CONFIRMATION OF DOMOIC ACID PRODUCTION BY PSEUDO‐NITZSCHIA AUSTRALIS (BACILLARIOPHYCEAE) ISOLATED FROM IRISH WATERS1

A nonaxenic isolate of the potentially toxic diatom Pseudo‐nitzschia australis (Frenguelli) from Irish waters was tested in two separate batch culture experiments. When grown under a low irradiance (～12 μmol photons·m ^{? 2}·s ^{? 1} 1 Received 20 March 2001. Accepted 21 August 2002.
; 16:8‐h light:dark cycle) for up to 40 days, the culture produced only trace amounts of the neurotoxin domoic acid (DA) during late stationary phase. Growth at a higher irradiance (～115 μmol photons·m ^{? 2}·s ^{? 1} 1 Received 20 March 2001. Accepted 21 August 2002.
; 12:12‐h light:dark cycle) resulted in DA production starting during late exponential phase and reaching a maximum concentration of 26 pg DA·cell ^{? 1} 1 Received 20 March 2001. Accepted 21 August 2002.
during late stationary phase. Liquid chromatography coupled to mass spectrometry was used to confirm the identity of DA in the culture. Irradiance and photoperiod could be important factors that contribute directly or indirectly to the control of DA production in P. australis. This is the first record of a DA‐producing diatom in Irish waters, and results indicate P. australis may have been the source of DA that has recently contaminated shellfisheries in this area.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

INTER‐ AND INTRASPECIFIC COMMUNITY STRUCTURE WITHIN THE DIATOM GENUS PSEUDO‐NITZSCHIA (BACILLARIOPHYCEAE)1

Pseudo‐nitzschia‐specific PCR primers (PnAll F/R) were designed to amplify a polymorphic region of the internal transcribed spacer 1 (ITS1) from at least 11 Pseudo‐nitzschia species. The primers were used to generate environmental clone libraries from Puget Sound, Washington, and Vancouver Island, British Columbia, to confirm that the primers were specific for Pseudo‐nitzschia and to determine the extent of ITS1 sequence diversity within individual species. All environmental ITS1 sequences generated with PnAll primers displayed the greatest similarity to known Pseudo‐nitzschia ITS1 sequences. The length of cloned ITS1 fragments differed among species but was conserved within a species. Intraspecific genotypes exhibited <3% sequence divergence for seven of the 10 species detected in clone libraries. Several ITS1 genotypes unique to the Pacific Northwest were identified in environmental samples, and other genotypes were more broadly distributed. The Pseudo‐nitzschia primers were also used to develop an automated ribosomal intergenic spacer analysis (ARISA) to rapidly identify Pseudo‐nitzschia species in environmental samples based on species‐specific variation in the length of the targeted ITS1 region. The ARISA peaks were then associated with the environmental clone sequences for Pseudo‐nitzschia species. Surveying the genetic composition of communities at both the inter‐ and intraspecific levels will enhance our understanding of Pseudo‐nitzschia bloom dynamics.     

GROWTH AND DOMOIC ACID PRODUCTION BY PSEUDO‐NITZSCHIA SERIATA (BACILLARIOPHYCEAE) UNDER PHOSPHATE AND SILICATE LIMITATION1

Pseudo‐nitzschia seriata (Cleve) H. Peragallo isolated from Scottish west coast waters was studied in batch culture with phosphate (P) or silicate (Si) as the yield‐limiting nutrient at 15°C. This species produced the neurotoxin domoic acid (DA) when either nutrient was limiting but produced more when stressed by Si limitation during the stationary phase. Under P‐limiting conditions, exponential growth stopped after P was reduced to a low threshold concentration. Under Si‐limiting conditions, fast exponential growth was followed by a period of slower exponential growth, until Si became exhausted. A stationary phase was observed in the P‐limited but not the Si‐limited cultures, the latter showing a rapid decrease in cell density after the second exponential growth phase. Si‐limited cultures exhibited a further period of active metabolism (as indicated by increases in chl and carbon per cell) late in the experiment, presumably fueled by regenerated Si. DA production was low in exponential phase under both conditions. In P‐limited cultures, most DA was produced during the immediate postexponential phase, with little or no new DA produced during later cell senescence. In contrast, although a substantial amount of DA was produced during the slower exponential phase of the Si‐limited cultures, DA production was even greater near the end of the experiment, coincident with the period of chl synthesis and increase in carbon biomass. Comparison of the magnitude of toxin production in the two nutrient regimes indicated a greater threat of P. seriata‐generated amnesic shellfish poisoning events under Si rather than P nutrient limitation.     

