Parasitic Dinoflagellate Research Articles

Ecological and morphological features of Amyloodinium ocellatum occurrences in cultivated gilthead seabream Sparus aurata L.; A case study

Understanding the patterns of occurrence of the ectoparasite Amyloodinium ocellatum and the conditions that result in its maintenance at non-dangerous levels for gilthead seabream Sparus aurata could be very useful, since outbreaks of heavy infestation by this parasitic dinoflagellate can cause severe mortality in temperate aquaculture. We have evaluated the interactions between A. ocellatum and related environmental variables for the first time. Biotic and abiotic parameters of water quality in production ponds from a temperate aquaculture (Sado Estuary, Portugal) were monitored and subsequently analysed. Dissolved oxygen, water temperature, pH, phytoplankton biomass and salinity were closely related to A. ocellatum occurrences; dissolved oxygen, water temperature, pH and phytoplankton biomass had significant negative relationships with A. ocellatum trophonts, while salinity had a significant positive relationship with A. ocellatum trophonts in fish gills. Phytoplankton biomass was significantly correlated with increases of dissolved oxygen in production ponds. An increase of rate of water renewal increased salinity, due to persistence of low water levels in production ponds during the water renewal procedure. Salinity negatively affected phytoplankton biomass and consequently the level of dissolved oxygen, raising the probability of A. ocellatum occurrences. Fish biomass in production ponds was correlated with the average and the maximum number of trophonts found in fish gills, highlighting the importance of defining stocking levels and production values in ponds. The present results help to improve understanding of the interactions between biotic and abiotic variables, fish farm management practices and parasite incidence in temperate terrestrial pond aquaculture. A morphological feature of the A. ocellatum tomonts cells in division phase, collected from the most infected fish gills, is discussed. We also give a description and illustration of the phases of the A. ocellatum life cycle.

Hematodinium sp. (Alveolata, Syndinea) detected in marine decapod crustaceans from waters of Denmark and Greenland

Five decapod crustacean species were examined for presence of the parasitic dinoflagellate Hematodinium spp. (Alveolata, Syndinea) by morphological methods (colour and pleopod methods) as well as by PCR and nested PCR with Hematodinium-specific primers. Nephrops norvegicus, Pagurus bernhardus and Liocarcinus depurator were sampled by trawling in Danish waters and Chionoecetes opilio and Hyas araneus were sampled by trapping off the west coast of Greenland. The existence of Hematodinium has not previoiusly been documented in Danish waters, but it was detected in all 3 decapod species examined in the present study. Hematodinium sp. was also detected for the first time in H. araneus and the existence of Hematodinium sp. in Greenlandic C. opilio was documented by PCR. Analyses of 26 Hematodinium sp. ITS1 sequences, including sequences from all 5 host species sampled, revealed more than 95% sequence similarity between 24 of the sequences. Two Hematodinium sp. ITS1 sequences from C. opilio were only 81% similar to the 24 other ITS1 sequences. The nested PCR approach resulted in the highest reported percentages of positive samples for Hematodinium sp.in the hosts investigated (between 45 and 87.5%). However, no decapods were found to be infected with Hematodinium sp. based on morphological methods. Consequently, Hematodinium sp. may be more common than previously believed, and, assuming that the DNA found originated from viable and infectious parasite cells, infections may not always be fatal. We suggest that the hosts investigated may have been subject to latent infections that could develop into a fatal disease only if the hosts were physiologically stressed due to other factors.

Growth and Survival of Siamese Fighting Fish, Betta Splendens, Larvae at Low Salinity and With Different Diets

We evaluated the survival and growth of 3-d post-hatch Siamese fighting fish, Betta splendens larvae, raised during 15 d in salt-free or salt-added freshwater (5 ppt using common table salt) and fed with a sequence of traditional live diets (Chlorella sp., rotifers, and Artemia) or with one formulated diet complemented with a single or daily additions of Chlorella. The highest survival were those of natural-fed larvae, at 5 and 0 ppt (100 and 96.67 ± 5.77%), and there were no significant differences in final length and weight, although these were more variable in the case of the larvae maintained at 5 ppt. The use of table salt had a negative effect on the trophonts of the external parasitic dinoflagellate Piscinoodinium sp., which colonized the body surface of the larvae raised at 0 ppt and were absent on those kept at 5 ppt.

