundances. High spatial disparities in future abundances are observed. Namely, O. cf. ovata abundances could increase on the Mediterranean coasts of France, Spain and the Adriatic Sea while a decrease is expected in the Tyrrhenian Sea. The bloom period could be extended, starting earlier and continuing later in the year. From a methodological point of view, this study highlights best practices of EHMs in the context of climate change to identify sensitive areas for current and future harmful algal blooms.

From micro to mesoscale: Understanding the influence of macroalgal communities on Ostreopsis Schmidt blooms

Harmful Algal Blooms (HABs) are increasing in temperate areas, and the growth rates of benthic harmful dinoflagellates may be favoured in the context of global climate change. Benthic dinoflagellates, including species belonging to the Ostreopsis Schmidt genus, are known to develop on the surface of macroalgae and different macroalgal morphotypes and communities could host higher or lower cell abundances. The physical structure of the macroalgal substrate at the small scale (cm, microhabitat scale) and the structural complexity of the macroalgal community at the medium scale (few m, mesohabitat scale) could play a relevant role in bloom facilitation: the hypothesis that Ostreopsis species could be associated with macroalgal turfs and shrubs, structurally less complex communities than canopy-forming macroalgae, is especially under discussion and, if confirmed, could link bloom occurrence to regime shifts in temperate ecosystems. The present study, performed in two locations of the Ligurian Sea (Rochambeau, France and Vernazzola, Italy) aimed at understanding marine vegetation's role at the micro and mesohabitat scales in controlling the distribution and abundance of Ostreopsis. The abundance of the microalgal cells was quantified at different spatial scales, from cm to a few m, on different macroalgal species and communities, including artificial substrates, to tease apart the micro and mesohabitat effects. The results obtained show a high spatio-temporal variability, potentially hiding habitat-related patterns. The substrate's preferences diminish when cell abundances are very high, as in the case of Rochambeau, while in presence of moderate cell abundances as in Vernazzola or the first phases of blooms, it is possible to appreciate differences in abundances among substrates (in our study, Dictyota fasciola (Roth) Lamouroux supporting higher abundances). Our results open new research topics such as the study of blooms at a larger scale (macrohabitat) and testing different sampling methods to standardise the cells' abundances independently on the substrate.

Ecophysiological responses of Ostreopsis towards temperature: A case study of benthic HAB facing ocean warming

Reports of the benthic dinoflagellate Ostreopsis spp. have been increasing in the last decades, especially in temperate areas. In a context of global warming, evidences of the effects of increasing sea temperatures on its physiology and its distribution are still lacking and need to be investigated. In this study, the influence of temperature on growth, ecophysiology and toxicity was assessed for several strains of O. cf. siamensis from the Bay of Biscay (NE Atlantic) and O. cf. ovata from NW Mediterranean Sea. Cultures were acclimated to temperatures ranging from 14.5 °C to 32 °C in order to study the whole range of each strain-specific thermal niche. Acclimation was successful for temperatures ranging from 14.5 °C to 25 °C for O. cf. siamensis and from 19 °C to 32 °C for O. cf. ovata, with the highest growth rates measured at 22 °C (0.54-1.06 d-1) and 28 °C (0.52-0.75 d-1), respectively. The analysis of cellular content of pigments and lipids revealed some aspects of thermal acclimation processes in Ostreopsis cells. Specific capacities of O. cf. siamensis to cope with stress of cold temperatures were linked with the activation of a xanthophyll cycle based on diadinoxanthin. Lipids (neutral reserve lipids and polar ones) also revealed species-specific variations, with increases in cellular content noted under extreme temperature conditions. Variations in toxicity were assessed through the Artemia franciscana bioassay. For both species, a decrease in toxicity was observed when temperature dropped under the optimal temperature for growth. No PLTX-like compounds were detected in O. cf. siamensis strains. Thus, the main part of the lethal effect observed on A. franciscana was dependent on currently unknown compounds. From a multiclonal approach, this work allowed for defining specificities in the thermal niche and acclimation strategies of O. cf. siamensis and O. cf. ovata towards temperature. Potential impacts of climate change on the toxic risk associated with Ostreopsis blooms in both NW Mediterranean Sea and NE Atlantic coast is further discussed, taking into account variations in the geographic distribution, growth abilities and toxicity of each species.

Effects of culture conditions on the growth of the benthic dinoflagellates Ostreopsis cf. ovata, Prorocentrum lima and Coolia malayensis (Dinophyceae): A global review

Benthic dinoflagellates produce potent toxins that may negatively affect humans and the marine biota. Understanding the factors that stimulate their growth is important for management strategies and to reduce their potential negative impacts. Laboratory cultures have been extensively used to study microalgae physiology and characterize life cycles, nutrition, growth rates, among other processes. A systematic review of the literature on the growth parameters of the benthic dinoflagellates Ostreopsis cf. ovata, Prorocentrum lima species complex and Coolia malayensis obtained in laboratory cultures of strains isolated from all over the world was performed. The effects of temperature, light intensity, photoperiod, salinity and culture media on the growth rate of these species were evaluated using multiple regressions and a model selection approach, based on the Akaike Information Criteria (AIC). The potential effects of the initial culture abundance and the media volume used on the growth of the species were also assessed. Data from 50 articles (25 for O. cf. ovata, 21 for P. lima and 6 for C. malayensis), resulting in 399 growth parameter values (growth rate, doubling time and maximum yield) were compiled in a database. The genetic clades of O. cf. ovata and P. lima species complex were also noted. Growth rate was the most frequently reported growth parameter for the three species, and 127 values were retrieved for O. cf. ovata, 90 for P. lima and 56 for C. malayensis. Temperature was the factor that best explained the growth response of P. lima and C. malayensis, whereas for O. cf. ovata, temperature and salinity were equally important. Light intensity and photoperiod were included among the six best models for the studied species but presented a weaker effect on growth. Given the observed and future projected climate change, increasing ocean temperature will promote the growth of these species, likely leading to an expansion of their impacts on ecosystems and human health. The use of common garden experiments using multiple strains from different geographic domains, particularly addressing underrepresented lineages is recommended, as they will provide more balanced insight regarding the species physiological responses to environmental drivers.

