Phylogenomics including new sequence data of phytoplankton-infecting chytrids reveals multiple independent lifestyle transitions across the phylum.

Parasitism is the most common lifestyle on Earth and has emerged many times independently across the eukaryotic tree of life. It is frequently found among chytrids (Chytridiomycota), which are early-branching unicellular fungi that feed osmotrophically via rhizoids as saprotrophs or parasites. Chytrids are abundant in most aquatic and terrestrial environments and fulfil important ecosystem functions. As parasites, they can have significant impacts on host populations. They cause global amphibian declines and influence the Earth's carbon cycle by terminating algal blooms. To date, the evolution of parasitism within the chytrid phylum remains unclear due to the low phylogenetic resolution of rRNA genes for the early diversification of fungi, and because few parasitic lineages have been cultured and genomic data for parasites is scarce. Here, we combine transcriptomics, culture-independent single-cell genomics and a phylogenomic approach to overcome these limitations. We newly sequenced 29 parasitic taxa and combined these with existing data to provide a robust backbone topology for the diversification of Chytridiomycota. Our analyses reveal multiple independent lifestyle transitions between parasitism and saprotrophy among chytrids and multiple host shifts by parasites. Based on these results and the parasitic lifestyle of other early-branching holomycotan lineages, we hypothesise that the chytrid last common ancestor was a parasite of phytoplankton.

Dinoflagellate hosts determine the community structure of marine Chytridiomycota: Demonstration of their prominent interactions.

The interactions of parasitic fungi with their phytoplankton hosts in the marine environment are mostly unknown. In this study, we evaluated the diversity of Chytridiomycota in phytoplankton communities dominated by dinoflagellates at several coastal locations in the NW Mediterranean Sea and demonstrated the most prominent interactions of these parasites with their hosts. The protist community in seawater differed from that in sediment, with the latter characterized by a greater heterogeneity of putative hosts, such as dinoflagellates and diatoms, as well as a chytrid community more diverse in its composition and with a higher relative abundance. Chytrids accounted for 77 amplicon sequence variants, of which 70 were found exclusively among different blooming host species. The relative abundance of chytrids was highest in samples dominated by the dinoflagellate genera Ostreopsis and Alexandrium, clearly indicating the presence of specific chytrid communities. The establishment of parasitoid-host co-cultures of chytrids and dinoflagellates allowed the morphological identification and molecular characterization of three species of Chytridiomycota, including Dinomyces arenysensis, as one of the most abundant environmental sequences, and the discovery of two other species not yet described.

Host preferences of coexisting Perkinsea parasitoids during coastal dinoflagellate blooms.

Parasites in aquatic systems are highly diverse and ubiquitous. In marine environments, parasite-host interactions contribute substantially to shaping microbial communities, but their nature and complexity remain poorly understood. In this study, we examined the relationship between Perkinsea parasitoids and bloom-forming dinoflagellate species. Our aim was to determine whether parasite-host species interactions are specific and whether the diversity and distribution of parasitoids are shaped by their dinoflagellate hosts. Several locations along the Catalan coast (NW Mediterranean Sea) were sampled during the blooms of five dinoflagellate species and the diversity of Perkinsea was determined by combining cultivation-based methods with metabarcoding of the V4 region of 18S rDNA. Most known species of Parviluciferaceae, and others not yet described, were detected, some of them coexisting in the same coastal location, and with a wide distribution. The specific parasite-host interactions determined for each of the studied blooms demonstrated the host preferences exhibited by parasitoids in nature. The dominance of a species within the parasitoid community is driven by the presence and abundances of its preferred host(s). The absence of parasitoid species, often associated with a low abundance of their preferred hosts, suggested that high infection rates are reached only under conditions that favour parasitoid propagation, especially dinoflagellate blooms.

Quantifying long-term recurrence in planktonic microbial eukaryotes.

