Allelopathic effects of diatom filtrates on the toxic benthic dinoflagellate Ostreopsis cf. ovata

Exposure to the mixotrophic dinoflagellate Lepidodinium sp. and its cues increase toxin production of Pseudo-nitzschia multiseries

The present study examined the defense responses of toxigenic Pseudo-nitzschia species (P. multiseries) to a mixotrophic dinoflagellate, Lepidodinium sp., and its associated cues. We evaluated their responses to different predation risks, including direct physical contact and indirect interactions facilitated by cues from Lepidodinium sp. during active feeding on heterospecific prey (Rhodonomas salina), limited feeding on conspecific prey (P. multiseries) and non-feeding (autotrophic growth in f/2 medium) states. This study is the first investigation of these trophic interactions. Our results demonstrated a significant increase in cellular domoic acid (cDA) in P. multiseries when exposed to Lepidodinium sp. and its associated cues, which was 1.38 to 2.42 times higher than the non-induced group. Notably, this increase was observed regardless of Lepidodinium sp. feeding on this toxic diatom and nutritional modes. However, the most significant increase occurred when they directly interacted. These findings suggest that P. multiseries evaluates predation risk and increases cDA production as a defensive strategy against potential grazing threats. No morphological changes were observed in P. multiseries in response to Lepidodinium sp. or its cues. P. multiseries cultured in flasks of Group L+P-P showed a decrease in growth, but Group L-P and Group L+R-P did not exhibit any decrease. These results suggest a lack of consistent trade-offs between the defense response and growth, thus an increase in cDA production may be a sustainable and efficient defense strategy for P. multiseries. Furthermore, our findings indicate that P. multiseries had no significant impact on the fitness (cell size, growth and/or grazing) of Lepidodinium sp. and R. salina, which suggests no evident toxic or allelopathic impacts on these two phytoplankton species. This study enhances our understanding of the trophic interactions between toxic diatoms and mixotrophic dinoflagellates and helps elucidate the dynamics of Harmful Algal Blooms, toxin transmission, and their impact on ecosystem health.

The succession of epiphytic microalgae conditions fungal community composition: how chytrids respond to blooms of dinoflagellates

This study aims to investigate the temporal dynamics of the epiphytic protist community on macroalgae, during the summer months, with a specific focus on fungi, and the interactions between zoosporic chytrid parasites and the proliferation of the dinoflagellates. We employed a combination of environmental sequencing techniques, incubation of natural samples, isolation of target organisms and laboratory experiments. Metabarcoding sequencing revealed changes in the dominant members of the epiphytic fungal community. Initially, fungi comprised < 1% of the protist community, mostly accounted for by Basidiomycota and Ascomycota, but with the emergence of Chytridiomycota during the mature phase of the biofilm, the fungal contribution increased to almost 30%. Chytridiomycota became dominant in parallel with an increase in the relative abundance of dinoflagellates in the community. Microscopy observations showed a general presence of chytrids following the peak proliferation of the dinoflagellate Ostreopsis sp., with the parasite, D. arenysensis as the dominant chytrid. The maximum infection prevalence was 2% indicating host-parasite coexistence. To further understand the in-situ prevalence of chytrids, we characterised the dynamics of the host abundance and prevalence of chytrids through co-culture. These laboratory experiments revealed intraspecific variability of D. arenysensis in its interaction with Ostreopsis, exhibiting a range from stable coexistence to the near-extinction of the host population. Moreover, while chytrids preferentially parasitized dinoflagellate cells, one of the strains examined displayed the ability to utilize pollen as a resource to maintain its viability, thus illustrating a facultative parasitic lifestyle. Our findings not only enrich our understanding of the diversity, ecology, and progression of epiphytic microalgal and fungal communities on Mediterranean coastal macroalgae, but they also shed light on the presence of zoosporic parasites in less-explored benthic habitats.