Progress on the Link between Nutrient Availability and Toxin Production by Ostreopsis cf. ovata: Field and Laboratory Experiments

This study aimed to improve the understanding of the nutrient modulation of Ostreopsis cf. ovata toxin content. During the 2018 natural bloom in the NW Mediterranean, the total toxin content (up to ca. 57.6 ± 7.0 pg toxin cell−1) varied markedly. The highest values often coincided with elevated O. cf. ovata cell abundance and with low inorganic nutrient concentrations. The first culture experiment with a strain isolated from that bloom showed that cell toxin content was higher in the stationary than in the exponential phase of the cultures; phosphate- and nitrate-deficient cells exhibited similar cell toxin variability patterns. The second experiment with different conditions of nitrogen concentration and source (nitrate, urea, ammonium, and fertilizer) presented the highest cellular toxin content in the high-nitrogen cultures; among these, urea induced a significantly lower cellular toxin content than the other nutrient sources. Under both high- and low-nitrogen concentrations, cell toxin content was also higher in the stationary than in the exponential phase. The toxin profile of the field and cultured cells included ovatoxin (OVTX) analogues -a to -g and isobaric PLTX (isoPLTX). OVTX-a and -b were dominant while OVTX-f, -g, and isoPLTX contributed less than 1-2%. Overall, the data suggest that although nutrients determine the intensity of the O. cf. ovata bloom, the relationship of major nutrient concentrations, sources and stoichiometry with cellular toxin production is not straightforward.

Rapid Biotic and Abiotic Transformation of Toxins produced by Ostreopsis. cf. ovata

The dinoflagellate Ostreopsis cf. ovata produces several families of toxic polyketides. Despite only a few field measurements of these phycotoxins in seawater and aerosols, they are believed to be responsible for dermatitis and the toxic inhalations reported during blooms of this species. Therefore, the stability of these compounds in seawater is essential to understanding the causes of these symptoms, however, this has never been assessed. In the current study, the optimization of a solid phase extraction (SPE) procedure was first performed to ensure the most efficient extraction of all phycotoxins known to be produced by this strain, including the recently described liguriatoxins. The SPE cartridge SDBL® under non acidified conditions offered the best option. The stability of the ovatoxins and the liguriatoxins under biotic and abiotic stress was assessed by exposing the spent medium of a culture of Ostreopsis cf. ovata to its bacterial consortium and natural sunlight. A rapid biotic transformation was detected for both families of compounds. When exposed to bacteria, the half-lives of the ovatoxins were reached before 10 h and at 36 h, 97% of these toxins had been transformed. The half-lives of the liguriatoxins were 10 h under these conditions. Photolysis (abiotic degradation) of the ovatoxins (T1/2 < 36 h) was faster than for the liguriatoxins (T1/2 > 62 h). Although none of the catabolites of these phycotoxins were thoroughly identified, an untargeted metabolomics approach combined with molecular networking highlighted the presence of several compounds exhibiting structural similarities with the ovatoxins. Additional work should confirm the preliminary findings on these potential ovatoxins’ catabolites and their biological properties. The rapid transformation of O. cf. ovata’s phycotoxins introduces questions concerning their presence in seawater and their dispersion in the sea spray aerosols. The compounds involved in the toxic inhalations and dermatitis often experienced by beachgoers may stem from the catabolites of these toxins or even unrelated and as yet unidentified compounds.

Occurrence of three dominant epibenthic dinoflagellates (Ostreopsis spp., Coolia monotis and Prorocentrum lima) in relation to biotic substrates and environmental factors in a highly dynamic ecosystem, the Strait of Gibraltar (Southwestern Mediterranean)

No studies have been carried out on the benthic harmful algal blooms (BHABs) along the Strait of Gibraltar in the Mediterranean, and little is known about the diversity of blooming species. Here, epibenthic dinoflagellates were monitored at least biweekly over 18 months (May 2019-November 2020) in Oued Lihoud, Cap Malabata and Dalia on the thalli of five dominant macrophytes and in the water column. This is the first report on the seasonal distribution of BHAB species hosted by natural biotic substrates in the Strait of Gibraltar, which is known for high hydrodynamics, major entry of Atlantic waters and important maritime traffic. Three BHAB dinoflagellates were observed in the surveyed areas: Ostreopsis spp., Coolia monotis and Prorocentrum lima. The analysis of all data at the three sites showed that Dictyota dichotoma was the most favourable macroalgae host for these benthic dinoflagellates. The highest cell densities were observed in Cap Malabata for Ostreopsis spp. (2.7 × 10⁵ cells/g fresh weight in September 2020), P. lima (4.57 × 10⁴ cells/g FW in September 2020) and C. monotis (4.07 × 10⁴ cells/g FW in June 2019). Phosphate and temperature were positively correlated to the abundances of the studied thermophilic BHAB species. In contrast, negative correlations were recorded with salinity, ammonium, nitrite, nitrate, DIN, nitrogen/phosphate ratio and suspended material, attesting of the complex relationships between environmental factors and BHAB species dynamic in each marine ecosystem. Toxin analyses of the natural phytoplankton assemblage during BHABs showed the presence of only lipophilic toxins, namely okadaic acid and dinophysistoxins produced by P. lima. These BHABs species have to be isolated to establish monoclonal cultures for ribotyping and ecophysiological investigations.

Current Trends and New Challenges in Marine Phycotoxins

Marine phycotoxins are a multiplicity of bioactive compounds which are produced by microalgae and bioaccumulate in the marine food web. Phycotoxins affect the ecosystem, pose a threat to human health, and have important economic effects on aquaculture and tourism worldwide. However, human health and food safety have been the primary concerns when considering the impacts of phycotoxins. Phycotoxins toxicity information, often used to set regulatory limits for these toxins in shellfish, lacks traceability of toxicity values highlighting the need for predefined toxicological criteria. Toxicity data together with adequate detection methods for monitoring procedures are crucial to protect human health. However, despite technological advances, there are still methodological uncertainties and high demand for universal phycotoxin detectors. This review focuses on these topics, including uncertainties of climate change, providing an overview of the current information as well as future perspectives.