How much temporal recurrence is present in microbial assemblages is still an unanswered ecological question. Even though marked seasonal changes have been reported for whole microbial communities, less is known on the dynamics and seasonality of individual taxa. Here, we aim at understanding microbial recurrence at three different levels: community, taxonomic group and operational taxonomic units (OTUs). For that, we focused on a model microbial eukaryotic community populating a long-term marine microbial observatory using 18S rRNA gene data from two organismal size fractions: the picoplankton (0.2-3 µm) and the nanoplankton (3-20 µm). We have developed an index to quantify recurrence in particular taxa. We found that community structure oscillated systematically between two main configurations corresponding to winter and summer over the 10 years studied. A few taxonomic groups such as Mamiellophyceae or MALV-III presented clear recurrence (i.e., seasonality), whereas 13%-19% of the OTUs in both size fractions, accounting for ~40% of the relative abundance, featured recurrent dynamics. Altogether, our work links long-term whole community dynamics with that of individual OTUs and taxonomic groups, indicating that recurrent and non-recurrent changes characterize the dynamics of microbial assemblages.

Morphological and molecular characterization of Bysmatrum subsalsum (Dinophyceae) from the western Mediterranean Sea reveals the existence of cryptic species.

Bysmatrum subsalsum is a cosmopolitan dinoflagellate species that inhabits marine and transitional habitats. Despite its wide distribution, information on the morphological variability, phylogeny and ecology of B. subsalsum is scarce. In this study, we provide morphological and molecular data on B. subsalsum strains and wild cells from different locations in the Mediterranean Basin. The dynamics of cell abundances and the associated environmental conditions during a field bloom are also described. Genetic sequences of B. subsalsum obtained in this study showed large intraspecific differences, clustering in two well-differentiated clades. Despite a certain degree of variation with respect to cell size, apical pore complex (APC) morphology and size, and cingulum displacement, cells from the two clades showed similar morphological traits. These findings indicated the occurrence of cryptic species. Comparisons of the morphology of our B. subsalsum specimens with the few descriptions available in the literature revealed larger than previously known intraspecific morphological variability. Phylogenetic trees inferred from the concatenated SSU, 5.8S-ITS, and LSU rRNA and the individual 5.8S-ITS regions suggested the inclusion of Bysmatrum in the Peridiniales and a close phylogenetic relationship with Peridinium sensu stricto. However, the low statistical support prevented the assignment of Bysmatrum to a particular family of Peridiniales. Ecological data obtained from a bloom in La Pletera salt marshes (Catalan Coast, Spain) suggested the species reaches high cell abundances at water temperatures >20°C and salinity levels >30. Our results add new information regarding the morphology, phylogeny, and ecology of B. subsalsum.

Newly identified diversity of Dinomycetaceae (Rhizophydiales, Chytridiomycota), a family of fungal parasites of marine dinoflagellates.

We identified two new parasite species of Chytridiomycota isolated during blooms of the dinoflagellate Alexandrium minutum in the coastal Mediterranean Sea. Light and electron microscopy together with molecular characterization of the nuclear 18S, ITS, and 28S rDNA regions led to their identification as two new species, Dinomyces gilberthii and Paradinomyces evelyniae, both belonging to the family Dinomycetaceae, order Rhizophydiales. Dinomyces gilberthii differs from the previously described D. arenysensis by the presence of discharge papillae and the development of a drop-shaped sporangium. Paradinomyces evelyniae differs from the previously described P. triforaminorum by the prominent lipid globule present in early sporangia and by the pointed end producing a rhizoid. The two chytrids differed in their geographical distribution. Dinomyces gilberthii was detected in several Mediterranean habitats, including harbours and beaches, and was particularly prevalent during summer dinoflagellate blooms. Its widespread occurrence in coastal ecosystems suggested a high level of adaptability to this environment. Paradinomyces evelyniae had a more restricted distribution in the coastal-marine environment, occurring in harbour sediments and only occasionally in the water column during winter and early spring. Paradinomyces evelyniae has previously been detected in the Baltic Sea, suggesting that its distribution encompasses contrasting coastal environments, although its presence is rare.

Cryo-electron microscopy of extracellular vesicles associated with the marine toxic dinoflagellate Alexandrium minutum.

Extracellular Vesicles (EVs) are likely an important strategy of transport and communication in marine microbial community. Their isolation and characterization from axenic culture of microbial eukaryotes represents a technological challenge not fully solved. Here, for the first time, we isolated EVs from a near-axenic culture of the toxic dinoflagellate Alexandrium minutum. Pictures of the isolated vesicles were done with Cryo TEM (Cryogenic Transmission Electron Microscopy). Based on their morphotype the EVs were clustered in five major groups (rounded, rounded electron-dense, lumen electron-dense, double and irregular) and each EV was measured resulting in an average size of 0.36 µm of diameter. Taking in account that in prokaryotes it has been demonstrated that EVs play an important role in the mechanism of toxicity, this descriptive work aims to be the first step to study the possible role of EVs in the toxicity of dinoflagellates.