Bacterial communities and toxin profiles of Ostreopsis (Dinophyceae) from the Pacific Island of Okinawa, Japan

Variations in toxicity of the benthic dinoflagellate Ostreopsis Schmidt 1901 have been attributed to specific molecular clades, biogeography of isolated strains, and the associated bacterial community. Here, we attempted to better understand the biodiversity and the basic biology influencing toxin production of Ostreopsis. Nine clonal cultures were established from Okinawa, Japan, and identified using phylogenetic analysis of the ITS-5.8S rRNA and 28S rRNA genes. Morphological analysis suggests that the apical pore complex L/W ratio could be a feature for differentiating Ostreopsis sp. 2 from the O. ovata species complex. We analyzed the toxicity and bacterial communities using liquid chromatography-mass spectrometry, and PCR-free metagenomic sequencing. Ovatoxin was detected in three of the seven strains of O. cf. ovata extracts, highlighting intraspecies variation in toxin production. Additionally, two new potential analogs of ovatoxin-a and ostreocin-A were identified. Commonly associated bacteria clades of Ostreopsis were identified from the established cultures. While some of these bacteria groups may be common to Ostreopsis (Rhodobacterales, Flavobacteria-Sphingobacteria, and Enterobacterales), it was not clear from our analysis if any one or more of these plays a role in toxin biosynthesis. Further examination of biosynthetic pathways in metagenomic data and additional experiments isolating specific bacteria from Ostreopsis would aid these efforts.

Environmental, human health and socioeconomic impacts of Ostreopsis spp. Blooms in the NW Mediterranean

This paper summarizes the research conducted by the partners of the EU co-funded CoCliME project to ascertain the ecological, human health and economic impacts of Ostreopsis (mainly O. cf. ovata) blooms in the NW Mediterranean coasts of France, Monaco and Spain. This knowledge is necessary to design strategies to prevent, mitigate and, if necessary, adapt to the impacts of these events in the future and in other regions. Ostreopsis proliferations in the Mediterranean have been related to massive mortalities of benthic organisms and to symptoms of respiratory and cutaneous irritation in humans. A six-year epidemiologic study in a Ostreopsis hot spot in Catalonia and the accumulated experience of the French Mediterranean National Ostreopsis Surveillance Network confirm the main effects of these blooms on human health in the NW Mediterranean. The impacts are associated to direct exposure to seawater with high Ostreopsis cell concentrations and to inhalation of aerosols containing unknown irritative chemicals produced under certain circumstances during the blooms. A series of mild acute symptoms, affecting the entire body as well as the ophthalmic, digestive, respiratory and dermatologic systems have been identified. A main remaining challenge is to ascertain the effects of the chronic exposure to toxic Ostreopsis blooms. Still, the mechanisms involved in the deletereous effects of Ostreopsis blooms are poorly understood. Characterizing the chemical nature of the harmful compounds synthesized by Ostreopsis as well as the role of the mucus by which cells attach to benthic surfaces, requires new technical approaches (e.g., metabolomics) and realistic and standardized ecotoxicology tests. It is also necessary to investigate how palytoxin analogues produced by O. cf. ovata could be transferred through the marine food webs, and to evaluate the real risk of seafood poisonings in the area. On the other hand, the implementation of beach monitoring and surveillance systems in the summer constitutes an effective strategy to prevent the impacts of Ostreopsis on human health. In spite of the confirmed noxious effects, a survey of tourists and residents in Nice and Monaco to ascertain the socioeconomic costs of Ostreopsis blooms indicated that the occurrence of these events and their impacts are poorly known by the general public. In relationship with a plausible near future increase of Ostreopsis blooms in the NW Mediterranean coast, this survey showed that a substantial part of the population might continue to go to the beaches during Ostreopsis proliferations and thus could be exposed to health risks. In contrast, some people would not visit the affected areas, with the potential subsequent negative impacts on coastal recreational and touristic activities. However, at this stage, it is too early to accurately assess all the economic impacts that a potentially increasing frequency and biogeographic expansion of the events might cause in the future.

Changes in Toxin Production, Morphology and Viability of Gymnodinium catenatum Associated with Allelopathy of Chattonella marina var. marina and Gymnodinium impudicum

Allelopathy between phytoplankton organisms is promoted by substances released into the marine environment that limit the presence of the dominating species. We evaluated the allelopathic effects and response of cell-free media of Chattonella marina var. marina and Gymnodinium impudicum in the toxic dinoflagellate Gymnodinium catenatum. Additionally, single- and four-cell chains of G. catenatum isolated from media with allelochemicals were cultured to evaluate the effects of post exposure on growth and cell viability. Cell diagnosis showed growth limitation and an increase in cell volume, which reduced mobility and led to cell lysis. When G. catenatum was exposed to cell-free media of C. marina and G. impudicum, temporary cysts and an increased concentration of paralytic shellfish toxins were observed. After exposure to allelochemicals, the toxin profile of G. catenatum cells in the allelopathy experiments was composed of gonyautoxins 2/3 (GTX2/3), decarcarbamoyl (dcSTX, dcGTX2/3), and the sulfocarbamoyl toxins (B1 and C1/2). A difference in toxicity (pg STXeq cell−1) was observed between G. catenatum cells in the control and those exposed to the filtrates of C. marina var. marina and G. impudicum. Single cells of G. catenatum had a lower growth rate, whereas chain-forming cells had a higher growth rate. We suggest that a low number of G. catenatum cells can survive the allelopathic effect. We hypothesize that the survival strategy of G. catenatum is migration through the chemical cloud, encystment, and increased toxicity.

