Biological and Molecular Geochemical Evidence for Dinoflagellate Ancestors in the Upper Sinian-Cambrian

Dinoflagellates are single celled organisms that reflect the ecological conditions in modern oceans and lakes. Their earliest undisputed fossil record suggests that dinoflagellates originated from the Middle Triassic (c. 240 Ma ago). However, the presence of molecular biomarkers (dinosterane, 4α-methyl-24-ethylcholestane and tria-romatic dinosteroids) in rock extracts and coccoid dinoflagellate fossils from the upper Sinian to Cambrian of the Tarim basin confirms the hypothesis that dinoflagellates have an ancient origin, and predate the oldest undisputed dinoflagellate fossils at least by 300 Ma, as early as the late Sinian-Cambrian.

Dinoflagellate cysts in recent marine sediment from Guangxi, China

Total of 33 species of dinoflagellate cysts were discovered from surface sediment in the searegion of Guangxi, among them 12 cyst types (Diplopsalopsis sp. 1, D. sp.2, D. sp.3, Cochlodiniumsp., Protoperidinium sp. 1, P. sp. 2, P.compressum, Scrippsiella sp. 1, S. sp. 2, Alexandriumsp. 1, A. sp. 2, A. sp. 3) were first reported from the South China Sea. And one cyst type(Cochlodinium sp. ) was first reported in the world. Scrippsiella trochoidea is the dominant species inthis area, accounting for 45 % of all the cysts. There are 2 cysts of toxic dinoflagellate (Alexandriumtamarensis and Gymnodinium catenatum ). But there is no relationship between cyst number and grainsize distribution.

Validation of reference genes for gene expression studies in the dinoflagellate Akashiwo sanguinea by quantitative real-time RT-PCR

The accurate measurement of gene expression via quantitative real-time reverse transcription PCR(q RT-PCR)heavily relies on the choice of valid reference gene(s) for data normalization. Resting cyst is the dormant stage in the life cycle of dinoflagellate, which plays crucial roles in HAB-forming dinoflagellate ecology. However, only limited investigations have been conducted on the reference gene selection in dinoflagellates. Gap remained in our knowledge about appropriate HKGs for normalizing gene expression in different life stages, which laid obstacles for the application of q RT-PCR to the HAB-forming group. In this study, six candidate reference genes,18 S ribosomal RNA(18S), glyceraldehyde-3-phosphate dehydrogenase(GAPDH), α-tubulin(TUA), β-tubulin(TUB), actin(ACT) and cytochrome oxidase subunit 1(COX1), were evaluated for their expression stability with q RT-PCR and three statistical algorithms(Ge Norm, Norm Finder, and Best Keeper) for the cosmopolitan, harmful algal bloom-forming dinoflagellate Akashiwo sanguinea. Expression patterns were observed across 18 biological samples, including cells at resting stages(resting cysts), different growth stages, in darkness, exposed to abscisic acid(ABA) and exposed to temperature stress. The results indicated that TUA, 18 S and GAPDH were relatively stable across all tested scenarios. While the best-recommended reference genes differed across experimental groups, the pairs of ACT and TUA, 18 S and GAPDH were the most reliable for cells at different growth stages and darkness treatment. The combination of TUA and TUB was the best choice for normalization in resting cysts and in ABA treatment, respectively. The pair of ACT and COX1 was suitable for temperature treatments. This study was the first to investigate the stable internal reference genes in dinoflagellates at different stages of life cycle,particularly in resting cysts. Our results provided useful information for selection of reference genes in dinoflagellates regarding quantification of gene expression at different experimental scenarios, which will facilitate more accurate and widespread use of q RT-PCR in gene analysis of dinoflagellates and help to design primers targeting orthologous genes in other algal species.