Hematodinium sp. infection of red Paralithodes camtschaticus and blue Paralithodes platypus king crabs from the Sea of Okhotsk, Russia

A disease caused by a parasitic dinoflagellate of the genus Hematodinium was identified in red, Paralithodes camtschaticus, and blue, Paralithodes platypus, king crabs from the north-east region of the Sea of Okhotsk, Russia, during annual stock surveys. No carapace color change was observed even in heavily infected crabs, but diseased crabs possessed creamy-yellow hemolymph, which was visible through the arthrodial membranes of the abdomen and appendages. Several stages of the parasite’s life history, including trophonts, plasmodia, sporonts and macrodinospores, were observed in tissues of infected king crabs. Numerous parasite cells were observed in the lumina of the myocardium, the gills, the connective tissue of antennal glands and the sinuses of nerve ganglia, eyestalks and gastrointestinal tract of king crabs with gross signs of infection. Based on sequencing of the 18S rDNA, it appears that the Hematodinium sp. found in red and blue king crabs is identical or closely related to Hematodinium sp. isolated from crabs of the genera Chionoecetes and Lithodes. Observed prevalences were 0.33% in sublegal male red king crabs, 0.18% in female red king crabs, 0.34% in sublegal male blue king crabs and 0.31% in female blue king crabs.

Reproductive plasticity and local adaptation in the host–parasite system formed by the toxic Alexandrium minutum and the dinoflagellate parasite Parvilucifera sinerae

A parasite threat stimulates adaptive shifts in the life-history strategy (sexual recombination rate) of the toxic bloom-forming dinoflagellate Alexandrium minutum Halim. This microalgae divides asexually when clonal but can also form mobile zygotes (planozygotes) when compatible clones are crossed. Planozygotes usually form resistant dormant stages (resting cysts) although they can also divide. In this study, asexual and sexual cultures were infected with the parasite Parvilucifera sinerae (Perkinsozoa) and the resulting clones classified as susceptible (S), low susceptible (LS), or resistant (R) to the infection. R and LS clones were never of Mediterranean origin, pointing to local adaptation of the parasite. (S × S) crosses were infected faster than either of the parental clones growing asexually. By contrast, (S × R) crosses were resistant to the parasite and produced no resting cysts, even when planozygotes were formed. Therefore, in infected cultures, the planozygotes mainly divided instead of encysting, thus increasing the rate at which recombinant progeny formed. This strategy against infection seems to combine the benefits of quickly producing asexual offspring and increasing recombination. As the susceptibility of the crosses was dependent on parental sexual compatibility, and cultures established by the division of (R × S) planozygotes (F1 offspring) also formed R or LS cultures, resistance may be regulated by several genes or through maternal effects.

Detection and quantification of the free-living stage of the parasitic dinoflagellate Hematodinium sp. in laboratory and environmental samples

Species of Hematodinium are highly pathogenic parasitic dinoflagellates and can cause substantial mortalities in many decapod crustaceans. Periodic outbreaks of Hematodinium disease have significantly impacted stocks of the blue crab Callinectes sapidus throughout the Delmarva Peninsula. Using a primer set targeting the internal transcribed spacer region 1 (ITS1) of the ribosomal RNA (rRNA) gene complex of Hematodinium, we confirmed the presence of the parasite's DNA in water and suspended sediment samples collected from Virginia estuaries of the Delmarva Peninsula by polymerase chain reaction (PCR) on DNA extracted from these environmental samples. Amplicon sequences were highly conserved and 99–100% similar to the ITS1 sequence of the endemic Hematodinium sp. reported from blue crabs in the region. A sensitive real-time PCR assay was developed and validated for the detection and quantification of the parasite DNA. The assay has a limit of detection of less than one parasite cell in 100 ml of environmental water or one gram of sediment; it will provide a valuable tool for our ongoing studies on the transmission dynamics of this parasite. The real-time PCR assay was further tested on a free-living stage of Hematodinium. Sporulation resulting in the release of millions of Hematodinium dinospores from heavily diseased crabs was observed in the laboratory. Some dinospores survived up to 7 days in aquaria outside of the crab host. We speculate that the Hematodinium sp. dinospores have a short free-living stage in the water column and that transmission to new hosts must occur relatively quickly in estuarine habitats.