MICROSATELLITE MARKER DEVELOPMENT AND GENETIC VARIATION IN THE TOXIC MARINE DIATOM PSEUDO‐NITZSCHIA MULTISERIES (BACILLARIOPHYCEAE)1

The genetic structure of phytoplankton populations is largely unknown. In this study we developed nine polymorphic microsatellite markers for the domoic acid–producing marine diatom Pseudo‐nitzschia multiseries (Hasle) Hasle. We then used them in the genotyping of 25 physiologically diverse field isolates and six of their descendants: 22 field isolates originated from eastern Canadian waters, two from European waters, and one from Russian waters. The numbers of alleles per locus ranged from three to seven and the observed heterozygosities from 0.39 to 0.70. A substantial degree of genetic variation was observed within the field isolates, with 23 different genotypes detected. The Russian isolate was the most genetically distinct, although there was also evidence of genetic differentiation at a more local scale. Mating experiments demonstrated that alleles were inherited in a Mendelian manner. Pseudo‐nitzschia multiseries primer pairs were tested on DNA from four congeners: P. calliantha Lundholm, Moestrup et Hasle; P. fraudulenta (P. T. Cleve) Hasle; P. pungens (Grunow ex P. T. Cleve) Hasle; and P. seriata (P. T. Cleve) H. Peragallo. Cross‐reactivity was only observed in P. pungens. Our results are a first step in understanding the genetic variation present at the Pseudo‐nitzschia“species” level and in determining the true biogeographic extent of Pseudo‐nitzschia species.     

CHLOROPHYLL C PIGMENT PATTERNS IN 18 SPECIES (51 STRAINS) OF THE GENUS PSEUDO‐NITZSCHIA (BACILLARIOPHYCEAE)1

The pigment composition of 18 species (51 strains) of the pennate diatom Pseudo‐nitzschia was examined using HPLC. The carotenoid composition was typical for diatoms, with fucoxanthin (the major xanthophyll), diadinoxanthin, diatoxanthin, and β,β‐carotene. However, a diverse array of chl c pigments was observed in the studied strains. All Pseudo‐nitzschia strains contained chl a and chl c₂, traces of Mg‐2,4‐divinyl phaeoporphyrin a₅ monomethyl ester (MgDVP), and traces of a chl c₂–like pigment originally found in the haptophyte Pavlova gyrans. The distribution of chl c₁ and chl c₃ was variable among species (present in seven and 14 species, respectively). Based on chl c distribution, three major pigment types were defined: type 1 (chl c₁ + c₂, four species: P. australis, P. brasiliana, P. multiseries, and P. seriata), type 2 (chl c₁ + c₂ + c₃, three species: P. fraudulenta, P. multistriata, and P. pungens), and type 3 (chl c₂ + c₃, 11 species: P. arenysensis, P. calliantha, P. cuspidata, P. decipiens, P. delicatissima, P. galaxiae, P. mannii, P. pseudodelicatissima, P. subcurvata, P. cf. subpacifica, and a novel Pseudo‐nitzschia species). Type 1 and 2 species also shared the absence of a particular morphological character, the central nodule in the raphe, with the only exception of P. fraudulenta. The implications of such pigment diversity in chemotaxonomy, HAB monitoring, ecology, and phylogeny of Pseudo‐nitzschia species are discussed.     

CRYPTIC AND PSEUDO‐CRYPTIC DIVERSITY IN DIATOMS—WITH DESCRIPTIONS OF PSEUDO‐NITZSCHIA HASLEANA SP. NOV. AND P. FRYXELLIANA SP. NOV.1