Mechanism of the allelopathic effect of macroalgae Gracilaria bailiniae on Nitzschia closterium

With the rapid development of the seaweed industry in China, the scale and production of its commercial seaweed are ranked among the most significant worldwide. Consequently, the control of algal blooms, especially fouling diatoms, during macroalgae industrialisation is an important issue. Many diatom bloom studies have focused on physical and chemical controls, with limited economic and eco-friendly biological controls reported. In our study, Gracilaria bailiniae fresh thalli and aqueous extract profoundly suppressed Nitzschia closterium growth (50% inhibition concentration of the fourth day (IC_50-4 day) was 0.667 × 10^-3 g·mL^-1 and 3.889 × 10^-3 g·mL^-1, respectively). The cellular morphology changes of N. closterium exposed to the G. bailiniae aqueous extract were severe atrophies and plasmolysis and dissolution of endocellular structures. To explore more potential allelochemicals to control N. closterium, the intracellular compounds of G. bailiniae were detected and screened. Three organic acids (citrate, hydroxyethanesulfonic acid (HA) and taurine) had allelopathic potential against N. closterium. Our results showed that citrate and HA markedly suppressed N. closterium (IC_50-4 day: 1.035 mM and 1.151 mM, respectively); however, taurine poorly suppressed N. closterium (IC_50-4 day: 2.500 mM). Therefore, HA is one of the main allelopathic compounds in G. bailiniae. Further, the allelopathic mechanism of HA against the N. closterium photosynthetic system broke its photosynthetic apparatus (oxygen-evolving complex, reaction centres, the effective antenna size and the donor side of photosystem II) and hindered electron transport. The experimental results provide a new and eco-friendly strategy to control diatom blooms.

Towards the Prediction of Favourable Conditions for the Harmful Algal Bloom Onset of Ostreopsis ovata in the Ligurian Sea Based on Satellite and Model Data

In recent years, the occurrence of Ostreopsis ovata (O. ovata) Harmful Algal Blooms (HAB) has increased in the coasts of the Ligurian Sea, causing problems to the marine environment and human health. Favourable conditions for O. ovata bloom are triggered by many drivers, many of which are still under investigation, but we hypothesize that this phenomenon can be simulated using a reduced number of major meteo-marine factors, namely water temperature and remixing. Satellite and model data obtained and derived from Copernicus service, namely Sea Surface Temperature (SST) and Significant Wave Height (SWH), were therefore investigated as possible proxies of these local factors. A simplified conceptual model, built on such proxies, was developed and applied to yield a synthetic indicator informative on O. ovata abundance. The model was tested in two study areas in the Ligurian Sea, Marina di Pisa and Marina di Massa in Tuscany, Italy. The results obtained show that the synthetic indicator is able to account for about 35% of the temporal variability of O. ovata bloom occurrence in the two study areas.

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 allelopathy and underlying mechanism of Skeletonema costatum on Karenia mikimotoi integrating transcriptomics profiling

The roles of allelopathy for succession of marine phytoplankton communities remain controversial, especially for the development of blooms. Physiological parameters measurement (Fv/Fm value, MDA content, SOD activity, Na⁺/K⁺, Ca²⁺/ Mg²⁺-ATPase activity, cell size, chlorophyll content, apoptosis and cell cycle) and whole transcriptome profiling analysis were used to investigate allelopathy effect of Skeletonema costatum on Karenia mikimotoi. Filtrate and extracts from S. costatum culture inhibited the growth of K. mikimotoi. Allelopathic effects were dose-dependent for filtrate culture and extract culture. K. mikimotoi scavenged excessive ROS and adapted to the stress fastly and easily, so oxidative damage was not the main cause of the growth inhibition. Allelochemicals of S. costatum were found to influence the structure and function of cell membrane of K. mikimotoi by damaging membrane structure till to cell necrosis, which caused high mortality. Coupled with the sensitivity of algal cells to environmental stress and restricted cell cycle, allelopathy was suggested to be deeply detrimental to the development of competition algal population.