Preliminary study on taxonomy of dinoflagellate cysts from major estuary and bays of Fujian Province, China

According to the morphology, wall structure, color, ornamentation, etc., 25 speciesbelonging to 9 genera are identified and described from 144 sediment samples of the Xiamen Harbor, themouth of the Minjiang River and the Sansha Bay. Among them there are 2 toxic species: Alexandriumminutun,A.tamarenes, 4 harmful species: Alexandrium affine,Lingulodinium polyedrum,Scrippsiella trochoide,Gonyaulax spinifera. It shows that 11 species of dinoflagellate cysts( Alexandrium a ffine , A . minutum , Diplopelta cf. parva, Polykrikos cf. schwartzii , protoceratiumreticulatum , Protoperidinium minutum , P. cf. minutum , P. cf americanum and Alexandrium sp.,Protoperidinium sp. 1, P. sp. 2) are first recorded along the coast of Fujian Province, China.These newly discovered species might be transported to the coastal sea of Fujian Province by ballast waterof international trade vessels.

Blooms of Prorocentrum donghaiense reduced the species diversity of dinoflagellate community

Most of reported harmful algal blooms(HABs)of microalgae(75%)have been caused by dinoflagellates.Studies on the negative effects of HABs have generally focused on animals,valuable organisms in particular,and environmental factors such as dissolved oxygen and nutrients,but relatively fewer on community level,particularly that using metagenomic approach.In this study,we reported an investigation on the effects of a HAB caused by the dinoflagellate Prorocentrum donghaiense on the species diversity and community structure of the dinoflagellate sub-community via a pyrosequencing approach for the samples taken before,during,and after the bloom season of P.donghaiense in the East China Sea.We sequenced partial 28S rRNA gene of dinoflagellates for the field samples and evaluated the species richness and diversity indices of the dinoflagellate community,as a sub-community of the total phytoplankton.We obtained 800185 valid sequences(categorized into 560 operational taxonomic units,OTUs)of dinoflagellates from 50 samples and found that the biodiversity of dinoflagellate community was significantly reduced during the blooming period in comparison to that in pre-and after-blooming periods,as reflected in the four diversity indices:the species richness expressed as the number of OTUs,Chao1 index,Shannon index(evenness),and Gini-Simpson index.These four indices were all found to be negatively correlated to the cell density of the bloom species P.donghaiense.Correlation analyses also revealed that the P.donghaiense cell abundance was correlated negatively with NO3--N,and NO2--N,but positively with total nitrogen(TN)and total phosphorus(TP).Principal coordinates analysis(PCoA)showed that the community structure of dinoflagellates was markedly different among the different sampling periods,while the redundancy analysis(RDA)revealed P.donghaiense abundance,salinity,NO3--N,and SiO32-were the most four significant factors shaping the dinoflagellate community structure.Our results together demonstrated that HABs caused by the dinoflagellate P.donghaiense could strongly impact the aquatic ecosystem on the sub-community level which the blooming species belongs to.

Effects of Ultraviolet(UV)Radiation on Outdoor-and Indoor-Cultured Prorocentrum lima,a Toxic Benthic Dinoflagellate

The effects of different types of ultraviolet(UV)radiation(UVR,wavelength=280-400 nm)and light intensities on cell growth,pigment composition,UV-absorbing compounds(UVACs),and chlorophyll a(Chl a)fluorescence were studied in dinoflag-ellate Prorocentrum lima cultured outdoors for 16 days and indoors for 18 days.In the outdoor experiment,UVA radiation(320-400 nm)increased the growth rate of this dinoflagellate when solar light intensities were<12%;decreased growth rates were observed when intensities were>12%.Exposure to UVB radiation(280-320 nm)alleviated the negative effects of UVA.In the indoor ex-periment,UVA and low doses of UVB enhanced growth rates.Addition of low doses of UVB to UVA exposure resulted in higher contents of Chl a and photoprotective pigments compared with UVA exposure only.The results of both experiments showed that UVB is the primary signal of UVAC synthesis.High-dose UVB exposure accelerated growth rates when UVAC contents were maintained at high levels,suggesting that the latter plays a key role in UVR damage protection.Furthermore,the repair rate was en-hanced by UVB exposure after 16 days of culture.This study confirms the positive effects of UVA and UVB on the growth of P.lima,with the latter enhancing the photoprotective and recovery pathways of the species.