A high degree of pseudo‐cryptic diversity was reported in the well‐studied diatom genus Pseudo‐nitzschia. Studies off the coast of Washington State revealed the presence of hitherto undescribed diversity of Pseudo‐nitzschia. Forty‐one clonal strains, representing six different taxa of the P. pseudodelicatissima complex, were studied morphologically using LM and EM, and genetically using genes from three different cellular compartments: the nucleus (D1–D3 of the LSU of rDNA and internal transcribed spacers [ITSs] of rDNA), the mitochondria (cytochrome c oxidase 1), and the plastids (LSU of RUBISCO). Strains in culture at the same time were used in mating studies to study reproductive isolation of species, and selected strains were examined for the production of the neurotoxin domoic acid (DA). Two new species, P. hasleana sp. nov. and P. fryxelliana sp. nov., are described based on morphological and molecular data. In all phylogenetic analyses, P. hasleana appeared as sister taxa to a clade comprising P. calliantha and P. mannii, whereas the position of P. fryxelliana was more uncertain. In the phylogenies of ITS, P. fryxelliana appeared to be most closely related to P. cf. turgidula. Morphologically, P. hasleana differed from most other species of the complex because of a lower density of fibulae, whereas P. fryxelliana had fewer sectors in the poroids and a higher poroid density than most of the other species. P. hasleana did not produce detectable levels of DA; P. fryxelliana was unfortunately not tested. In P. cuspidata, production of DA in offspring cultures varied from higher than the parent cultures to undetectable.     

GENETIC POPULATION STRUCTURE OF PSEUDO‐NITZSCHIA PUNGENS (BACILLARIOPHYCEAE) FROM THE PACIFIC NORTHWEST AND THE NORTH SEA1

Several species of the diatom Pseudo‐nitzschia produce the neurotoxin domoic acid (DA). Consumption of fish and shellfish that have accumulated this potent excitotoxin has resulted in severe illness and even death in humans, marine mammals, and seabirds. Pseudo‐nitzschia pungens (Grunow ex Cleve) Hasle is a cosmopolitan diatom commonly occurring in the waters of the Pacific Northwest (PNW) and the eastern North Atlantic, including the North Sea. However, genetic and physiological relationships among populations throughout this large geographic distribution have not been assessed. Population genetic parameters (e.g., Hardy–Weinberg equilibrium, linkage equilibrium, F_ST) calculated for P. pungens collected from the Juan de Fuca eddy region in the PNW indicated the presence of two distinct groups that were more divergent from each other than either was from a P. pungens sample from the North Sea. Geographic heterogeneity was also detected within each of the two PNW groups. These results suggested that the populations of P. pungens recently mixed in the Juan de Fuca eddy region (a seasonally retentive feature off the coasts of Washington State, USA, and Vancouver Island, Canada) but did not exchange genetic material by sexual reproduction. Alternatively, these two groups may be cryptic (morphologically identical, but reproductively isolated) species. Identifying cryptic diversity in Pseudo‐nitzschia is important for bloom prediction and aiding the identification of molecular markers that can be used for rapid detection assay development.     

UV‐EXCITED BLUE AUTOFLUORESCENCE OF PSEUDO‐NITZSCHIA MULTISERIES (BACILLARIOPHYCEAE)1

A flow cytometer coupled to a scanning monochromator and a fluorescence microscope were used to characterize the fluorescence spectrum of Pseudo‐nitzschia multiseries (Hasle) Hasle, a pennate diatom that produces the neurotoxin domoic acid, a lethal amnesic. In this research, we characterize the fluorescence spectrum of P. multiseries in vivo over the wavelength range of 360 to 850 nm and show that this diatom autofluoresces blue when excited with UV light (350–365 nm). The autofluorescence characterization of Pseudo‐nitzschia may provide new methods for rapid in situ monitoring of diatom populations and reiterates the usefulness of flow cytometry in the analysis and study of marine phytoplankton.     

EFFECT OF SALINITY ON PSEUDO‐NITZSCHIA SPECIES (BACILLARIOPHYCEAE) GROWTH AND DISTRIBUTION1

Salinity varies widely in coastal areas that often have a high abundance of Pseudo‐nitzschia H. Peragallo. Pseudo‐nitzschia is abundant in Louisiana waters, and high cellular domoic acid has been observed in natural samples but no human illness has been reported. To assess the threat of amnesic shellfish poisoning (ASP), we examined the effect of salinity on Pseudo‐nitzschia occurrence in the field and growth in the laboratory with special emphasis on the salinity range where oysters are harvested (10–20 psu). In Louisiana coastal waters, Pseudo‐nitzschia spp. occurred over a salinity range of 1 to >35 psu, but they occurred more frequently at higher rather than lower salinities. Seven species were identified, including toxigenic species occurring at low salinities. In culture studies, seven clones of three species grew over a salinity range of 15 to 40 psu, some grew at salinities down to 6.25 psu, and most grew at salinities up to 45 psu. Tolerance of low salinities decreased from Pseudo‐nitzschia delicatissima (Cleve) Heiden to P. multiseries (Hasle) Hasle to P. pseudodelicatissima (Hasle) Hasle emend. Lundholm, Hasle et Moestrup. In conclusion, although Pseudo‐nitzschia was more prevalent in the field and grew better in the laboratory at higher salinities, it grew and has been observed at low salinities. Therefore, the probability of ASP from consumption of oysters harvested from the low salinity estuaries of the northern Gulf of Mexico is low but not zero; animal mortality events from toxin vectors other than oysters at higher salinity on the shelf are more likely.     