The role of habitat in the facilitation of Ostreopsis spp. blooms

In recent decades, recurrent Ostreopsis spp. blooms have been recorded throughout the globe, causing public health issues and mass mortalities of invertebrates. Ostreopsis species are benthic and develop in shallow waters in close relation with a substrate, but possible substrate preferences are still ambiguous. Bloom develops on both living and dead substrates and several interacting biotic and abiotic factors acting at different spatial scales can potentially foster or regulate Ostreopsis spp. development. The objective of this review is to collect and summarize information on Ostreopsis spp. blooms related to the habitat at different spatial scales, in order to assess preferences and trends. References including Ostreopsis spp. samplings in the field were analysed in this review, as potentially including information about the micro- (substrate), meso‑ (community) and macrohabitat (ecosystem) related to Ostreopsis spp. blooms. The sampled substrate and the ecosystem where Ostreopsis spp. were collected were generally reported and described in the studies, while the description of the mesohabitat was rarely reported. Ostreopsis spp. were generally described as attached to biotic substrates and in particular, macroalgae, even in studies conducted in coral reefs, where macroalgae are generally not dominant (but they can be in case of coral reef degradation). In both temperate and tropical areas, Ostreopsis spp. were mostly sampled on algal species usually forming medium or low complexity communities (erect or turf-forming algae), often characteristic from post-regime shift scenarios, and rarely on canopy-forming species (such as fucoids and kelps). This literature review highlights the need of collecting more information about the mesohabitat where important Ostreopsis spp. blooms develop, as much as of the underlying mechanisms driving eventual differences on Ostreopsis spp. abundances. This knowledge would allow a better risk assessment of Ostreopsis spp. blooms, identifying areas at high risk on the base of the benthic habitats.

Allelopathic interactions between phytobenthos and meiofaunal community in an Adriatic benthic ecosystem: Understanding the role of aldehydes and macroalgal structural complexity

Macroalgae produce several allelopathic substances, including polyunsaturated aldehydes (PUAs), which may inhibit photosynthesis and growth rates of other algal species, and grazing. Additionally, macroalgal structural complexity is an important factor in determining abundance patterns and size structure of epiphytic organisms. In this study the PUAs production of two Mediterranean macroalgae, Dictyopteris polypodioides, (DP, Phaeophyceae, Dictyotales) and Cystoseira compressa (CC, Phaeophyceae, Fucales), was characterized to clarify the relationships between the meiobenthic and microphytobenthic communities. Results showed a higher PUAs production and a diverse qualitative profile for DP, which reported long-chain compounds (i.e. C14-C16) as main aldehydes, than CC, with the short-chain C6:2 as the main compound, as well as variability among sampling times. A clear separation of the meiofauna and microphytobenthos assemblages was found for the macroalgae, but with different temporal trends. Dissimilarities were due to five microalgal orders, namely Naviculales, Lyrellales, Gonyaulacales (i.e. Ostreopsis), Bacillariales, and Licmophorales, and to the meiofaunal groups nematodes, copepods, and copepod nauplii, which were more abundant on DP than on CC. Results indicate that macroalgal complexity is a major determinant of the meiofaunal community structure (accounting for 26% of the variation), rather than PUAs production itself (17%). PUAs effects seem species-specific, thus affecting some grazers instead of the entire community. Conversely, microphytobenthos affected the meiofauna assemblages, particularly harpacticoids, confirming the role of these organisms as the primary food source of all marine food chain producers. Since PUAs are produced also by several epiphytic diatoms, the understanding of their effects on the community structure and on the relationships among taxa in the field is complicated and requires further in-depth investigations in simplified systems (i.e. microcosms).

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.

Combined effects of temperature and light intensity on growth, metabolome and ovatoxin content of a Mediterranean Ostreopsis cf. ovata strain

Ostreopsis cf. ovata is a benthic and ovatoxin-producing dinoflagellate proliferating yearly along the Mediterranean coasts where blooms have been related to human illness and unusual mortality of marine organisms. The spreading of O. cf. ovata in this temperate area has been linked to global changes and its consequences such as the increase of temperature or light intensities. In the present study, an experimental design using batch cultures of pre-acclimated cells of a strain of O. cf. ovata isolated from Villefranche-sur-Mer (NW Mediterranean Sea, France), was implemented to investigate the combined effect of temperature (23, 27 and 30 °C) and light intensity (200, 400 and 600 µmol m^-2s^-1) on the growth, metabolome and OVTX content. Both light intensity and temperature affected the growth as significantly higher growth rates were obtained under 400 and 600 µmol m^-2s^-1 while the maximum values were obtained at 27 °C (0.48 d^-1). Metabolomic analyses highlighted a clear effect only for temperature that may correspond to two different strategies of acclimation to suboptimal temperatures. Significant features (such as carotenoid and lipids) modified by the temperature and/or light conditions were annotated. Only temperature induced a significant change of OVTX content with higher values measured at the lowest temperature of 23 °C (29 - 36 pg cell^-1). In a context of global changes, these results obtained after acclimation suggest that the increase of temperature might favor the proliferation of less toxic cells. However, in the light of the intraspecific variability of O. cf. ovata, further studies will be necessary to test this hypothesis. This study also highlighted the lack of knowledge about the metabolome composition of such non-model organisms that impairs data interpretation. There is a need to study more deeply the metabolome of toxic dinoflagellates to better understand how they can acclimate to a changing environment.

Dynamics of the genus Ostreopsis (Gonyaulacales, Dinophyceae) in a Mediterranean fish farm

This study revealed the dynamics of the genus Ostreopsis in the south-western Mediterranean Sea fish farm during the 2016 and 2017 summers. This phytoplankton is known to produce palytoxin-like compounds, listed among the most potent marine toxins known, and can pose a serious concern for humans in the Mediterranean area. Principal component analysis (PCA) explained the significance of temperature, salinity, and dissolved inorganic nitrogen in the proliferation of this toxic dinoflagellate. The peak of the Ostreopsis sp. (6.34 × 10³ cells L^-1) was recorded at 28.4 °C, at a salinity of 38.3 PSU, and the dissolved inorganic nitrogen had a value of 0.60 μmol L^-1. Our results highlight the importance of monitoring the proliferation of this harmful dinoflagellate in southern Mediterranean waters.

Functional profiles of phycospheric microorganisms during a marine dinoflagellate bloom

Harmful algal blooms (HABs) are an ecological concern but relatively few studies have investigated the functional potential of bacterioplankton over a complete algal bloom cycle, which is critical for determining their contribution to the fate of algal blooms. To address this point, we carried out a time-series metagenomic analysis of the functional features of microbial communities at three different Gymnodinium catenatum bloom stages (pre-, peak-, and post-bloom). Different microbial composition were observed during the blooming stages. The environmental parameters and correlation networks co-contribute to microbial variability, and the former explained 38.4% of total variations of the bacterioplankton community composition. Functionally, a range of pathways involved in carbon, nitrogen, phosphorus and sulfur cycling were significantly different during the various HAB stages. Genes associated with carbohydrate-active enzymes, denitrification, and iron oxidation were enriched at the pre-bloom stage; genes involved in reductive citrate cycle for carbon fixation, carbon degradation, nitrification and phosphate transport were enhanced at the peak stage; and relative gene abundance related to sulfur oxidation, vitamin synthesis, and iron transport and storage was increased at the post-bloom stage. The ecological linkage analysis has shown that microbial functional potential especially the C/P/Fe metabolism were significantly linked to the fate of the algal blooms. Taken together, our results demonstrated that microorganisms displayed successional patterns not only at the community level, but also in the metabolic potential on HAB's progression. This work contributes to a growing understanding of microbial structural elasticity and functional plasticity and shed light on the potential mechanisms of microbial-mediated HAB trajectory.