The impact of biotic and abiotic factors on the distribution of surface sediment dinoflagellate cyst assemblages on the Nanji Island in the East China Sea

The dinoflagellate cyst assemblages on the Nanji Island in the East China Sea, are documented at the first time to construct a quantitative overview of the cyst bank from 2014 to 2015. Thirty-four morphotypes from six groups are identified and quantified at eight sampling sites around the island, including a high proportion of potentially toxigenic species(14%). Autotrophic dinocysts constitute 74% of the total cyst counts, which is relatively low(two to thirty-three per millilitre sediment) compared with previous studies in adjacent areas. Scrippsiella trochoidea and Protoperidinium avellana are the most abundant autotrophic and heterotrophic species, respectively. A multivariate analysis is performed to assess associations between dinocysts and abiotic or biotic variables.Differentiation among seasons is evident in the detrended correspondence analysis(DCA) ordination plot, while a spatial pattern is not clearly revealed despite heterogeneity of the hydrodynamic conditions between sampling sites. Soluble reactive phosphate, the ratio of nitrogen to phosphorus concentrations and Karenia mikimotoi bloom are the three factors significantly(P<0.05) related to surface sediment cyst assemblage defined by the canonical correspondence analysis(CCA), highlighting the importance of nutrient regime to a dinocyst distribution in this area. Although attempts to address the origin of HAB events in recent years using seed banks have failed, knowledge can be valuable for further investigation of dinocyst dynamics and potential toxin threats on the Nanji Island.

Reviving and characterizing three species of dinoflagellate cysts dormant for about 70 years in the East China Sea:Biecheleria brevisulcata,Biecheleriopsis adriatica,and Scrippsiella donghaienis

Many marine dinoflagellates can form resting cysts as a part of their life cycle,and the cysts could be buried in sediment and remained viable for as long as over 150 years.However,only a very limited number of cyst species have been revived from long-buried sediments and investigated in regard to a possible shift in the intra-specific genetic structure of a species detected from the historical record at a particular location.Here,we report a successful germination of three species of resting cysts that were sampled from the depth dated back to 1941±18 AD from a 44-cm sediment core from the East China Sea.Seven isolates were established from germination of single cyst isolation or multi-cyst germinations.LSU rRNA gene or ITS sequences of these strains were obtained,then they were identified to be Biecheleria brevisulcata(five strains),Biecheleriopsis adriatica(one strain),and Scrippsiella donghaienis(one strain)in terms of morphology and rRNA gene sequence.Biecheleria brevisulcata strain 1,Bps.adriatica strain 21,and S.donghaienis strain 23 were examined in detail with light microscope(LM)and scanning electron microscope(SEM),and analyzed with high performance liquid chromatography(HPLC)for their pigment compositions,and genetic diversity.We also confirmed the presence of a resting cyst of Bps.adriatica in the field for the first time.The LSU rRNA gene-based genetic distances of Bps.adriatica from that obtained from water sample,single-cell PCR sequencing for the cysts isolated from the surface sediment of the same sea area and that reported from other regions during the recent years,and ITS-based genetic distances of S.donghaienis from that obtained from cysts isolated from the surface sediment of the same location and that reported from other regions during the recent years indicated that the intra-specific genetic structure of each species in the sampling area may have shifted during the last 70 years.Our work confirms that B.brevisulcata,Bps.adriatica,and S.donghaienis,all described as new species around 2010,have inhabited the East China Sea for about 70 years.The present work reports for the first time the revival of dinoflagellate resting cysts long-buried in the coastal sediments of China,which facilitates further study on the historical occurrences of other harmful dinoflagellates and their relevance to the regional climate and environmental changes in China.