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.

Bloom of Gymnodinium catenatum in Bahía Santiago and Bahía Manzanillo, Colima, Mexico.

Gymnodinium bloom events are of concern, since they produce toxins, which have unfavorable consequences to marine ecosystems, human health and the economy. This report describes the physico-chemical conditions that were present during the algal bloom event on May 2010 in Bahía Manzanillo and Bahía Santiago, Colima, Mexico. For this, seawater nutrient analysis, phytoplankton counts, identification, and toxicity tests were undertaken. Nutrients in seawater were determined using colorimetric techniques, the higher concentrations (8.88 microM DIN, 0.78 microM PO4 and 24.34 microM SiO2) were related with upwelling waters that promoted the algal bloom that began after registering the year lowest sea-surface temperature, favoring the rapid growth of G. catenatum (up to 1.02 x 10(7) cells/L). Phytoplankton counting was carried out using sedimentation chambers and cells enumerated on appropriated area. The bloom persisted in the bays for approximately two weeks and was associated with toxicity (determined with HPLC) in local oysters (1525.8 microg STXeq/100g), and in phytoplankton (10.9 pg STXeq/cells) samples. Strong variations in cell toxicity (1.4 to 10.9pg STXeq/cells), most likely reflected the availability of inorganic nutrients. The toxin profile of the phytoplankton samples consisted of 11 toxins and resembled those recorded for several strains of G. catenatum isolated from other coastal areas of Mexico.

Seasonal niche differentiation among closely related marine bacteria.

Bacteria display dynamic abundance fluctuations over time in marine environments, where they play key biogeochemical roles. Here, we characterized the seasonal dynamics of marine bacteria in a coastal oligotrophic time series station, tested how similar the temporal niche of closely related taxa is, and what are the environmental parameters modulating their seasonal abundance patterns. We further explored how conserved the niche is at higher taxonomic levels. The community presented recurrent patterns of seasonality for 297 out of 6825 amplicon sequence variants (ASVs), which constituted almost half of the total relative abundance (47%). For certain genera, niche similarity decreased as nucleotide divergence in the 16S rRNA gene increased, a pattern compatible with the selection of similar taxa through environmental filtering. Additionally, we observed evidence of seasonal differentiation within various genera as seen by the distinct seasonal patterns of closely related taxa. At broader taxonomic levels, coherent seasonal trends did not exist at the class level, while the order and family ranks depended on the patterns that existed at the genus level. This study identifies the coexistence of closely related taxa for some bacterial groups and seasonal differentiation for others in a coastal marine environment subjected to a strong seasonality.

The new chytridiomycete Paradinomyces triforaminorum gen. et sp. nov. co-occurs with other parasitoids during a Kryptoperidinium foliaceum (Dinophyceae) bloom in the Baltic Sea.

A new chytrid genus and species was isolated and cultured from samples obtained in the Baltic Sea during a dinoflagellate bloom event. This species is characterized by having a spherical sporangium without papillae and zoospores of 2-3 µm in diameter that are released through 3 discharge pores. Molecular phylogeny based on ribosomal operon showed its sister position to the Dinomyces cluster in Rhizophydiales. Zoospores lack fenestrated cisternae but contain a paracrystalline inclusion, found in a Rhizophydiales representative for the first time. Additionally, the kinetid features are uncommon for Rhizophydiales and only observed in Dinomyces representatives so far. These morphological features and its phylogenetic relationships justify the description of the new genus and speciesParadinomyces triforaminorum gen. nov. sp. nov. belonging to the family Dinomycetaceae. The chytrid was detected during a high-biomass bloom of the dinoflagellate Kryptoperidinium foliaceum. Laboratory experiments suggest this species is highly specific and demonstrate the impact it can have on HAB development. The chytrid co-occurred with three other parasites belonging to Chytridiomycota (Fungi) and Perkinsea (Alveolata), highlighting that parasitic interactions are common during HABs in brackish and marine systems, and these multiple parasites compete for similar hosts.