Molecular detection of Hematodinium sp. in Northeast Pacific Chionoecetes spp. and evidence of two species in the Northern Hemisphere

Hematodinium is a genus of parasitic dinoflagellates that infects crustaceans worldwide including Tanner crabs Chionoecetes bairdi and snow crabs C. opilio in the Northeast Pacific Ocean. The present study describes the optimization of a PCR-based assay for the detection and monitoring of Hematodinium sp. in snow and Tanner crabs. Two fragments, 1682 and 187 bp, were amplified from the 18S ribosomal DNA region of the parasite. The assay performed well in 6 additional decapod species (1 lobster and 5 crabs) infected with Hematodinium spp., suggesting that it could be used to detect Hematodinium spp. in other decapods. We also report Hematodinium spp. infections in the majid crab, Hyas coarctatus, and the lithodid crab, Lithodes couesi. Sequencing of 18S rDNA and the adjacent internal transcribed spacer 1 (ITS1) region of Hematodinium spp. isolated from 7 host species in the present study revealed the presence of 2 Hematodinium clades, one in the blue crab Callinectes sapidus and a second in all other host species. The ITS1 sequences of the 2 clades could not be aligned, but showed a conserved secondary structure that may be related to a functional diversification during a host switch. Comparison of our data with 18S and ITS1 sequence data available in GenBank placed the north Pacific Hematodinium sp. in a clade separate from the Hematodinium sp. infecting the portunoids, C. sapidus, Liocarcinus depurator and Scylla serrata, and within a second clade that infected all other decapod hosts located in the North Pacific and North Atlantic Oceans.

Hematodinium infections in cultured ridgetail white prawns, Exopalaemon carinicauda, in eastern China

Parasitic dinoflagellates in the genus Hematodinium are highly pathogenic to crustaceans. This study presents the first report of Hematodinium infection in shrimps in China. Various forms of the parasite were observed in affected ridgetail white prawn Exopalaemon carinicauda — uninucleate, binucleate and multinucleate sporonts, and occasionally motile dinospores. The parasite caused profound pathological changes in gills, hepatopancreas, heart, and muscles of infected hosts. This parasite was the primary causative agent of the milky shrimp disease which occurred in cultured ridgetail white prawn in eastern China in 2008, and resulted in up to 100% mortality in affected polyculture ponds in Zhoushan, Zhejiang province, China. The pathogen apparently can be transmitted among susceptible hosts in the polyculture ponds. The parasite shared similar antigenic determinants with the Hematodinium sp. identified in the Chinese swimming crab Portunus trituberculatus. Results of histopathology, ultrastructure examination, immunological and molecular identification indicated that this parasite is the same species as that identified in the Chinese swimming crab, P. trituberculatus and mud crab, Scylla serrata from China. Our preliminarily phylogenetic analysis indicated that the parasite is closely related to the Hematodinium sp. identified in C. sapidus and has distinct genetic distance from the Hematodinium sp. identified in northern Europe.

10.1007/s11179-008-1004-5

Five species of parasitic dinoflagellates of the genera Protoodinium, Paulsenella, Duboscquella, Syndinium, and Dissodinium and two species of ellobiopsids of the genus Ellobiopsis are described. Figures and distribution data are provided. All species, except for Dissodinium pseudolunula Swift ex Elbrachter et Drebes, are first records for the Russia Far-Eastern seas.