Mechanisms of high-frequency dinoflagellate blooms of Scrippsiella trochoidea in Daya Bay,South China Sea

Harmful algal blooms(HABs),such as dinoflagellate blooms,have adverse eff ects on coastal water environments,causing seafood contamination and aquaculture economic losses.Based on historical HAB records data(1983-2017)and phytoplankton data(1982-2008),field investigation and laboratory experiments on frequently bloomed dinoflagellate species,we analyzed the possible mechanisms of high frequency dinoflagellate blooms in Daya Bay,South China Sea.The HAB records data show that the proportion of dinoflagellate blooms increased from 15% before 1994 to 77.27% after 2011.The bloom occurred mostly in summer(57.58%)in Yaling Bay(76.92%).The species inducing dinoflagellate blooms tended to be singular over time,with high proportion of Scrippsiella trochoidea(42.86%-88.88%).Laboratory experiment and field investigation of S.trochoidea bloom showed that this species grew rapidly at 30℃ and 32.1℃(r=0.47,P<0.05),respectively.Results indicate that temperature rise in Yaling Bay was aff ected by nearby nuclear plant effluent,which favored the S.trochoidea bloom.Coincidently,nearly 70% of S.trochoidea blooms occurred in about 7 days after local spring tide and strong wind(wind velocity>7 m/s),which disturbed the resting cysts in the sediment and bring them into surface layer.Taking into account of environmental parameters,algae source,and atmospheric conditions,the probable mechanism of S.trochoidea bloom could be explained.The cysts were resuspended by the spring tide or strong wind,germinated at the bottom layer,reproduced rapidly at higher temperature,and then aggregated in the inner bay(Yaling Bay),resulting the frequent S.trochoidea bloom.Considering the influences of global warming and human activities,it should be taken more attention that dinoflagellate blooms might increase along the coastal waters.

Biodiversity and distribution of benthic dinoflagellates in tropical Zhongsha Islands,South China Sea

Benthic dinoflagellates have attracted increasing attention in recent years because of their toxicity and ability to form extensive harmful algal blooms.Ostreopsis producing palytoxin and its analogs,Gambierdiscus producing ciguatera toxins,and Prorocentrum producing okadaic acid and dinophysis toxins,have been concerned as serious human poisonings.We explored the benthic dinoflagellate biodiversity and distribution characteristics of a series of tropical reefs in 20-40-m water depth in wet season in the Zhongsha Islands in South China Sea using morphological,phylogenetic,and cell counting methods.Results show that benthic dinoflagellates in the islands are rich in biodiversity and 15 species from genera Amphidinium,Coolia,Ostreopsis,and Prorocentrum were identifi ed:Amphidinium carterae,A.magnum,A.massartii,A.operculatum,Coolia canariensis,C.malayensis,C.palmyrensis,C.tropicalis,Ostreopsis cf.ovata,Prorocentrum concavum,P.cf.sculptile,P.emarginatum,P.hof fmannianum,P.lima,and P.rhathymum.Among them,A.magnum is reported for the first time in Chinese waters.The abundance of benthic dinoflagellates was relatively low at 88-4345 cells/100 cm^(2) on sediment and 10-91 cells/g on macroalgae.Prorocentrum and Amphidinium were the dominant and subdominant genera,respectively.It is speculated that the low abundance of benthic dinoflagellates is closely related to the scarcity of macroalgae and stronger water motion at the depth>15 m in Zhongsha Islands.This study expanded the study in biodiversity of benthic dinoflagellates in Chinese waters,and revealed the distribution characteristics of harmful benthic microalgae in reef habitats.

Molecular Assessment of Athecate Dinoflagellates of the Order Gymnodiniales (Dinophyceae) in Todos Santos Bay, Baja California México

The Gymnodiniales are the most important group of athecate dinoflagellates both for its abundance and distribution and for the harmful potential of several of its species. Although morphologically it is well known, phylogenetically it has been very little studied. HABs impact important coastal activities in Todos Santos Bay, so a detailed characterization of potentially toxic phytoplankton species is crucial in monitoring surveys. Therefore, the aim of the present study was to carry out a molecular assessment to the morphospecies of Gymnodiniales currently recorded in Todos Santos Bay, from which discussed their phylogenetic relationships. 120 samples were obtained from monthly samplings during 2019 to 2020. For all of them, 18 s small subunit (SSU) and 28 s large subunit (LSU) genes of rDNA were amplified and sequenced using single-cell PCR. Also, they were photographed. A phylogenetic analysis was performed on MrBayes from the combined 18 s - 28 s data set. The presence of 15 phylogenetic entities was confirmed, four of which constituted new records for the Mexican Pacific, as well as three harmful species, one toxic and two bloom-forming. Our molecular results evidenced the need to re-evaluate the current taxonomic system of athecate dinoflagellates at all hierarchical levels, integrating them with morphological evidence without preponderance of this criterion over taxonomic decisions.