MORPHOLOGICAL,TOXICOLOGICAL, AND GENETIC DIFFERENCES AMONG PSEUDO‐NITZSCHIA (BACILLARIOPHYCEAE) SPECIES IN INLAND EMBAYMENTS AND OUTER COASTAL WATERS OF WASHINGTON STATE,USA1

Plankton samples from three inland embayments and several outer coastal sites of Washington State were collected from 1997 through 1999 and were examined for the presence of diatoms of the genus Pseudo‐nitzschia and levels of the toxin, domoic acid (DA). Seven species were observed, including Pseudo‐nitzschia pungens (Grunow ex Cleve) Hasle, P. multiseries (Hasle) Hasle, P. australis Frenguelli, P. fraudulenta (Cleve) Hasle, P. cf. heimii Manguim, P. pseudodelicatissima (Hasle) Hasle, and P. delicatissima (Cleve) Heiden. The coastal Pseudo‐nitzschia species assemblages differed significantly from those observed within embayments. The dominant species observed at coastal sites were P. pseudodelicatissima and P. cf. heimii. Pseudo‐nitzschia assemblages found in embayments included one or more of the following species: P. pungens, P. multiseries, P. australis, P. pseudodelicatissima, and P. fraudulenta. The nuclear large subunit rRNA gene was sequenced for six of the seven species identified. This sequence revealed that P. multiseries, P. pungens, P. australis, and P. heimii were genetically similar to those found in California, whereas P. delicatissima and P. pseudodelicatissima were distinct from the California isolates. Although the concentrations of DA in razor clams along Washington State coasts have exceeded regulatory limits several times since 1991, levels of DA in shellfish from Washington State embayments have not yet exceeded regulatory limits. The widespread presence of toxin‐producing Pseudo‐nitzschia species suggests, however, that toxic blooms are likely to occur within embayments in the future. In conjunction with the monitoring of environmental conditions conducive to toxic bloom formation, the development of species‐specific probes for rapid and accurate detection of potentially toxic Pseudo‐nitzschia species in this region would enable the forecasting of a toxic event before DA accumulates in shellfish, thereby reducing the impacts to coastal communities.     

MASS SEXUAL REPRODUCTION IN THE TOXIGENIC DIATOMS PSEUDO‐NITZSCHIA AUSTRALIS AND P. PUNGENS (BACILLARIOPHYCEAE) ON THE WASHINGTON COAST,USA1

Sexual reproduction is documented for the first time in field populations of the pennate diatoms Pseudo‐nitzschia australis Freng. and P. pungens (Grunow ex Cleve) Hasle (var. cingulata Villac and hybrids between var. cingulata and var. pungens). A bloom dominated by these species began on June 26, 2006, along Kalaloch Beach, Washington, USA, coincident with a drop in the Si(OH)₄:NO₃ ratio to below two. Multimodal size distributions were detected for both species, and synchronous auxosporulation occurred within the smallest size class during a 3‐week window. Auxospores and initial cells created a new class of large cells, and cells in the intermediate size classes increased in abundance during auxosporulation. Mating cells of both species were attached to colonies of surf‐zone diatoms. Paired gametangia, gametes, zygotes, auxospores, and large initial cells were found. Auxosporulation began first for P. pungens (June 30), apparently once a critical, high cell concentration was reached, followed by P. australis (July 5), when the total Pseudo‐nitzschia cell concentration reached 929,000 cells · L^?1. Low frequencies of auxosporulation occurred throughout the bloom but increased 4‐fold for P. australis and 3‐fold for P. pungens when macronutrients were reduced to low levels on July 11. A 2‐year life cycle was estimated for P. australis and 3 years for P. pungens, both with annual auxosporulation. Domoic acid (DA) in razor clams reached a maximum of 38 μg DA · g^?1 on July 18. A significant relationship existed between the percent of cells within the new size range and DA concentrations in razor clams on the same beach.     