Expressed Sequence Tags from Amoebophrya sp. Infecting Karlodinium veneficum: Comparing Host and Parasite Sequences

Parasitic dinoflagellates of the genus Amoebophrya play important roles in the ecology of estuaries and open ocean environments. Little is known of the cell and molecular biology of Amoebophrya, but the genus is intermediate on phylogenetic trees between apicomplexans and typical dinophycean dinoflagellates. Here, we constructed four cDNA libraries, from different stages after infecting the host, Karlodinium veneficum, with Amoebophrya sp. These libraries were used to generate 898 expressed sequence tags (ESTs), with sequences attributed to either the host or parasite, based on AT bias, codon usage, and occurrence during infection. Overall, 209 sequences were attributable to the parasite and 685 to the host. The 50 putative parasite sequences with good protein matches in GenBank were used to find the same protein from host ESTs. For 26 genes, both host and parasite sequences were identified, of which 20 encoded ribosomal proteins. PCR for seven predicted parasite and two host genes were used to confirm attributions. The most common host and parasite ESTs were compared to see if multiple gene copies were present. The host plastocyanin gene had multiple sequence variants, but parasite rps27a contained only one polymorphism, likely due to an amplification error. Amplification, cloning, and sequencing of five parasite protein-coding genes suggested that the parasite has a single sequence for each gene, but three host genes were found to have multiple variants. The genome of Amoebophrya sp. infecting K. veneficum appears to have an organization more similar to other eukaryotes than to the tandem gene arrangements found in dinoflagellates.

Transmission of the parasitic dinoflagellate Hematodinium sp. infection in blue crabs Callinectes sapidus by cannibalism

Infection with the parasitic dinoflagellate Hematodinium sp. can be devastating to blue crab Callinectes sapidus populations. Morbidity and mortality appear to depend on the burden of parasitic organisms. Heavily infected crabs become lethargic and, if not preyed upon, succumb to overwhelming infection. We report on the transmission of Hematodinium sp. into blue crabs that were fed pieces of infected tissues and examined for evidence of infection at time periods from 1 to 48 h and for the general state of their health after 4 d. During the first 16 h after feeding, Hematodinium sp. was found in the gut, followed by large increases in hemolymph hemocytes and the appearance of hemocytic nodules in tissues. By 16 h, the hemocytic nodules appeared poorly circumscribed and disorganized. No nodules were seen in a heavily infected crab after 24 h. By the end of the 48 h after feeding, 73% (11 of 15) of the crabs had shown evidence of infection with Hematodinium sp. Those crabs with infection intensities (Hematodinium sp. as percent of cells in hemolymph) higher than 20% were dead within 4 d.

Life cycle and molecular phylogeny of the dinoflagellates Chytriodinium and Dissodinium, ectoparasites of copepod eggs

The dinoflagellates Chytriodinium affine, C. roseum and Dissodinium pseudolunula are ectoparasites of crustacean eggs. Here, we present new observations regarding their life cycle based on coastal plankton samples and incubations and analyze their molecular phylogeny using the small subunit ribosomal RNA gene (SSU rDNA) as a marker. In contrast to the typical stages already documented for its life cycle, we observed that D. pseudolunula dinospores may exceptionally differentiate inside a globular cyst. Despite its parasitic life style, the cysts and dinospores of D. pseudolunula contain chlorophyll a. We obtained the first SSU rDNA sequences for the genera Chytriodinium (the type C. roseum and C. affine) and Dissodinium ( D. pseudolunula). Classical taxonomical schemes have ascribed these genera to the order Blastodiniales. However, our SSU rDNA-based phylogenetic analysis shows that these ectoparasites form a clade in the Gymnodinium sensu stricto group, unarmored dinokaryotic dinoflagellates of the order Gymnodiniales. They branch in a subgroup composed of warnowiids, polykrikoids, the type of Gymnodinium, G. fuscum and G. aureolum. Although Chytriodinium and Dissodinium appear to be relatives based on SSU rDNA phylogeny, feeding and host specificity, their life cycles are substantially different. Based on these data we consider that the type of life cycle is a poor criterion for classification at the family level. We suggest that the morphology of the infective cell is probably the most reliable phenotypic characteristic to determine the systematic position of parasitic dinoflagellates.