Spatio-temporal Variability of Dinoflagellates in Different Salinity Regimes in the Coast of Rakhine

Regarding the spatial and seasonal variations of dinoflagellates in different salinities regimes,Prorocentrum rostratum showed a strong correlation with high salinity(≥29 ppt.).However,P.micans had a negative correlation with salinity.In Dinophysoids,Dinophysis caudata showed a wide salinity tolerance than other species in the group.D.miles,Ornithocercus magnificus,O.steinii,and O.thumii showed a strong correlation with salinity.In Gonyaulacoids,Ceratium furca,C.fusus,C.horridum,C.trichoceros,C.tripos,Gonyaulax polygramma,G.spinifera,and Pyrophacus magnificus showed a strong correlation with salinity.In Peridinoids,Protoperidinium depressum,P.oblongum,P.oceanicum,P.pyreforme,and Podolampus palmipes showed a strong correlation with salinity.In Gymnodinoid and Noctilucoid,Gyrodinium estuariale and N.scintillans showed a strong correlation with salinity.

青海湖1.8万年以来甲藻Dinoflagellate群落变化及其古气候环境的指示意义

近年来,有学者通过古DNA方法成功地重建了海洋沉积记录中古微生物群落变化,并借此反演了当地古环境—气候变化。然而,此种方法对于陆地湖泊沉积记录是否适用仍然有待研究。采用聚合酶链式反应（PCR,Polymerase Chain Reaction）、变性梯度凝胶电泳（DGGE,Denaturing Gradient Gel Electrophoresis）与实时荧光定量聚合酶链式反应（Q-PCR,Quantitative PCR）相结合的综合分析技术手段,系统研究青海湖5.8 m（时间跨度为~18 500 a）沉积柱中的甲藻（Dinoflagellate）多样性和丰度变化。研究结果显示,青海湖Dinoflagellate藻18S rRNA基因序列主要与海洋型藻类Woloszynskiahalophila和Scrippsiellahangoei相近（~98%序列相似性）。定量Q-PCR结果显示,每克沉积物含有dinoflagellate藻类18S rRNA基因丰度范围为2.27×10~3~8.55×10~6拷贝。另外,Dinoflagellate藻类18S rRNA基因丰度与总有机碳含量成显著正相关（R=0.408,p=0.0001）。对比分析揭示,较高的藻类丰度对应高总有机碳含量和较低的可溶解性盐电导率;反之,较低的藻类丰度对应较高的可溶解性盐电导率和较低的总有机碳含量。在青海湖区,总有机碳指示着季风降雨变化,并间接地指示着外源输入和湖泊营养状况变化,然而可溶性盐电导率则指示着湖泊盐度变化。综上所述,青海湖沉积柱Dinoflagellate藻类丰度可能反应了历史时期湖泊营养状况和盐度波动情况。

Preliminary Palynostratigraphic Data on the Middle Albian-Lower Cenomanian of the Tarfaya Basin, Southwest Morocco