DEVELOPMENT OF SEQUENCE CHARACTERIZED AMPLIFIED REGION MARKERS FROM INTERSIMPLE SEQUENCE REPEAT FINGERPRINTS FOR THE MOLECULAR DETECTION OF TOXIC PHYTOPLANKTON ALEXANDRIUM CATENELLA (DINOPHYCEAE) AND PSEUDO‐NITZSCHIA PSEUDODELICATISSIMA (BACILLARIOPHYCEAE) FROM FRENCH COASTAL WATERS1

Harmful algal blooms are a serious threat to shellfish farming and human health all over the world. The monitoring of harmful algae in coastal waters originally involved morphological identification through microscopic examinations, which was often difficult unless performed by specialists and even then often did not permit identification of toxic species. More recently, specific molecular markers have been used to identify specific phytoplankton species or strains. Here we report on the use of the intersimple sequence repeat (ISSR) technique to develop specific sequence characterized amplified region markers (SCAR) and to identify with these tools two toxic species in French coastal waters, the diatom Pseudo‐nitzschia pseudodelicatissima (Hasle) Hasle and the dinoflagellate Alexandrium catenella (Whedon and Kofoid 1936), Balech 1985. Six polymorphic ISSR regions were selected among amplified fingerprints of a representative sample of phytoplankton species. After cloning and sequencing the selected polymorphic ISSR regions, pairs of internal primers were designed to amplify a unique and specific sequence designed as a SCAR marker. Of the six selected SCAR markers, three were specific to P. pseudodelicatissima and one for A. catenella. The SCAR marker specificity was confirmed by using basic local alignment search tool comparison, by experimental assays on different strains from 11 countries, and by checking that the sequence amplified was the expected one. When tested on water samples collected along the French shores, the four specific SCAR markers proved to be efficient tools for fast and low‐cost detection of toxic phytoplankton species.     

LIFE CYCLE,SIZE REDUCTION PATTERNS,AND ULTRASTRUCTURE OF THE PENNATE PLANKTONIC DIATOM PSEUDO‐NITZSCHIA DELICATISSIMA (BACILLARIOPHYCEAE)1

Pseudo‐nitzschia delicatissima (Cleve) Heiden is a very common pennate planktonic diatom found in temperate marine waters, where it is often responsible for blooms. Recently, three distinct internal transcribed spacer types have been recorded during a P. delicatissima bloom in the Gulf of Naples (Mediterranean Sea, Italy), which suggests the existence of cryptic diversity. We carried out mating experiments with clonal strains belonging to the most abundant internal transcribed spacer type. Pseudo‐nitzschia delicatissima is heterothallic and produces two functional anisogametes per gametangium. The elongated auxospore possesses a transverse and a longitudinal perizonium. The sexual phase was observed to occur over a wide size spectrum, spanning 19–80 μm and corresponding to almost the whole range of cell length observed for P. delicatissima. We also investigated cell morphology, valve ultrastructure and morphometry of parental, F1‐generation strains, and the progeny of crosses between parental and F1 strains. Although ultrastructural features match those described for P. delicatissima, variability in cell shape was recorded in the largest cells of the F1 generation as well as in valves with an abnormal arrangement of poroids. As many other diatoms, P. delicatissima undergoes size reduction over its life cycle, and cells of different size showed differences in growth rates and the amount of size reduction per cell cycle. Cells between 60 and 30 μm in length showed the fastest growth and the slowest rates of size reduction per generation. In culture, P. delicatissima cells can decrease to 8 μm in length; however, such small cells (≤30 μm) are not recorded in the sea, and this raises interesting questions about the factors that control their survival in the natural environment.     

DOMOIC ACID PRODUCTION IN NITZSCHIA SP. (BACILLARIOPHYCEAE) ISOLATED FROM A SHRIMP-CULTURE POND IN DO SON, VIETNAM

Domoic acid (DA), a neuroexcitatory amino acid, was detected in batch culture of the newly recognized species Nitzschia navis-varingica Lundholm et Moestrup . The production of DA by this diatom was confirmed by electrospray ionization mass spectrometry. The diatom was collected from a shrimp-culture pond in Do Son, Vietnam. The production of DA (1.7 pg·cell ^{− 1}) is within the levels reported for Pseudo-nitzschia multiseries (Hasle) Hasle. The DA production started during the late exponential growth phase and reached a maximum during the early stationary growth phase. Maximum DA levels in the axenic culture decreased to about half that of the nonaxenic culture (0.9 pg·cell ^{− 1} vs. 1.7 pg·cell ^{− 1}), suggesting that DA production by the new species is influenced by bacteria.