Toxin content of Ostreopsis cf. ovata depends on bloom phases, depth and macroalgal substrate in the NW Mediterranean Sea

Over the last fifteen years, blooms of the genus Ostreopsis have been reported more frequently and at higher abundances in the Mediterranean area. Ostreopsis cf. ovata is known to produce ovatoxins (OVTXs), structural analogues of palytoxin, which is one of the most potent non-polymeric toxins. However, the production of OVTXs is poorly characterized in situ. The present study focuses on toxin content and profile according to the bloom phase during summer 2017 in Villefranche-sur-Mer, France (NW Mediterranean Sea), depth (from 0.5 to 5 m) and three different macroalgal substrates of this epiphytic dinoflagellate (Padina pavonica, Dictyota spp. and Halopteris scoparia). Ovatoxin quantification of all samples was performed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). The bloom started at the end of June and declined in mid-July, showing the typical seasonal pattern of the NW Mediterranean Sea area. The peak was observed on the 10 July with 1.8 × 10⁶ cells/g FW and 1.7 × 10⁴ cells/L for benthic and planktonic cells, respectively. Total toxin content of cells, collected using artificial substrates, increased during the exponential and stationary growth phases. After reaching a maximum concentration of 9.2 pg/cell on 18 July, toxin concentration decreased and remained stable from 25 July until the end of monitoring. A decreasing trend of the abundance and of the associated total toxin content was noted with depth. Finally, the decreasing order of maximal epiphytic concentration of O. cf. ovata was: Dictyota spp. (8.3 × 10⁵ cells/g FW), H. scoparia (3.1 × 10⁵ cells/g FW) and P. pavonica (1.6 × 10⁵ cells/g FW). Interestingly, the highest OVTX quota was obtained in cells present on Halopteris scoparia, then on Dictyota spp. and Padina pavonica. This suggests that the nature of the macroalgal substrate influences both growth and toxin production of O. cf. ovata and further work will be required to understand the underlying mechanisms (e.g., competition for nutrition, pH or allelopathic interaction). However, the toxin profiles (i.e., the proportion of each ovatoxin analogue) were not affected by any of the studied parameters (bloom phase, depth, macroalgae or artificial substrates).

Ostreopsis cf. ovata (Dinophyceae) Molecular Phylogeny, Morphology, and Detection of Ovatoxins in Strains and Field Samples from Brazil

Recurrent blooms of Ostreopsis cf. ovata have been reported in Brazil and the Mediterranean Sea with associated ecological, and in the latter case, health impacts. Molecular data based on the D1–D3 and D8–D10 regions of the LSU rDNA and ITS loci, and the morphology of O. cf. ovata isolates and field populations from locations along the Brazilian tropical and subtropical coastal regions and three oceanic islands are presented. Additional ITS sequences from three single cells from the tropical coast are provided. Toxin profiles and quantities of PLTX and their analogues; OVTXs; contained in cells from two clonal cultures and two field blooms from Rio de Janeiro were investigated. Morphology was examined using both light and epifluorescence microscopy. Morphometric analysis of different strains and field populations from diverse locations were compared. Molecular analysis showed that six of the seven sequences grouped at the large “Atlantic/Mediterranean/Pacific” sub-clade, while one sequence branched in a sister clade with sequences from Madeira Island and Greece. The toxin profile of strains and bloom field samples from Rio de Janeiro were dominated by OVTX-a and -b, with total cell quotas (31.3 and 39.3 pg cell⁻¹) in the range of that previously reported for strains of O. cf. ovata.

Climate change and harmful benthic microalgae

Sea surface temperatures in the world's oceans are projected to warm by 0.4-1.4 °C by mid twenty-first century causing many tropical and sub-tropical harmful dinoflagellate genera like Gambierdiscus, Fukuyoa and Ostreopsis (benthic harmful algal bloom species, BHABs) to exhibit higher growth rates over much of their current geographic range, resulting in higher population densities. The primary exception to this trend will be in the tropics where temperatures exceed species-specific upper thermal tolerances (30-31 °C) beyond which growth slows significantly. As surface waters warm, migration to deeper habitats is expected to provide refuge. Range extensions of several degrees of latitude also are anticipated, but only where species-specific habitat requirements can be met (e.g., temperature, suitable substrate, low turbulence, light, salinity, pH). The current understanding of habitat requirements that determine species distributions are reviewed to provide fuller understanding of how individual species will respond to climate change from the present to 2055 while addressing the paucity of information on environmental factors controlling small-scale distribution in localized habitats. Based on the available information, we hypothesized how complex environmental interactions can influence abundance and potential range extensions of BHAB species in different biogeographic regions and identify sentinel sites appropriate for long-term monitoring programs to detect range extensions and reduce human health risks.

Occurrence of epibenthic dinoflagellates in relation to biotic substrates and to environmental factors in Southern Mediterranean (Bizerte Bay and Lagoon, Tunisia): An emphasis on the harmful Ostreopsis spp., Prorocentrum lima and Coolia monotis

Harmful events associated with epibenthic dinoflagellates, have been reported more frequently over the last decades. Occurrence of potentially toxic benthic dinoflagellates, on the leaves of two magnoliophytes (Cymodocea nodosa and Zostera noltei) and thalli of the macroalgae (Ulva rigida), was monitored over one year (From May 2015 to April 2016) in the Bizerte Bay and Lagoon (North of Tunisia, Southern Mediterranean Sea). The investigated lagoon is known to be highly anthropized. This is the first report on the seasonal distribution of epibenthic dinoflagellates hosted by natural substrates, from two contrasted, adjacent coastal Mediterranean ecosystems. The environmental factors promoting the development of the harmful epibenthic dinoflagellates Ostreopsis spp., Prorocentrum lima and Coolia monotis were investigated. The highest cell densities were reached by Ostreopsis spp. (1.9 × 10³ cells g^-1 FW, in October 2015), P. lima (1.6 × 10³ cells g^-1 FW, in June 2015) and C. monotis (1.1 × 10³ cells g^-1 FW, in May 2015). C. nodosa and Z. noltei were the most favorable host macrophytes for C. monotis (in station L2) and Ostreopsis spp. (in station L3), respectively. Positive correlations were recorded between Ostreopsis spp. and temperature. Densities of the epibenthic dinoflagellates varied according to the collection site, and a great disparity was observed between the Bay and the Lagoon. Maximum concentrations were recorded on C. nodosa leaves from the Bizerte Bay, while low epiphytic cell abundances were associated with macrophytes sampled from the Bizerte Lagoon. The observed differences in dinoflagellate abundances between the two ecosystems (Bay-Lagoon) seemed not related to the nutrients, but rather to the poor environmental conditions in the lagoon.