The Tarfaya-Laayoune-Boujdour basin is of great interest to oil exploration;on this aspect, there has been exploration by the Moroccan National Office of Hydrocarbons and Mining, which has shown indices of oil and/or gas recorded in most wells drilled in the Moroccan Atlantic margin. FD-1 well, object of this study, is one of these wells located in the offshore Tarfaya-Laayoune-Boujdour basin. A palynological examination of the processed samples revealed the existence of organic palynomorphs, well conserved and rich in dinoflagellate cysts. The latter are composed of worldwide marker taxa of high stratigraphic resolutions. Thus, the middle upper Albian (3408 - 3408.5 m) is determined by the presence of Cribroperidinium tensiftense, Tehamadinium coummia, Spiniferella cornuta, Chichaouadinium vestitum, Dinopterygium alatum and Litosphaeridium arundum. The upper Albian (3406.5 - 3408 m) is characterized by the FOs of marker taxa: Cyclonephelium chabaca, Cribroperidinium auctificum and Chichaouadinium vestitum. Lastly, the upper Albian-lower Cenomanian transition is defined between 3404 m and 3406.50 m, as suggested by the FOs of Chichaouadinium boydii, Dinopterygium tuberculatum, Sepispinula ancorifera, Litosphaeridium conispinum, Prolixosphaeridium conulum and Xenascus ceratioides.

VISUALIZATION OF THE CHROMOSOME SCAFFOLD IN A PRIMITIVE EUKARYOTE DINOFLAGELLATE Crypthecoainium cohnii

The chromosome scaffolds in higher eukaryotic nuclei have been described elsewhere. Butit is unknown when they evolved. The dinoflagellates are the primitive organisms that maybe the intermediate between prokaryotes and eukaryotes. Combining chromosome scaffold prep-aration methods with embedment-free section microscopy,we demonstrate that the dino-flagellate Crypthecodinium cohnii chromosome retains a protein scaffold after the depletionof DNA and soluble proteins. This scaffold preserves the morphology characteristic of thechromosome. Two-dimensional electrophoreses show that the chromosome scaffolds are mainlycomposed of acidic proteins. Our results suggest that a framework similar to the chromosomescaffold in the mammalian cell appeared in the primitive eukaryote. We propose that thechromosome scaffold possibly originated from the early stages of eukaryote evolution.

First harmful algal bloom record of tycoplanktonic dinoflagellate Prorocentrum lima(Ehrenberg)F.Stein,1878 in the Dardanelles(Turkish Straits System,Turkey)

This study focused on daily variations and harmful algal bloom features of toxic dinoflagellate Prorocentrum lima(Ehrenberg)F.Stein(P.lima),1878 in middle summer period(9th July-6th August 2013)in the Dardanelles.Harmful algal bloom of P.lima was recorded for the first time in the Turkish Straits System.Density of P.lima reached to 2.40×10^(6)cells/L and exhibited four excessive blooms over 1.0×10^(6)cells/L during the study.The contribution of P.lima to both Prorocentrum spp.and dinoflagellates reached to 100%,particularly at the moment of the harmful algal bloom attested by regresion(R≥0.70)and correlation findings(R≥0.80).Nutrient concentrations were lower than previous levels due to excessive blooms.Concentrations of NO^(-)_(2)+NO^(-)_(3),PO^(-3)_(4)and SiO_(4)varied between 0.20 and 0.78μmol/L[(0.44±0.17)μmol/L],0.08 and 0.18μmol/L[(0.12±0.03)μmol/L]and 0.25 and 0.65μmol/L[(0.41±0.09)μmol/L]respectively.During the bloom,nutrient ratios were more different than redfield ratios due to eutrophication(NO^(-)_(2)+NO^(-)_(3)/PO^(-3)_(4)=4.04±1.74;SiO_(4)/PO^(-3)_(4)=3.79±1.24;SiO_(4)/NO^(-)_(2)+NO^(-)_(3)=1.04±0.36).Chlorophyll a concentration reached to 8.52μg/L(average:4.82±2.29 mg/L)in the bloom period.Temperature[(24.70±0.44)°C],salinity[(22.9±0.49)ppt],pH(8.23±0.15)and dissolved oxygen levels(7.35±0.60 mg/L)were approximately constant.The compact bloom of P.lima,similar to excessive blooms of other dinoflagellates and diatoms,was associated not only with eutrophication,but also with ocean warming interactions.Results revealed that it will be possible to reach to millions of cell number of P.lima(2.40×10^(6)cells/L)in eutrophied waters characterized by high chlorophyll a biomass(8.52μg/L).