Long-term patterns of an interconnected core marine microbiota.

BACKGROUND: Ocean microbes constitute ~ 70% of the marine biomass, are responsible for ~ 50% of the Earth's primary production and are crucial for global biogeochemical cycles. Marine microbiotas include core taxa that are usually key for ecosystem function. Despite their importance, core marine microbes are relatively unknown, which reflects the lack of consensus on how to identify them. So far, most core microbiotas have been defined based on species occurrence and abundance. Yet, species interactions are also important to identify core microbes, as communities include interacting species. Here, we investigate interconnected bacteria and small protists of the core pelagic microbiota populating a long-term marine-coastal observatory in the Mediterranean Sea over a decade. RESULTS: Core microbes were defined as those present in > 30% of the monthly samples over 10 years, with the strongest associations. The core microbiota included 259 Operational Taxonomic Units (OTUs) including 182 bacteria, 77 protists, and 1411 strong and mostly positive (~ 95%) associations. Core bacteria tended to be associated with other bacteria, while core protists tended to be associated with bacteria. The richness and abundance of core OTUs varied annually, decreasing in stratified warmers waters and increasing in colder mixed waters. Most core OTUs had a preference for one season, mostly winter, which featured subnetworks with the highest connectivity. Groups of highly associated taxa tended to include protists and bacteria with predominance in the same season, particularly winter. A group of 13 highly-connected hub-OTUs, with potentially important ecological roles dominated in winter and spring. Similarly, 18 connector OTUs with a low degree but high centrality were mostly associated with summer or autumn and may represent transitions between seasonal communities. CONCLUSIONS: We found a relatively small and dynamic interconnected core microbiota in a model temperate marine-coastal site, with potential interactions being more deterministic in winter than in other seasons. These core microbes would be essential for the functioning of this ecosystem over the year. Other non-core taxa may also carry out important functions but would be redundant and non-essential. Our work contributes to the understanding of the dynamics and potential interactions of core microbes possibly sustaining ocean ecosystem function.

Quantitative PCR coupled with melt curve analysis for detection of selected pseudo-nitzschia spp. (Bacillariophyceae) from the Northwestern Mediterranean sea.

The frequency and intensity of Pseudo-nitzschia spp. blooms along the coast of Catalonia have been increasing over the past 20 years. As species from this genus that are documented as toxigenic have been found in local waters, with both toxic and nontoxic species cooccurring in the same bloom, there is a need to develop management tools for discriminating the difference. Currently, differentiation of toxic and nontoxic species requires time-consuming electron microscopy to distinguish taxonomic features that would allow identification as to species, and cryptic species can still remain misidentified. In this study, cells of Pseudo-nitzschia from clonal cultures isolated from seawater were characterized to their species identity using scanning electron microscopy, and subsamples of each culture were used to create an internal transcribed spacer 1 (ITS-1), 5.8S, and ITS-2 ribosomal DNA database for development of species-specific quantitative PCR (qPCR) assays. Once developed, these qPCR assays were applied to field samples collected over a 2-year period in Alfaques Bay in the northwestern Mediterranean Sea to evaluate the possibility of a comprehensive surveillance for all Pseudo-nitzschia spp. using molecular methods to supplement optical microscopy, which can discern taxonomy only to the genus level within this taxon. Total Pseudo-nitzschia cell density was determined by optical microscopy from water samples collected weekly and compared to results obtained from the sum of eight Pseudo-nitzschia species-specific qPCR assays using duplicate samples. Species-specific qPCR followed by melt curve analysis allowed differentiation of amplicons and identification of false positives, and results correlated well with the total Pseudo-nitzschia cell counts from optical microscopy.

New Perkinsea Parasitoids of Dinoflagellates Distantly Related to Parviluciferaceae Members.