Inhibitory effects of Prorocentrum donghaiense allelochemicals on Sargassum fusiformis zygotes probed by JIP-test based on fast chlorophyll fluorescence kinetics

The macro- and microalgae have been found to inhibit the growth and photosynthesis of one another due to allelopathic interactions between them. Sargassum fusiformis is a common and commercially cultivated seaweed in coastal waters of the East China Sea (ECS) and usually encounters dense harmful algal blooms (HABs) formed by dinoflagellates during their sexual reproduction period. In the present study, the effects of Prorocentrum donghaiense lipophilic extracted allelochemicals on the growth and photosynthesis of S. fusiformis zygotes were probed by fast chlorophyll fluorescence rise kinetics and chlorophyll a transient analysis (JIP-test). It was found that exposure to the allelochemicals led to decreased chlorophyll a content and photosynthetic rates of the zygotes in comparison to the ones in the control. In addition, using the JIP-test, it was found that the inhibitory effects of allelochemicals on photosynthesis of the zygotes were mainly exerted on the electron transport within PSII. The decrease of photosynthetic parameters such as VJ, Mo, ϕPo, ϕo, ϕEo, PI, PTR, PET in the zygotes exposed to the allelochemicals all revealed that the obstruction of electron transport, and the dominant decrease in PET, both implied that inhibition on the dark reaction contributed to the highest photosynthetic reduction. In addition, some reaction centers (RCs) in the zygotes exposed to the allelocamicals were inactivated, which led to higher dissipation of excitation energy, as demonstrated by the significant enhancement of the photosynthetic parameter DIo/RC. All the results indicated that the lipophilic extracts contained the allelochemicals of P. donghaiense which could inhibit the growth and photosynthesis of S. fusiformis zygotes by damaging the electron acceptors and inactivating RCs, and finally block the electron transport.

Interspecies competition between Scrippsiella acuminata and three marine diatoms: Growth inhibition and allelopathic effects

Interactions between Scrippsiella acuminata and three diatoms, Chaetoceros curvisetus, Phaeodactylum tricornutum, and Skeletonema dohrnii, were investigated using bi-algal co-cultures and cell-free and sonicated-cell filtrates in this study. Volatile aldehydes in sonicated filtrates of the three diatoms were analyzed by GC/MS. Furthermore, effects of 2E, 4E-decadienal (2,4-D) on the growth and the photosynthetic efficiency of the four microalgal species were studied. The growth of Sc. acuminata was significantly inhibited by the three diatoms in all co-cultures, and the inhibitory effects were higher under nutrient-rich conditions. Both cell-free and sonicated-cell filtrates of the three diatoms showed significant inhibitions on the growth of Sc. acuminata, which highlighted that diatoms produce allelopathic compounds not only to the surrounding environments but also inside the cells. Fifteen aldehydes were detected in the sonicated-cell filtrates of the three diatoms, and 5, 5, and 12 types of aldehydes were detected in C. curvisetus, P. tricornutum, and Sk. dohrnii, respectively. Polyunsaturated aldehydes (PUAs) composition differed among the three diatom species. Phenylglyoxal (C₈H₆O₂) dominated in C. curvisetus, 2,4-D (C₁₀H₁₆O) predominated in P. tricornutum, and high proportions of 2-hexenal (C₆H₁₀O), 2E, 4E-heptadienal (C₇H₁₀O), and 2,4-D were detected in Sk. dohrnii. 2,4-D showed significantly inhibitory effects on the growth of algal cells including diatoms themselves in a dose-dependent manner, and photosynthetic efficiency was significantly decreased as well. Sc. acuminata was the most sensitive species. The 96 h EC₅₀ values of 2,4-D on the growth of the four microalgae were 1.64 μmol/L for Sc. acuminata, 3.09 μmol/L for C. curvisetus, 4.93 μmol/L for P. tricornutum, and 8.54 μmol/L for Sk. dohrnii, respectively. The results suggest that PUAs produced by diatoms may help them to take the competitive advantages in phytoplankton community, and thus to sustain diatom-dominated community structure in nutrient rich coastal waters.