Sporogenesis of an extracellular cell chain from the spheroidal radiolarian host Haliommilla capillaceum (Haeckel), Polycystina, Protista

We present the first record of a living solitary spherical polycystine radiolarian, Haliommilla capillaceum (Haeckel), with a chain of extracellular cells. The extracellular cells divided seven times over about 4 h from the 8-cell to the 1024-cell stage and then swam away. At the 1024-cell stage, the cells were very small isospores (ca. 12 μm in length). After final cytokinesis, the cells remained essentially immobile, with only occasional movement. After this immobile stage, they abruptly swam out in a burst into seawater. The cell division patterns and cell shape are similar to the sporogenesis of the host-specific parasitic dinoflagellate Duboscquella sp. infecting a tintinnid. Precise identification, however, needs molecular analysis, transmitted scanning images and proliferation test.

Molecular Phylogeny of Parvilucifera prorocentri (Alveolata, Myzozoa): Insights into Perkinsid Character Evolution

Perkinsids and colpodellids are lineages that diverged near the origins of dinoflagellates and apicomplexans, respectively, and provide compelling insights into the earliest stages of alveolate evolution. Perkinsids, including Perkinsus and Parvilucifera, are intracellular parasites of animals and dinoflagellates and possess traits also known in syndineans, dinokaryotes (mainly free living dinoflagellates), and colpodellids. An improved understanding of perkinsid biodiversity and phylogeny is expected to shed considerable light on the evolutionary origins of syndineans and dinokaryotes as well as the cellular identities of environmental sequences derived from marine and freshwater habitats. Accordingly, the small subunit (SSU) rDNA sequence from Parvilucifera prorocentri, a tube-forming intracellular parasite of the marine benthic dinoflagellate Prorocentrum fukuyoi, was determined. Molecular phylogenetic analyses demonstrated, with very high statistical support, that P. prorocentri branched as a sister lineage to a divergent clade consisting of Parvilucifera infectans and Parvilucifera sinerae. The entire Parvilucifera clade was nested within a more inclusive and modestly supported clade consisting of Perkinsus and several environmental sequences. Because P. prorocentri possessed a novel combination of ultrastructural features known in Perkinsus, Parvilucifera, and/or syndineans (i.e. germ tubes, trichocysts, and a syndinean-like nucleus), establishing the molecular phylogenetic position of this species enabled us to build a more comprehensive framework for understanding the earliest stages in the evolution of myzozoans.

Infection by Amoebophrya spp. parasitoids of dinoflagellates in a tropical marine coastal area

Infection of marine dinoflagellates by the parasitic dinoflagellate Amoebophrya spp. plays an important role in population dynamics and carbon flow in marine food webs. It has been extensively reported that Amoebophrya parasitoids occur in temperate coastal areas of the northern hemisphere; however, little is known about their distribution and importance in tropical areas and southern oceans. We used an rRNA-based, fluorescent in situ hybridization assay to detect Amoebophrya spp. infections during the decline of a late-summer dinoflagellate population dominated by Ceratium falcatiforme in a tropical coastal area of the southern Atlantic Ocean subjected to recurrent upwelling―downwelling cycles. Conditions during our survey were typical of downwelling when oligotrophic waters dominate the area. C. falcatiforme was the most infected host, with a prevalence averaging 2 % over the study area at the beginning of sampling. At a fixed sampling station monitored over 4 wk, Amoebophrya prevalence escalated from 1 to 7 % over a 6 d period, concomitant to a 94 % decrease in host cell numbers. Infection by Amoebophrya was estimated to have killed ca. 11 % of the host cell population within this period; thus, parasitism was not the main factor behind the C. falcatiforme population decline. Estimates based on biovolume calculations indicate that ca. 6.5 % of the carbon in the decaying C. falcatiforme population was transformed into parasitoid dinospores, which became available to tintinnid ciliates that were abundant during our survey. Such a trophic link might be relevant in tropical oligotrophic waters when the system is characterized by a microbial food web structure.