Perkinsea is a phylogenetic group of protists that includes parasites of distantly related hosts. However, its diversity is still mainly composed of environmental sequences, mostly obtained from freshwater environments. Efforts to isolate and culture parasitoids of dinoflagellates have led to the description of several phylogenetically closely related species constituting the Parviluciferaceae family. In this study, two new parasitoid species infecting dinoflagellates during recurrent coastal blooms are reported. Using the ribosomal RNA (rRNA) gene phylogenies, we show that both cluster within Perkinsea, one of them at the base of Parviluciferaceae and the other in a distinct branch unrelated to other described species. The establishment of host-parasite lab cultures of the latter allowed its morphological characterization, resulting in the formal description of Maranthos nigrum gen. nov., sp. nov. The life-cycle development of the two parasitoids is generally the same as that of other members of the Parviluciferaceae family but they differ in the features of the trophont and sporont stages, including the arrangement of zoospores during the mature sporangium stage and the lack of specialized structures that release the zoospores into the environment. Laboratory cross-infection experiments showed that the parasitoid host range is restricted to dinoflagellates, although it extends across several different genera. The maximum prevalence reached in the tested host populations was lower than in other Parviluciferaceae members. The findings from this study suggest that Perkinsea representatives infecting dinoflagellates are more widespread than previously thought.

Emerging Parasitic Protists: The Case of Perkinsea.

The last century has witnessed an increasing rate of new disease emergence across the world leading to permanent loss of biodiversity. Perkinsea is a microeukaryotic parasitic phylum composed of four main lineages of parasitic protists with broad host ranges. Some of them represent major ecological and economical threats because of their geographically invasive ability and pathogenicity (leading to mortality events). In marine environments, three lineages are currently described, the Parviluciferaceae, the Perkinsidae, and the Xcellidae, infecting, respectively, dinoflagellates, mollusks, and fish. In contrast, only one lineage is officially described in freshwater environments: the severe Perkinsea infectious agent infecting frog tadpoles. The advent of high-throughput sequencing methods, mainly based on 18S rRNA assays, showed that Perkinsea is far more diverse than the previously four described lineages especially in freshwater environments. Indeed, some lineages could be parasites of green microalgae, but a formal nature of the interaction needs to be explored. Hence, to date, most of the newly described aquatic clusters are only defined by their environmental sequences and are still not (yet) associated with any host. The unveiling of this microbial black box presents a multitude of research challenges to understand their ecological roles and ultimately to prevent their most negative impacts. This review summarizes the biological and ecological traits of Perkinsea-their diversity, life cycle, host preferences, pathogenicity, and highlights their diversity and ubiquity in association with a wide range of hosts.

Description of two new coexisting parasitoids of blooming dinoflagellates in the Baltic sea: Parvilucifera catillosa sp. nov. and Parvilucifera sp. (Perkinsea, Alveolata).

Perkinsea are a group of intracellular protist parasites that inhabit all types of aquatic environments and cause significant population declines of a wide variety of hosts. However, the diversity of this lineage is mostly represented by environmental rDNA sequences. Complete descriptions of Perkinsea that infect marine dinoflagellates have increased in recent literature due to the identification, isolation and culturing of representatives during bloom events, contributing to expand the knowledge on the diversity and ecology of the group. Shallow coastal areas in the Baltic Sea suffer seasonal dinoflagellate blooms. In summer 2016, two parasitoids were isolated during a Kryptoperidinium foliaceum bloom in the Baltic Sea. Morphological features and sequences of the small and large subunit of the ribosomal DNA gene revealed these two parasitoids were new species that belong to the genus Parvilucifera. This is the first time that Parvilucifera infections are reported in the Inner Baltic Sea. The first species, Parvilucifera sp. has some morphological and phylogenetic features in common with P. sinerae and P. corolla, although its ultrastructure could not be studied and the formal description could not be done. The second new species, named Parvilucifera catillosa, has several distinct morphological features in its zoospores (e.g. the presence of a rostrum), and in the shape and size of the apertures in the sporangium stage, which are larger and more protuberant than in the other species of the genus. Infections observed in the field and cross-infection experiments determined that the host range of both Parvilucifera species was restricted to dinoflagellates, each one showing a different host preference. The coexistence in the same environment by the two closely related parasitoids with very similar life cycles suggests that their niche separation is the preferred host.

Assessment of microbial plankton diversity as an ecological indicator in the NW Mediterranean coast.