The current state of knowledge on taxonomy, modulating factors, ecological roles, and mode of action of phytoplankton allelochemicals

Allelopathy is widespread in marine, brackish, and freshwater habitats. Literature data indicate that allelopathy could offer a competitive advantage for some phytoplankton species by reducing the growth of competitors. It is also believed that allelopathy may affect species succession. Thus, allelopathy may play a role in the development of blooms. Over the past few decades, the world's coastal waters have experienced increases in the numbers of cyanobacterial and microalgal blooming events. Understanding how allelopathy is implicated with other biological and environmental factors as a bloom-development mechanism is an important topic for future research. This review focuses on a taxonomic overview of allelopathic cyanobacteria and microalgae, the biological and environmental factors that affect allelochemical production, their role in ecological dynamics, and their physiological modes of action, as well as potential industrial applications of allelopathic compounds.

Evaluation of co-culturing a diatom and a coccolithophore using different silicate concentrations

Globally, the demand for sustainable energy production and high-value biological compounds have become intertwined in an attempt to improve the feasibility of sustainable algal cultivation. Marine microalgae, especially diatoms and coccolithophores, represent viable cultures that can produce biofuels and high-value compounds. Growing them in co-culture offers the potential to produce lipids and pigments, while also generating CaCO₃ for C sequestration. The main objective of this work was to investigate competition or co-existence of the diatom Chaetoceros gracilis and the coccolithophore Pleurochrysis Carterae. The focus was on the effects of silicate and co-culturing on the growth rate, productivity, pigment production, and ash production for C. gracilis and P. carterae in laboratory conditions. The results showed that, in monoculture, 2-mM Si enhanced the specific growth rate of C. gracilis, but did not affect P. carterae. Regardless of silicate concentration, C. gracilis was more productive than P. carterae. In co-culture, P. carterae had a slower growth rate, indicating an inhibitory effect of C. gracilis on P. carterae. Neither silicate concentration nor co-culturing had an impact on the contents of pigments fucoxanthin, chlorophyll-a, and chlorophyll-c, which means that pigment productivity was proportional to biomass productivity. Finally, the ash content increased in all cultures with the lower silicate concentration (0.2 mM) in the medium. With one exception, the ash content was dominated by SiO₂ regardless of silicate amount, and CaCO₃ was a major part of the ash only when P. carterae was grown separately with the higher silicate level. These results highlight that co-culturing did not provide an advantage for improving biomass, pigments, or CaCO₃ productivity.

High predictability of direct competition between marine diatoms under different temperatures and nutrient states

The distribution of marine phytoplankton will shift alongside changes in marine environments, leading to altered species frequencies and community composition. An understanding of the response of mixed populations to abiotic changes is required to adequately predict how environmental change may affect the future composition of phytoplankton communities. This study investigated the growth and competitive ability of two marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana, along a temperature gradient (9-35°C) spanning the thermal niches of both species under both high-nitrogen nutrient-replete and low-nitrogen nutrient-limited conditions. Across this temperature gradient, the competitive outcome under both nutrient conditions at any assay temperature, and the critical temperature at which competitive advantage shifted from one species to the other, was well predicted by the temperature dependencies of the growth rates of the two species measured in monocultures. The temperature at which the competitive advantage switched from P. tricornutum to T. pseudonana increased from 18.8°C under replete conditions to 25.3°C under nutrient-limited conditions. Thus, P. tricornutum was a better competitor over a wider temperature range in a low N environment. Being able to determine the competitive outcomes from physiological responses of single species to environmental changes has the potential to significantly improve the predictive power of phytoplankton spatial distribution and community composition models.

The Importance of Allelopathic Picocyanobacterium Synechococcus sp. on the Abundance, Biomass Formation, and Structure of Phytoplankton Assemblages in Three Freshwater Lakes