Real-time PCR-based assay for quantitative detection of Hematodinium sp. in the blue crab Callinectes sapidus

Hematodinium sp. is a parasitic dinoflagellate infecting the blue crab Callinectes sapidus and other crustaceans. PCR-based assays are currently being used to identify infections in crabs that would have been undetectable by traditional microscopic examination. We therefore sought to define the limits of quantitative PCR (qPCR) detection within the context of field collection protocols. We present a qPCR assay based on the Hematodinium sp. 18S rRNA gene that can detect 10 copies of the gene per reaction. Analysis of a cell dilution series vs. defined numbers of a cloned Hematodinium sp. 18S rRNA gene suggests a copy number of 10,000 per parasite and predicts a sensitivity of 0.001 cell equivalents. In practice, the assays are based on analysis of 1% of the DNA extracted from 200 microl of serum, yielding a theoretical detection limit of 5 cells ml(-1) hemolymph, assuming that 1 cell is present per sample. When applied to a limited field survey of blue crabs collected in Maryland coastal bays from May to August 2005, 24 of 128 crabs (18.8%) were identified as positive for Hematodinium sp. infection using qPCR. In comparison, only 6 of 128 crabs (4.7%) were identified as positive using traditional hemolymph microscopic examination. The qPCR method also detected the parasite in gill, muscle, heart and hepatopancreas tissues, with 17.2% of the crabs showing infection in at least one of these tissues. Importantly, it is now possible to enumerate parasites within defined quantities of crab tissue, which permits collection of more detailed information on the epizootiology of the pathogen.

Hematodinium sp. in Irish Cancer pagurus fisheries: infection intensity as a potential fisheries management tool

Infection of Cancer pagurus by a parasitic dinoflagellate of the genus Hematodinium is described for the first time in Ireland. An industry-based monitoring programme was established to determine seasonality of infection intensity and prevalence in the country's 3 largest brown crab fisheries in the southwest, north and southeast. The parasite was present in all areas for the majority of sampling periods, with highest prevalences recorded in pre-recruit animals of both sexes. Microscopic examination of haemolymph revealed trophont, plasmodial and dinospore stages of the parasite. Overall prevalence in males (16%) was higher than in females (9%). Prevalence of Hematodinium sp. infection ranged from 0 to 51%, but a distinct seasonal trend was not apparent. Infection intensity was seasonal with significantly higher peaks occurring in late autumn/early winter months than in other quarters, corresponding to industry reports of moribund and dead pink-shelled crabs in commercial catches. We postulate that seawater temperature or a temperature-linked process is a key factor in triggering the final stages of infection, as significant autumn peaks were followed by a reduction in infection intensity as temperature decreased in the late winter/early spring months with no increase in intensity again until the following autumn. We propose that infection intensity, rather than prevalence, provides a more appropriate indication of the period when there is greatest potential for biological and economic impacts; the parameter's application as a fisheries management tool is discussed.

Prevalence and phylogeny of parasitic dinoflagellates (genus Blastodinium) infecting copepods in the Gulf of California.

Plankton samples collected from Bahía de La Paz, Baja California Sur, Mexico in June 2008 revealed infection of copepods by Blastodinium spp. Of eight copepod genera encountered in the samples, only one (Paracalanus) was parasitized by Blastodinium. Infection prevalence was low, 0.6 % to 2.0 %, withall parasitized individuals being females tentatively identified as Paracalanus parvus. All but one infected host examined during the study was parasitized by Blastodinium crassum, the sole exception being a Paracalanus cf. parvus infected by Blastodinium contortum. A phylogeny constructed using a dataset of ~1.3 Kb sequences of the small subunit ribosomal DNA gene of B. contortum and B. crassum from Bahía de La Paz and 36 other dinoflagellate sequences available in GenBank, including all available for Blastodinium,placed Blastodinium as a monophyletic clade deep within the Dinokaryota. Sequence divergence among B. contortum ex Paracalanus cf. parvus from Bahía de La Paz, B. contortum ex P. parvus from the Mediterranean Sea, B. contortum ex Clausocalanus arcuicornis, B. crassum ex P. cf. parvus from Bahía de La Paz, and B. navicula ex Corycaeus giesbrechti supported separation of the three parasites as distinct species.