High-throughput sequencing of microbial assemblages has been proposed as an alternative methodology to the traditional ones used in marine monitoring and environmental assessment. Here, we evaluated pico- and nanoplankton diversity as ecological indicators in NW Mediterranean coastal waters by comparing their diversity in samples subjected to varying degrees of continental pressures. Using metabarcoding of the 16S and 18S rRNA genes, we explored whether alphadiversity indices, abundance of Operational Taxonomic Units and taxonomic groups (and their ratios) provide information on the ecological quality of coastal waters. Our results revealed that only eukaryotic diversity metrics and a limited number of prokaryotic and eukaryotic taxa displayed potential in assessing continental influences in our surveyed area, resulting thus in a restrained potential of microbial plankton diversity as an ecological indicator. Therefore, incorporating microbial plankton diversity in environmental assessment could not always result in a significant improvement of current marine monitoring strategies.

Morphological and phylogenetic data do not support the split of Alexandrium into four genera.

A recently published study analyzed the phylogenetic relationship between the genera Centrodinium and Alexandrium, confirming an earlier publication showing the genus Alexandrium as paraphyletic. This most recent manuscript retained the genus Alexandrium, introduced a new genus Episemicolon, resurrected two genera, Gessnerium and Protogonyaulax, and stated that: "The polyphyly [sic] of Alexandrium is solved with the split into four genera". However, these reintroduced taxa were not based on monophyletic groups. Therefore this work, if accepted, would result in replacing a single paraphyletic taxon with several non-monophyletic ones. The morphological data presented for genus characterization also do not convincingly support taxa delimitations. The combination of weak molecular phylogenetics and the lack of diagnostic traits (i.e., autapomorphies) render the applicability of the concept of limited use. The proposal to split the genus Alexandrium on the basis of our current knowledge is rejected herein. The aim here is not to present an alternative analysis and revision, but to maintain Alexandrium. A better constructed and more phylogenetically accurate revision can and should wait until more complete evidence becomes available and there is a strong reason to revise the genus Alexandrium. The reasons are explained in detail by a review of the available molecular and morphological data for species of the genera Alexandrium and Centrodinium. In addition, cyst morphology and chemotaxonomy are discussed, and the need for integrative taxonomy is highlighted.

Use of the dinoflagellate Karlodinium veneficum as a sustainable source of biodiesel production.

Microalgae are microscopic heterotrophic-autotrophic photosynthesizing organisms with enormous potential as a source of biofuel. Dinoflagellates, a class of microalgae, contain large amounts of high-quality lipids, the principal component of fatty acid methyl esters. The biotic characteristics of the dinoflagellate species Karlodinium veneficum include a growth rate of 0.14 day(-1), a wet biomass of 16.4 g/L, a growth period of approximately 30 days, and an approximate 97% increase in fatty acid content during the transition from exponential phase to stationary phase. These parameters make K. veneficum a suitable choice as a bioresource for biodiesel production. Similarly, two other species were also determined to be appropriate for biodiesel production: the Dinophyceae Alexandrium andersoni and the Raphidophyte Heterosigma akashiwo.

Description, host-specificity, and strain selectivity of the dinoflagellate parasite Parvilucifera sinerae sp. nov. (Perkinsozoa).

A new species of parasite, Parvilucifera sinerae sp. nov., isolated from a bloom of the toxic dinoflagellate Alexandrium minutum in the harbor of Arenys de Mar (Mediterranean Sea, Spain), is described. This species is morphologically, behaviourally, and genetically (18S rDNA sequence) different from Parvilucifera infectans, until now the only species of the genus Parvilucifera to be genetically analyzed. Sequence analysis of the 18S ribosomal DNA supported P. sinerae as a new species placed within the Perkinsozoa and close to P. infectans. Data on the seasonal occurrence of P. sinerae, its infective rates in natural and laboratory cultures, and intra-species strain-specific resistance are presented. Life-cycle studies in field samples showed that the dinoflagellate resting zygote (resting cyst) was resistant to infection, but the mobile zygote (planozygote) or pellicle stage (temporary cyst) became infected. The effects of light and salinity levels on the growth of P. sinerae were examined, and the results showed that low salinity levels promote both sporangial germination and higher rates of infection. Our findings on this newly described parasite point to a complex host-parasite interaction and provide valuable information that leads to a reconsideration of the biological strategy to control dinoflagellate blooms by means of intentional parasitic infections.