The contribution of picocyanobacteria to summer phytoplankton blooms, accompanied by an ecological crisis, is a new phenomenon in Europe. This issue requires careful investigation. We studied allelopathic activity of freshwater picocyanobacterium Synechococcus sp. on phytoplankton assemblages from three freshwater lakes. In this study, the allelopathic activity of the Synechococcus sp. on the total abundance, biomass, as well as structure of the phytoplankton assemblages were investigated. Our results indicated that addition of exudates obtained from Synechococcus sp. affected the number of cells and biomass of the phytoplankton communities; the degree of inhibition or stimulation was different for each species, causing a change in the phytoplankton abundance and dominance during the experiment. We observed that some group of organisms (especially cyanobacteria from the genus Aphanothece, Limnothrix, Microcystis, and Synechococcus) showed tolerance for allelopathic compounds produced and released by Synechococcus sp. It is also worth noting that in some samples, Bacillariophyceae (e.g., Amphora pediculus, Navicula pygmaea, and Nitzschia paleacea) were completely eliminated in the experimental treatments, while present in the controls. This work demonstrated that the allelopathic activity exhibited by the Synechococcus sp. is probably one of the major competitive strategies affecting some of the coexisting phytoplankton species in freshwater ecosystems. To our best knowledge this is the first report of the allelopathic activity of Synechococcus sp. in the freshwater reservoirs, and one of the few published works showing allelopathic properties of freshwater picocyanobacteria on coexisting phytoplankton species.

Co-occurrence patterns and assembly processes of microeukaryotic communities in an early-spring diatom bloom

The interaction and assembly processes of microeukaryotic community compositions (MECs) are rarely elucidated in environment with strong disturbance such as harmful algal blooms. To fill this gap, we analyzed changes of MECs induced by a diatom bloom using 18S rRNA gene amplicon sequencing. The MECs were mainly dominated by Cercozoa (average relative abundance, 49.2%), Diatom (25.5%) and Dinoflagellata (15.6%). MECs changed significantly (ANOSIM P < 0.01) in four-bloom stages. Environmental factors including pH, DO, nitrate and phosphate, together with bacterial communities could significantly influence the variation of MECs. Co-occurrence network analysis revealed a complex interaction between microeukaryotic and bacterial communities. Most OTUs in modules of the co-occurrence network were specific to one particular bloom stage. Phylogenetic based β-nearest taxon distance analyses revealed that stochastic processes mainly dominated microeukaryotic community assembly in the initial and after-bloom stage. However, microeukaryotic community assembly in middle and late stage of the bloom were driven by deterministic processes. In conclusion, both stochastic and deterministic processes play important roles in distinct bloom stages. These findings may expand current understandings of assembly mechanisms and microbial interactions underlying microeukaryotic dynamics in eutrophic aquatic ecosystems where harmful algal blooms occurred frequently.

Isolation of anti-algal substances from Cylindrotheca closterium and their inhibition activity on bloom-forming Prorocentrum donghaiense

This paper studied the allelopathic effect of Cylindrotheca closterium on the growth of Prorocentrum donghaiense, the model of harmful algal blooms in aquatic environment, by the co-culture tests and bioassay-guided fraction methods. The growth of P. donghaiense in co-cultures was observably suppressed by C. closterium, and P. donghaiense biomass in C/P = 3: 1 group increased slowly with a low growth rate of 0.18 d^-1 after 4 days. Petroleum ether (PE) extract derived from C. closterium filtrates was isolated by C18 column and the allelopathy of all isolated fractions for P. donghaiense was investigated. After 96 h cultivation, the inhibition ratio of PE-Ⅲ and PE-Ⅷ fractions on P. donghaiense could reach up to 70.2% and 64.3% at the concentration of 10-fold when compared to control, while the other fractions displayed relatively low inhibitory effects on P. donghaiense. PE-Ⅲ and PE-Ⅷ fractions also decreased the chlorophyll content and maximum quantum yield of photosystem II (Fv/Fm) of P. donghaiense cells. The activities of superoxide dismutase (SOD), one of antioxidant enzymes, reduced around 8.3% and 13.7% following exposure to 2-fold PE-Ⅲ and PE-Ⅷ, and was significantly decreased following higher exposure concentrations. After 96 h of 10-fold PE-Ⅲ and PE-Ⅷ treatments, Catalase (CAT) activity reduced to 44.86% and 46.42% of that observed in the control group. At the same time, a significant increase in malondialdehyde (MDA) contents was observed. These findings suggested that PE-Ⅲ and PE-Ⅷ fractions contained main allelochemicals and possibly acted as promising algistatic agents for emergency handling of P. donghaiense blooms.