Detection of Ostreopsis cf. ovata in environmental samples using an electrochemical DNA-based biosensor

Ostreopsis cf. ovata is a benthic microalga distributed in tropical and temperate regions worldwide which produces palytoxins (PlTXs). Herein, an electrochemical biosensor for the detection of this toxic microalga is described. The detection strategy involves isothermal recombinase polymerase amplification (RPA) of the target using tailed primers and a sandwich hybridisation assay on maleimide-coated magnetic beads immobilised on electrode arrays. The biosensor attained a limit of detection of 9 pg/μL of O. cf. ovata DNA (which corresponds to ~640 cells/L), with no interferences from two non-target Ostreopsis species (O. cf. siamensis and O. fattorussoi). The biosensor was applied to the analysis of planktonic and benthic environmental samples. Electrochemical O. cf. ovata DNA quantifications demonstrated an excellent correlation with other molecular methods (qPCR and colorimetric assays) and allowed the construction of a predictive regression model to estimate O. cf. ovata cell abundances. This new technology offer great potential to improve research, monitoring and management of O. cf. ovata and harmful algal blooms.

Ocurrence of Ostreopsis in two temperate coastal bays (SW iberia): Insights from the plankton

The benthic genus Ostreopsis contains toxic-bloom forming species and is an important cause of concern in warm-temperate and tropical waters. On the coast of Portugal, NE Atlantic, the occurrence of Ostreopsis cf. siamensis and Ostreopsis cf. ovata has been reported since 2008 and 2011, respectively. This work aims to understand the favorable conditions for high concentrations of Ostreopsis cells in the plankton at two sites, Lagos and Lisbon Bays, located in the South and West coast of Portugal, respectively. This study is based on weekly Ostreopsis abundance data in the plankton, from 2011 to 2017, daily satellite and in situ sea surface temperature (SST), and meteorological and sea state parameters, namely wind stress and significant wave height. The molecular identification of local Ostreopsis spp. is also presented. The maximum cell densities occur between late-summer and autumn. The distribution range of Ostreopsis cf. ovata is restricted to the South coast, while Ostreopsis cf. siamensis has a wider distribution range, being also present on the West coast. In the study period, there was only one occurrence of Ostreopsis spp., in Lagos Bay, with concentrations within the alert phase of monitoring. In Lagos Bay, high Ostreopsis spp. concentrations were related with positive SST anomalies. These high concentrations were often recorded after a period of almost 2-weeks to more than 4-weeks of low sea state (<0.6 m), followed by short time events of onshore wind and moderate waves (0.6-1 m). The former conditions are interpreted as favoring bloom development on the substrate and the latter as causing the re-suspension of Ostreopsis cells in the water column. In Lisbon Bay, O. cf. siamensis occurred in the plankton in few occasions and no clear relation could be established with the studied environmental variables. It is here hypothesized that the recent records of O. cf. siamensis in Lisbon Bay may correspond to an early colonization stage of an invasion process. Knowledge gained on Ostreopsis dynamics along the Portuguese coast can be used for both the improvement of benthic harmful algal blooms (BHABs) monitoring in the region and as a basis to design forecasting models.

Colorimetric DNA-based assay for the specific detection and quantification of Ostreopsis cf. ovata and Ostreopsis cf. siamensis in the marine environment

Ostreopsis is a toxic benthic dinoflagellate largely distributed worldwide in tropical and temperate areas. In the Mediterranean Sea, periodic summer blooms have been reported and have become a serious concern due to their direct impact on human health and the environment. Current microalgae identification is performed via light microscopy, which is time-consuming and is not able to differentiate among Ostreopsis species. Therefore, there is mature need for rapid, specific and easy-to-use detection tools. In this work, a colorimetric assay exploiting a combination of recombinase polymerase amplification (RPA) and a sandwich hybridisation assay was developed for O. cf. ovata and O. cf. siamensis detection and quantification. The specificity of the system was demonstrated by cross-reactivity experiments and calibration curves were successfully constructed using genomic DNA, achieving limits of detection of 10 and 14 pg/μL for O. cf. ovata and O. cf. siamensis, respectively. The assay was applied to the analysis of planktonic and benthic environmental samples from different sites of the Catalan coast. Species-specific DNA quantifications were in agreement with qPCR analysis, demonstrating the reliability of the colorimetric approach. Significant correlations were also obtained between DNA quantifications and light microscopy counts. The approach may be a valuable tool to provide timely warnings, facilitate monitoring activities or study population dynamics, and paves the way towards the development of in situ tools for the monitoring of harmful algal blooms.

Microbial dynamics during harmful dinoflagellate Ostreopsis cf. ovata growth: Bacterial succession and viral abundance pattern

Algal-bacterial interactions play a major role in shaping diversity of algal associated bacterial communities. Temporal variation in bacterial phylogenetic composition reflects changes of these complex interactions which occur during the algal growth cycle as well as throughout the lifetime of algal blooms. Viruses are also known to cause shifts in bacterial community diversity which could affect algal bloom phases. This study investigated on changes of bacterial and viral abundances, bacterial physiological status, and on bacterial successional pattern associated with the harmful benthic dinoflagellate Ostreopsis cf. ovata in batch cultures over the algal growth cycle. Bacterial community phylogenetic structure was assessed by 16S rRNA gene ION torrent sequencing. A comparison between bacterial community retrieved in cultures and that one co-occurring in situ during the development of the O. cf. ovata bloom from where the algal strain was isolated was also reported. Bacterial community growth was characterized by a biphasic pattern with the highest contributions (~60%) of highly active bacteria found at the two bacterial exponential growth steps. An alphaproteobacterial consortium composed by the Rhodobacteraceae Dinoroseobacter (22.2%-35.4%) and Roseovarius (5.7%-18.3%), together with Oceanicaulis (14.2-40.3%), was strongly associated with O. cf. ovata over the algal growth. The Rhodobacteraceae members encompassed phylotypes with an assessed mutualistic-pathogenic bimodal behavior. Fabibacter (0.7%-25.2%), Labrenzia (5.6%-24.3%), and Dietzia (0.04%-1.7%) were relevant at the stationary phase. Overall, the successional pattern and the metabolic and functional traits of the bacterial community retrieved in culture mirror those ones underpinning O. cf. ovata bloom dynamics in field. Viral abundances increased synoptically with bacterial abundances during the first bacterial exponential growth step while being stationary during the second step. Microbial trends also suggest that viruses induced some shifts in bacterial community composition.