Bitter crab syndrome in two species of king crabs from the Sea of Okhotsk

The disease caused by parasitic dinoflagellates of the genus Hematodinium was found in the red Paralithodes camtschaticus and blue P. platypus king crabs from the Sea of Okhotsk. The hemolymph of diseased crabs was cream colored, opaque, and dense. Numerous dinokaryotic trophonts and multinucleate plasmodia of the parasite were revealed in the hemolymph and internals of diseased animals. The parasitic infection was recorded in females and juvenile males from August to mid-October.

Responsibility of microzooplankton and parasite pressure for the demise of toxic dinoflagellate blooms

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 53:211-225 (2008) - DOI: https://doi.org/10.3354/ame01245 Responsibility of microzooplankton and parasite pressure for the demise of toxic dinoflagellate blooms David J. S. Montagnes1,*, Aurélie Chambouvet2,3, Laure Guillou2,3, Andy Fenton1 1School of Biological Sciences, University of Liverpool, Biosciences Building, Crown Street, Liverpool L69 7ZB, UK 2Université Pierre et Marie Curie – Paris 6, Station Biologique de Roscoff, Place Georges Teissier, BP 74, 29680 Roscoff, France 3CNRS, UMR 7144, Laboratoire Adaptation et Diversité en Milieu Marin, Place Georges Teissier, BP 74, 29680 Roscoff, France *Email: dmontag@liv.ac.uk ABSTRACT: Two mechanisms proposed to control dinoflagellate blooms—parasitism by eukaryotes (e.g. Amoebophrya sp.) and grazing by microzooplankton—have been explored through previous laboratory and field studies but lack quantitative assessment. We modelled the relative effect of these mechanisms. We used literature values to embed an Anderson-May host-microparasite model into a microbial food web model (nano- and microphytoplankton, nano- and microzooplankton, a toxic dinoflagellate, a dinoflagellate parasite). Three scenarios were examined to simulate the introduction of a toxic dinoflagellate and its parasite into an environment: (1) a food web, including autotrophic nano- and microplankton, heterotrophic flagellates, and ciliates; (2) as for Case 1 but with a toxic dinoflagellate; (3) as for Case 2 but with a dinoparasite. This mimics observations in a French estuary, where a toxic dinoflagellate began blooming in 1988; since 1998, blooms appear to have become regulated, and numerous parasitic infections by Amoebophrya sp. occurred from 2004 to 2006. After supporting parasite control of dinoflagellate blooms, we assessed the effects of observed ranges of key variables associated with the parasite and other components of the food web on parasite control of the dinoflagellate population. Population dynamics were examined over 50 d. In Case 1, all taxa had 10 to 20 d blooms. In Case 2, the toxic dinoflagellate population dynamics mimicked that of the microphytoplankton, and this dinoflagellate was reduced in numbers but not extirpated by microzooplankton grazing. In Case 3 population blooms occurred, and the parasite virtually eliminated the dinoflagellate over ~10 d. Sensitivity analysis indicated that our assessment was robust. We propose that the decline in toxic dinoflagellates in the French estuary may have been due to an introduced dinoparasite. In general, we suggest that parasites may have greater impact on toxic dinoflagellate blooms than microzooplankton grazers; the parasites have the potential to eliminate the toxic dinoflagellates. We recommend that such parasites be incorporated into more complex food web models. KEY WORDS: Amoebophrya · Alexandrium minutum · Dinospore · Harmful algal bloom · HAB · Microbial food web · Model · Plankton · Red tide Full text in pdf format PreviousNextCite this article as: Montagnes DJS, Chambouvet A, Guillou L, Fenton A (2008) Responsibility of microzooplankton and parasite pressure for the demise of toxic dinoflagellate blooms. Aquat Microb Ecol 53:211-225. https://doi.org/10.3354/ame01245 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 53, No. 2. Online publication date: October 31, 2008 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2008 Inter-Research.