The parasitic dinoflagellate Hematodinium infects marine crustaceans

Hematodinium is a type of parasitic dinoflagellate that infects marine crustaceans globally.The parasite lives mainly in the hemolymph or hemocoels of affected hosts,and results in mortalities due to malfunction or loss of functions of major organs.In recent years,the parasite had developed into an emerging epidemic pathogen not only affecting wild populations of economically valuable marine crustaceans in western countries but also the sustainable yield of aquaculture of major crabs in China.The epidemics of the parasitic diseases expanded recently in the coastal waters of China,and caused frequent outbreaks in aquaculture of major crab species,especially Portunus trituberculatus and Scylla paramamosain.In addition,the pathogen infected two species of co-cultured shrimps and multiple cohabitating wild crabs,implying it is a significant threat to the sustainable culture of commercially valuable marine crustaceans.In particular,the polyculture system that is widely used along the coast of China may facilitate the spread and transmission of the pathogen.Thus,to provide a better understanding of the biological and ecological characteristics of the parasitic dinoflagellate and highlight important directions for future research,we have reviewed the current knowledge on the taxonomy,life cycle,pathogenesis,transmission and epidemiology of Hematodinium spp.Moreover,ecological countermeasures have been proposed for the prevention and control of the emerging infectious disease.

Factor driving heterotrophic dinoflagellate in relation to environment conditions in Kerkennah Islands(eastern coast of Tunisa)

Objective:To study the seasonal variability of heterotrophic dinoflagellate in the station of Cercina(southern coast of Tunisia).Methods:Sampling was done in 2007 in Cercina station located in the western coast of Kerkennah(34°41'27''N;11°07'45''E)(Southern Tunisia).Three replicates of water samples were taken during 10 days of each month.Environmental variables and nutrients were measured in situ.Results:A significant seasonal difference was observed for temperature and water salinity.The highest values were observed in spring and summer.No significant seasonal difference was,however,detected for nitrite,nitrate,ammonia,silica and phosphate.Sixty-five species of dinoflagellate were identified in the station of Cercina.Abundance of dinoflagellates fluctuated between seasons with values showing a significant seasonal and monthly difference.The highest mean abundance was recorded in spring in April,while the lowest abundance was detected in December in winter.Protoperidinium granii was the main species contributing to the dissimilarity between spring and winter with 13.98%followed by Peridinium sp.with 12.5%of dissimilarity and by Polykrikos sp.with 10.58%.Conclusions:Heterotrophic dinoflagellates proliferate in spring and summer.This increase was justified by the nutrient availability.Protoperidinium granii and Polykrikos kofoidii were the main heterotrophic dinoflagellate making difference between seasons and their densities were positively correlated with both temperature and salinity.

Occurrences of the toxic dinoflagellate Ostreopsis ovata in relation with environmental factors in Kerkennah Island(Southern coast of Tunisia)

Objective:To study the seasonal and monthly variability of the toxic dinoflagellate Ostreopsis ovata(O.ovata)in relation to environmental parameters in Kerkennah Island.Methods:Three water samples replicate of one-litter were taken daily for ten consecutive days on 12 months.All sampling water was kept in the dark at ambient temperature until their microscopic observation.Environmental variables such as salinity and temperature were measured in the field concomitantly as phytoplankton sampling.Nutrients(ammonium,nitrite,nitrate,phosphate and silicate)were analysed in laboratory with Auto-analyser Luebbe type.Cell identification and enumeration in water samples were performed with an inverted microscope after the sedimentation.Results:The highest abundance of O.ovata was recorded in summer.Analysis of variance showed significant difference of abundance between seasons,whereas no significant difference for month was detected.Factorial analysis ordination showed a positive correlation of Ostreopsis mainly with temperature and low correlation with nitrite and nitrate whereas the second axis(with 26.30%of variance)showed that Ostreopsis was correlated with temperature and salinity.Conclusions:The maximum abundance of Ostreopsis was reached in summer when temperature was high and a low relationship between O.ovata and nutrient was detected.