Microbial Community Structure and Associations During a Marine Dinoflagellate Bloom

Interactions between microorganisms and algae during bloom events significantly impacts their physiology, alters ambient chemistry, and shapes ecosystem diversity. The potential role these interactions have in bloom development and decline are also of particular interest given the ecosystem impacts of algal blooms. We hypothesized that microbial community structure and succession is linked to specific bloom stages, and reflects complex interactions among taxa comprising the phycosphere environment. This investigation used pyrosequencing and correlation approaches to assess patterns and associations among bacteria, archaea, and microeukaryotes during a spring bloom of the dinoflagellate Alexandrium catenella. Within the bacterial community, Gammaproteobacteria and Bacteroidetes were predominant during the initial bloom stage, while Alphaproteobacteria, Cyanobacteria, and Actinobacteria were the most abundant taxa present during bloom onset and termination. In the archaea biosphere, methanogenic members were present during the early bloom period while the majority of species identified in the late bloom stage were ammonia-oxidizing archaea and Halobacteriales. Dinoflagellates were the major eukaryotic group present during most stages of the bloom, whereas a mixed assemblage comprising diatoms, green-algae, rotifera, and other microzooplankton were present during bloom termination. Temperature and salinity were key environmental factors associated with changes in bacterial and archaeal community structure, respectively, whereas inorganic nitrogen and inorganic phosphate were associated with eukaryotic variation. The relative contribution of environmental parameters measured during the bloom to variability among samples was 35.3%. Interaction analysis showed that Maxillopoda, Spirotrichea, Dinoflagellata, and Halobacteria were keystone taxa within the positive-correlation network, while Halobacteria, Dictyochophyceae, Mamiellophyceae, and Gammaproteobacteria were the main contributors to the negative-correlation network. The positive and negative relationships were the primary drivers of mutualist and competitive interactions that impacted algal bloom fate, respectively. Functional predictions showed that blooms enhance microbial carbohydrate and energy metabolism, and alter the sulfur cycle. Our results suggest that microbial community structure is strongly linked to bloom progression, although specific drivers of community interactions and responses are not well understood. The importance of considering biotic interactions (e.g., competition, symbiosis, and predation) when investigating the link between microbial ecological behavior and an algal bloom's trajectory is also highlighted.

Multiple stressors and benthic harmful algal blooms (BHABs): Potential effects of temperature rise and nutrient enrichment

Blooms of Ostreopsis cf. ovata, causing health incidence and mass human intoxications in the Mediterranean, gained special attention over the past decades. To study the potential effects of temperature and nutrient enrichment on this benthic dinoflagellate and other associated microalgae in situ, a multifactorial experiment was set up along a temperature gradient of a heat pump system in Monaco. Microalgae were quantified in experimental units, in the natural biofilm and in the water column. No significant interaction was observed between temperature and nutrients. A species- and bloom phase-dependent effect of the increased temperature was recorded, while the nutrient enrichment had a significant effect only at the end of the experiment (when cell abundances were low). Temperature effects were also visible in the biofilm and the surrounding water. The observed assemblages were mainly driven by changes in abundances of Ostreopsis cf. ovata and Actinocyclus sp., affected in different ways.

Influence of environmental factors on the toxin production of Ostreopsis cf. ovata during bloom events

Intense blooms of the toxic dinoflagellate Ostreopsis have been a recurrent phenomenon along several Mediterranean coasts. Blooms have been associated with noxious effects on human health and mortality of marine organisms, due to the production of palytoxin-like compounds. We analyzed the toxin concentrations throughout an O. cf. ovata bloom to highlight their relationships with environmental parameters in the Conero Riviera, northern Adriatic Sea. High temperature and balanced nutrient conditions were the optimal environmental conditions to start and sustain blooms as well as to maximize toxin production. Ostreopsis showed a gradual decrease of toxin content throughout the bloom ascribed to the occurring of the same non-optimal conditions that led to the bloom decline. Moreover, our results suggest that toxin fraction released during bloom could be higher than that released in batch culture. Results from this study pointed out that the first bloom phase is potentially the most dangerous to human health.

Fungal community dynamics during a marine dinoflagellate (Noctiluca scintillans) bloom

Contamination and eutrophication have caused serious ecological events (such as algal bloom) in coastal area. During this ecological process, microbial community structure is critical for algal bloom succession. The diversity and composition of bacteria and archaea communities in algal blooms have been widely investigated; however, those of fungi are poorly understood. To fill this gap, we used pyrosequencing and correlation approaches to assess fungal patterns and associations during a dinoflagellate (Noctiluca scintillans) bloom. Phylum level fungal types were predominated by Ascomycota, Chytridiomycota, Mucoromycotina, and Basidiomycota. At the genus level drastic changes were observed with Hysteropatella, Malassezia and Saitoella dominating during the initial bloom stage, while Malassezia was most abundant (>50%) during onset and peak-bloom stages. Saitoella and Lipomyces gradually became more abundant and, in the decline stage, contributed almost 70% of sequences. In the terminal stage of the bloom, Rozella increased rapidly to a maximum of 50-60%. Fungal population structure was significantly influenced by temperature and substrate (N and P) availability (P < 0.05). Inter-specific network analyses demonstrated that Rozella and Saitoella fungi strongly impacted the ecological trajectory of N. scintillans. The functional prediction show that symbiotrophic fungi was dominated in the onset stage; saprotroph type was the primary member present during the exponential growth period; whereas pathogentroph type fungi enriched in decline phase. Overall, fungal communities and functions correlated significantly with N. scintillans processes, suggesting that they may regulate dinoflagellate bloom fates. Our results will facilitate deeper understanding of the ecological importance of marine fungi and their roles in algal bloom formation and collapse.