Distribution and potential toxicity of benthic harmful dinoflagellates in waters of Florida Bay and the Florida Keys

Few studies have been carried out on benthic dinoflagellates along the Florida Keys, and little is known about their distribution or toxicity in Florida Bay. Here, the distribution and abundance of benthic dinoflagellates was explored in northern and eastern Florida Bay and along the bay and ocean sides of the Florida Keys. Isolates were brought into culture and their toxicity was tested with oyster larvae bioassays. Seven genera were detected, including Prorocentrum, Coolia, Ostreopsis, Amphidinium, Gambierdiscus, Fukuyoa (all included potentially toxic species) and Sinophysis. In general, distribution increased with water temperature and nutrient availability, especially that of phosphate. This study documented the first record of Coolia santacroce in the Florida Keys. Potential toxic effects of Gambierdiscus caribaeus, the abundance of which exceeded 1000 cells g^-1 fw at some sites, were established using oyster larvae as a bioassay organism. These findings suggest a potential risk of ciguatera fish poisoning in this area.

Physiological Effects on Coexisting Microalgae of the Allelochemicals Produced by the Bloom-Forming Cyanobacteria Synechococcus sp. and Nodularia Spumigena

Only a few studies have documented the physiological effects of allelopathy from cyanobacteria against coexisting microalgae. We investigated the allelopathic ability of the bloom-forming cyanobacteria Synechococcus sp. and Nodularia spumigena filtrates on several aspects related to the physiology of the target species: population growth, cell morphology, and several indexes of photosynthesis rate and respiration. The target species were the following: two species of green algae (Oocystis submarina, Chlorella vulgaris) and two species of diatoms (Bacillaria paxillifer, Skeletonema marinoi). These four species coexist in the natural environment with the employed strains of Synechococcus sp. and N. spumigena employed. The tests were performed with single and repeated addition of cyanobacterial cell-free filtrate. We also tested the importance of the growth phase in the strength of the allelopathic effect. The negative effects of both cyanobacteria were the strongest with repeated exudates addition, and generally, Synechococcus sp. and N. spumigena were allelopathic only in the exponential growth phase. O. submarina was not negatively affected by Synechococcus filtrates in any of the parameters studied, while C. vulgaris, B. paxillifer, and S. marinoi were affected in several ways. N. spumigena was characterized by a stronger allelopathic activity than Synechococcus sp., showing a negative effect on all target species. The highest decline in growth, as well as the most apparent cell physical damage, was observed for the diatom S. marinoi. Our findings suggest that cyanobacterial allelochemicals are associated with the cell physical damage, as well as a reduced performance in respiration and photosynthesis system in the studied microalgae which cause the inhibition of the population growth. Moreover, our study has shown that some biotic factors that increase the intensity of allelopathic effects may also alter the ratio between bloom-forming cyanobacteria and some phytoplankton species that occur in the same aquatic ecosystem.

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.

Variation in allelopathy of extracellular compounds produced by Cylindrotheca closterium against the harmful-algal-bloom dinoflagellate Prorocentrum donghaiense

Allelopathy between algae is an ecological strategy that can facilitate or inhibit the occurrence of algal blooms. The role of allelopathic effects of marine microalgae Cylindrotheca closterium in other phytoplankton population dynamics are still limited. In the current study, the effects of cell-free filtrates of diatom Cylindrotheca closterium on two common dinoflagellates (Prorocentrum donghaiense and Prorocentrum cordatum), a chrysophyceae (Isochrysis galbana) and a diatom (Chaetoceros curvisetus) were investigated within controlled laboratory experiments. It was observed that the growth of P. donghaiense was significantly suppressed and approximately 80% cells disappeared after 8-d exposure, while the other three algae was less sensitive. P. donghaiense was very sensitive to the exudates of C. closterium from the stationary phase by comparing various percentage (10, 30, 50, 70 and 100%) of filtrates. In addition, the allelopathic effects of extracellular compounds of C. closterium extracted by three different organic solvents (ethyl acetate, chloroform and petroleum ether) on P. donghaiense were explored by determining cell density, chlorophyll content and maximum photosystem II (PSII) quantum yield (F_v/F_m). It was found that the compounds extracted by ethyl acetate and chloroform appeared to exhibit less toxicity on P. donghaiense than that of petroleum ether. The present results indicated that the allelochemicals released by C. closterium might be concentrated effectively in the petroleum ether extraction phase, which provided a new perspective for controlling the red tides of P. donghaiense in the East China Sea by means of the ecological inhibitors extracted.

Antialgal effects of α-linolenic acid on harmful bloom-forming Prorocentrum donghaiense and the antialgal mechanisms

Harmful algal blooms (HABs) induced by Prorocentrum donghaiense occur frequently and cause a serious threat to the marine ecosystem. In this study, antialgal effects of α-linolenic acid (ALA) that is generally extracted from diverse macroalga on P. donghaiense were investigated. Specifically, the growth, cellular morphology and ultrastructure, reactive oxygen species (ROS) content, mitochondrial membrane potential (MMP), cytochrome C (Cyt-C), and caspase-9,3 activity of untreated and treated P. donghaiense were investigated. The results showed that ALA significantly inhibited the growth of P. donghaiense. Under ALA exposure, the cellular morphology and ultrastructure were damaged. ALA also induced ROS overproduction in the algal cells, decreased MMP, induced Cyt-C release, and activated caspase-9,3, which strongly relates to algal apoptosis. In summary, this study revealed the responses of morphology and physiology of P. donghaiense when exposed under ALA, and shows the potential of biotechnology on controlling P. donghaiense.

Diverse algicidal bacteria associated with harmful bloom-forming Karenia mikimotoi in estuarine soil and seawater

Algicidal bacteria associated with Karenia mikimotoi have been isolated, yet the distribution of the algicidal bacteria has been rarely studied. Here, we postulated and demonstrated that terrestrial environment harbors diverse algicidal bacteria, which can survive in seawater along water flowing into marine and suppress Karenia mikimotoi. In summary, 9 and 5 bacteria with algicidal activity on Karenia mikimotoi were isolated from seawater and estuarine soil, respectively. Similar with the marine bacteria (Alteromonas sp., Halomonas sp., Marinobacter sp., Paracoccus sp., Rhodobacteraceae, Idiomarina sp.), the soil strains (Pseudoalteromonas sp. and Flavobaterium sp.) showed high mortality in Karenia mikimotoi with the inhibitory rate of 87% and 93.5%, respectively, after two days co-cultivation. Algicidal activity of the two strains was detected in the cell-free filtrate not in bacterial cells. The results suggest that algicidal bacteria associated with Karenia mikimotoi widely exist in terrestrial and marine environments, and have application potential on controlling Karenia mikimotoi.

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.

Allelopathic interactions between the benthic toxic dinoflagellate Ostreopsis cf. ovata and a co-occurring diatom

For decades the microphytobenthos assemblage in the coastal Mediterranean Sea has been regularly colonized by the toxic benthic dinoflagellate Ostreopsis cf. ovata. This harmful algal species is a toxin producer and occupies the same ecological niche as various diatoms. Surprisingly, there are only few insights reported on the physiological responses of diatoms to blooms of O. cf. ovata The chemical interactions of O. cf. ovata with the co-occurring diatom Licmophora paradoxa was studied using a bioassay (measuring impact of cell-free culture filtrate) and a co-culture approach (separate by a membrane) to investigate the effects of the exometabolome and its mode of action. Bioassays highlighted a toxic effect of the exometabolome of O. cf. ovata on the diatom photosynthetic activity. However, the co-cultures revealed that these toxic effects do not occur through remote allelopathy. Contact or close interactions between cells of the two species is most likely needed to impair the diatom growth. Ovatoxins are suspected to be the toxic metabolites secreted by O. cf. ovata although the current set of data did not give confirmation of this assumption. Interestingly, the exometabolome of L. paradoxa impaired the growth and the photochemistry of O. cf. ovata in both bioassays and co-cultures. Some biomarkers possibly involved for the effect were identified using a metabolomic approach and may correspond to oxylipins, however a bacterial source of the bioactive metabolites is also considered.

Allelopathic and Bloom-Forming Picocyanobacteria in a Changing World

Picocyanobacteria are extremely important organisms in the world's oceans and freshwater ecosystems. They play an essential role in primary production and their domination in phytoplankton biomass is common in both oligotrophic and eutrophic waters. Their role is expected to become even more relevant with the effect of climate change. However, this group of photoautotrophic organisms still remains insufficiently recognized. Only a few works have focused in detail on the occurrence of massive blooms of picocyanobacteria, their toxicity and allelopathic activity. Filling the gap in our knowledge about the mechanisms involved in the proliferation of these organisms could provide a better understanding of aquatic environments. In this review, we gathered and described recent information about allelopathic activity of picocyanobacteria and occurrence of their massive blooms in many aquatic ecosystems. We also examined the relationships between climate change and representative picocyanobacterial genera from freshwater, brackish and marine ecosystems. This work emphasizes the importance of studying the smallest picoplanktonic fractions of cyanobacteria.