Management of harmful benthic dinoflagellates requires targeted sampling methods and alarm thresholds

Concern regarding Benthic Harmful Algal Blooms (BHABs) is increasing since some harmful benthic species have been identified in new areas. In the Mediterranean basin, the most common harmful benthic microalgae are Ostreopsis cf. ovata and Prorocentrum lima, which produce palytoxin-like compounds and okadaic acid respectively, and the need to implement monitoring activities has increased. However, a general agreement on appropriate strategies (e.g. sampling season, definition of alarm thresholds, etc.) is still lagging behind, especially for P. lima, whose proliferation dynamics are still poorly known.

Culturing Toxic Benthic Blooms: The Fate of Natural Biofilms in a Microcosm System

A microcosm designed for culturing aquatic phototrophic biofilms on artificial substrata was used to perform experiments with microphytobenthos sampled during summer toxic outbreaks of Ostreopsis cf. ovata along the Middle Tyrrhenian coast. This dynamic approach aimed at exploring the unique and complex nature of O. cf. ovata bloom development in the benthic system. Epibenthic assemblages were used as inocula for co-cultures of bloom organisms on polycarbonate slides at controlled environmental conditions. Biofilm surface adhesion, growth, and spatial structure were evaluated along with shifts in composition and matrix production in a low disturbance regime, simulating source habitat. Initial adhesion and substratum colonisation appeared as stochastic processes, then community structure and physiognomy markedly changed with time. Dominance of filamentous cyanobacteria and diatoms, and dense clusters of Amphidinium cf. carterae at the mature biofilm phases, were recorded by light and confocal microscopy, whilst O. cf. ovata growth was visibly limited in the late culture phases. Life-form strategies, competitiveness for resources, and possibly allelopathic interactions shaped biofilm structure during culture growth. HPLC (High Performance Liquid Chromatography) analysis of exopolysaccharidic matrix revealed variations in sugar total amounts and composition. No toxic compounds were detected in the final communities tested by LC-MS (Liquid Chromatography- Mass Spectrometry) and MALDI-TOF MS (Matrix Assisted Laser Desorption Ionization Time OF Flight Mass Spectroscopy) techniques.

Ostreopsis cf. ovata in the Gulf of Gabès (south-eastern Mediterranean Sea): morphological, molecular and ecological characterization

In the last few decades, the frequency of the toxic benthic dinoflagellate Ostreopsis cf. ovata proliferation has increased in the Mediterranean Sea. These blooms are associated with harmful effects on human health and the environment. The present work provides the first long term study on the spatio-temporal distribution of O. cf. ovata in relation to physical parameters in the Gulf of Gabès coastal waters (south-eastern Mediterranean Sea), as well as its morphological, molecular and physiological features. The strains of O. cf. ovata were identified morphologically by light and epifluorescence microscopy. The morphology and the size range of cultured strains were similar to those described regarding O. cf. ovata isolated from the Mediterranean Sea. The ultrastructural analysis of O. cf. ovata cells using the transmission electron microscopy showed the presence of numerous vesicles (VE) containing spirally coiled fibers (SCFs) connected to the mucus canal (CH). The phylogenetic tree based on the internal transcribed spacer region containing the 5.8S rDNA (ITS-5.8S rDNA) revealed that O. cf. ovata strains were placed into the Mediterranean/Atlantic clade. In addition, O. cf. ovata toxicity was evaluated by the mouse bioassay and a dose level≥4×10⁴ cells was found to be lethal to mice. The examination of the O. cf. ovata occurrence in the Gulf of Gabès at a large temporal scale (1997-2012) revealed a clear seasonal pattern with dominance from midsummer (July) to late autumn (November). Furthermore, a positive correlation was found between the abundance of O. cf. ovata and salinity, whereas no correlation was found as regards temperature. The occurrence of O. cf. ovata was only detected at salinity above 35 and the highest concentrations were observed at 45. Laboratory experiments confirmed such a result and showed that isolated O. cf. ovata strains had optimal growth at salinity ranging between 35 and 45, with its peak at 40.

Genetic and toxinological characterization of North Atlantic strains of the dinoflagellate Ostreopsis and allelopathic interactions with toxic and non-toxic species from the genera Prorocentrum, Coolia and Gambierdiscus

The genus Ostreopsis includes several toxic species that can develop blooms in benthic ecosystems, with potential harmful consequences for human health and marine invertebrates. Despite of this, little is known about the allelopathic interactions between these organisms and other co-occurring microalgae that exploit similar spatial and nutrient resources in benthic ecosystems. The aim of this study was to follow these interactions in cultures of two Ostreopsis ribotypes with different toxin profiles (O. cf. ovata contained ovatoxins-a, b, c and e, while only ovatoxin-d was found in O .sp. "Lanzarote-type"), mixed with species of three benthic dinoflagellate genera (Coolia, Prorocentrum and Gambierdiscus), isolated from the same area (North East Atlantic, Canary Islands). In a first experiment, the potential allelopathic effects on growth rates were followed, in mixed cultures of Coolia monotis (a non toxic species) exposed to the clarified medium and to cells of O. sp."Lanzarote-type" and O. cf. ovata. Growth delayed in C. monotis was observed specially in clarified medium, while the O. sp. "Lanzarote-type" strain attained much lower densities in mixed cultures. In a second experiment, we examined the potential effects of clarified media from O. sp."Lanzarote-type" and O. cf. ovata on the adherence capacity in two toxic species (Prorocentrum hoffmannianum and Gambierdiscus excentricus). Contrasting effects were found: a significant increase of adherence capacity in P. hoffmannianum vs attachment decline in G. excentricus, that experienced also severe deleterious effects (cell lysis). Our results suggest the existence of weak to moderate allelopathic interactions between the studied organisms, although the outcome is dependent on the species involved.

Ostreopsis fattorussoi sp. nov. (Dinophyceae), a new benthic toxic Ostreopsis species from the eastern Mediterranean Sea

The new benthic toxic dinoflagellate, Ostreopsis fattorussoi sp. nov., is described from the Eastern Mediterranean Sea, Lebanon and Cyprus coasts, and is supported by morphological and molecular data. The plate formula, Po, 3', 7″, 6c, 7s, 5‴, 2'''', is typical for the Ostreopsis genus. It differs from all other Ostreopsis species in that (i) the curved suture between plates 1' and 3' makes them approximately hexagonal, (ii) the 1' plate lies in the left half of the epitheca and is obliquely orientated leading to a characteristic shape of plate 6″. The round thecal pores are bigger than the other two Mediterranean species (O. cf. ovata and O. cf. siamensis). O. fattorussoi is among the smallest species of the genus (DV: 60.07 ± 5.63 μm, AP: 25.66 ± 2.97 μm, W: 39.81 ± 5.05 μm) along with O. ovata. Phylogenetic analyses based on the LSU and internal transcribed spacer rDNA shows that O. fattorussoi belongs to the Atlantic/Mediterranean Ostreopsis spp. clade separated from the other Ostreopsis species. Ostreopsis fattorussoi produces OVTX-a and structural isomers OVTX-d and -e, O. cf. ovata is the only other species of this genus known to produce these toxins. The Lebanese O. fattorussoi did not produce the new palytoxin-like compounds (ovatoxin-i, ovatoxin-j₁ , ovatoxin-j₂ , and ovatoxin-k) that were previously found in O. fattorussoi from Cyprus. The toxin content was in the range of 0.28-0.94 pg · cell^-1 . On the Lebanon coast, O. fattorussoi was recorded throughout the year 2015 (temperature range 18°C-31.5°C), with peaks in June and August.

Ostreopsis cf. ovata from western Mediterranean Sea: Physiological responses under different temperature and salinity conditions

The dinoflagellate Ostreopsis cf. ovata proliferates seasonally in the Mediterranean Sea, producing palytoxin-like compounds (ovatoxins) which are considered among the most potent marine toxins. Blooms have been related to several toxic events in which respiratory problems in humans and mortality of benthic marine organisms have been observed. In the coming decades, an increase in temperature and salinity is predicted in the Mediterranean Sea as a consequence of global warming that may provoke alterations in the dynamics of marine microorganisms. In this study, the physiological effects of changes in water temperature and salinity were analyzed, and their interaction through a multi-factorial experiment using two strains of O. cf. ovata in culture that had been isolated from the western Mediterranean Sea. In order to perform an accurate and reliable estimation of cell abundance, hydrochloric acid and sodium-ethylenediaminetetraacetic acid treatments were evaluated for the purpose of disaggregating cell clumps, with the former providing lower counting errors, especially after the stationary phase. Results of the physiological study showed that growth was inhibited at 19°C for all salinities. The highest growth rates were registered at 24°C for both strains (0.48±0.05divday^-1), and a significant variability in growth rate was found among salinities at 24°C and 28°C. Two groups were distinguished by cell size in all high temperature conditions and a positive correlation was found between the amount of small cells and growth rate. The concentration of palytoxin-like compounds in the cultures increased with time and significantly higher amounts of toxin were found at 28°C in comparison to 24°C. The results suggest that climate change may not affect intensity of blooms, but their toxicity may be enhanced.

Phylogenetic structure of bacterial assemblages co-occurring with Ostreopsis cf. ovata bloom

Extensive blooms of the toxic epiphytic/benthic dinoflagellate Ostreopsis cf. ovata are being reported with increasing frequency and spatial distribution in temperate coastal regions including the Mediterranean. These blooms are of human and environmental health concern due to the production of isobaric palytoxin and a wide range of ovatoxins by Ostreopsis cf. ovata. Bacterial-microalgal interactions are important regulators in algal bloom dynamics and potentially toxin dynamics. This study investigated the bacterial assemblages co-occurring with O. cf. ovata (OA) and from ambient seawaters (SW) during the early and peak phases of bloom development in NW Adriatic Sea. Fractions of the bacterial assemblages co-occurring with O. cf. ovata (OA) and more closely associated to the mucilage layer (LA) embedding O. cf. ovata cells were also reported. In total, 14 bacterial phyla were detected by targeted 454 pyrosequencing of the 16S rRNA gene. The dominant bacterial phyla in the OA assemblages were Proteobacteria and Bacteroidetes; while at the class level, Alphaproteobacteria were the most abundant (83 and 66%, relative abundance, early and peak bloom phases), followed by Flavobacteria (7 and 19%, early and peak phases). Actinobacteria and Cyanobacteria were of minor importance (<5% of the relative bacterial abundance each). Gammaproteobacteria showed a notably presence in OA assemblage only at the early phase of the bloom (genus Haliea, 13%). The Alphaproteobacteria were predominately composed by the genera Ruegeria, Jannaschia and Erythrobacter which represented about half of the total phylotypes' contribution of OA at both early and peak phases of the O. cf. ovata bloom, suggesting interactions between this consortium and the microalga. Moreover, the highest contribution of Ruegeria (30% of the total phylotypes) was observed at the early phase of the bloom in LA assemblage. Microbial assemblages associated with the ambient seawaters while being also dominated by Alphaproteobacteria and Flavobacteria were partially distinct from those associated with O. cf. ovata due to the presence of genera almost not retrieved in the latter assemblages.

Effects of N and P availability on carbon allocation in the toxic dinoflagellate Ostreopsis cf. ovata

Blooms of the toxic dinoflagellate Ostreopsis cf. ovata are usually associated with shallow and calm coastal waters, characterized by low nutrient concentrations. The algal cells typically cover the benthic substrates, such as the macroalgal and invertebrate communities and rocks, forming a mucilaginous film. Data reported on O. cf. ovata toxin production observed under both field and culture conditions show high variability in terms of toxic profile and cellular content; little is known about the environmental and physiological aspects which regulate the toxin dynamics. In this study, O. cf. ovata physiology was investigated using batch cultures supplied with nutrient concentrations similar to those found in the Adriatic Sea during the recurrent blooms and the observed cellular dynamics were compared with those found in a culture grown under optimal conditions, used as a reference. Data on the cellular C, N and P content during the growth highlighted a possible important role of the cellular nutritional status in regulating the toxin production that resulted to be promoted under specific intervals of the C:N and C:P ratios. The variable toxicity found for O. cf. ovata in various geographic areas could be related to the different in situ prevalent environmental conditions (e.g., nutrient concentrations) which affect the cellular elemental composition and carbon allocation. The obtained results strongly suggest that in the environment toxin production is steadily sustained by a low and constant nutrient supply, able to maintain appropriate cellular C:N (>12) or C:P (>170) ratios for a long period. These results explain to some extent the variability in toxicity and growth dynamics observed in blooms occurring in the different coastal areas.