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• First analysis of HAB species diversity for the Texas coast based on the HTS-metabarcoding. • Identification of 31 HAB OTUs with the 18S rDNA V4 marker; 29 OTUs with the V8-V9 marker. • A total of 36 HAB species were identified, including 5 HAB species recognized for the first time in the GoM. • Phylogenetic analysis is recommended over BLAST for species identification. Environmental-DNA (eDNA) for metabarcoding is a rapid and effective means to investigate microplankton community composition and species diversity. The objective of this study was to examine the genetic diversity of the phytoplankton community in the Gulf of Mexico, with particular emphasis on harmful algal bloom species. Samples were collected at stations along the coast of Texas in September-October 2017 that were inundated by low salinity waters in the aftermath of Hurricane Harvey. Metabarcodes were generated from the eDNA targeting both the V4 and V8-V9 regions of the 18S rDNA gene. Evaluation of the metabarcodes revealed an unexpectedly high number of harmful algal species during this short period, including five that had not been documented in this region previously. A total of 36 harmful algal species could be differentiated based on V4 and V8-V9 metabarcode markers. Using a phylogenetic approach, the taxonomic resolution of each marker differed and not all species could be differentiated using solely one marker. The V4 region resolved species within some genera (e.g., Heterocapsa), while the V8-V9 marker was necessary to resolve species within other genera (e.g., Chattonella). In other cases, species differentiation within a genus required a combination of both markers (e.g., Prorocentrum, Karenia), or another marker will be needed to resolve all species (e.g., Alexandrium, Dinophysis). We conclude that no single marker can delineate all species, so it is recommended HAB monitoring programs use more than one marker. Overall, the observed diversity of HAB species along the Texas coast using metabarcoding exceeded reports from other parts of the world. [ABSTRACT FROM AUTHOR]

Blooms of Dinophysis ovum and Mesodinium spp. have been observed in the Gulf of Mexico since 2007 using the Imaging FlowCytobot technology. Bloom dynamics of these two organisms in conjunction with ancillary environmental data for a 5-year period were analyzed to identify the conditions necessary for bloom initiation or presence with the goal of predicting future blooms of D. ovum. Using time-series analysis, we observed a positive time-lagged correlation between the two organisms in each year when both were present, which suggests that the presence of Mesodinium may be useful as a leading indicator for a D. ovum bloom. Although in some cases D. ovum and Mesodinium co-occurred, no strong predator–prey relationship was observed. We identified a narrow range of temperature and salinity that could be necessary for bloom initiation of D. ovum and Mesodinium in the Gulf of Mexico. Analysis of images over the time series revealed a wide range in the size of Mesodinium cells, which suggests that species other than M. rubrum may be present in the Gulf of Mexico. Based on the occurrence of a D. ovum bloom preceded by low abundances of Mesodinium, we suggest that D. ovum is able to utilize ciliates other than M. rubrum as prey. Our observations indicate that environmental conditions, as well as Mesodinium abundance and species composition, can affect initiation, presence or abundance of D. ovum and thus may help in the prediction of future blooms. [ABSTRACT FROM PUBLISHER]

With the increase in frequency of harmful algal blooms (HABs) worldwide, a better understanding of the mechanisms that influence toxin production is needed. Karenia brevis, the major HAB dinoflagellate in the Gulf of Mexico, produces potent neurotoxins, known as brevetoxins. Human health is directly impacted by blooms of K. brevis through consumption of shellfish contaminated by accumulated brevetoxins (neurotoxic shellfish poisoning) or from aerosolized brevetoxins in sea spray (reduced respiratory function); however, the reason for brevetoxin production has remained a mystery. Here we show that brevetoxin production increased dramatically in response to osmotic stress in three of the four K. brevis clones examined. By rapidly changing salinity to simulate a shift from oceanic conditions to a decreased salinity typical of coastal conditions, brevetoxin production was triggered. As a result, brevetoxin cell quota increased by >14-fold, while growth rate remained unchanged. Live images of K. brevis cells were also examined to assess changes in cell volume. In the K. brevis Wilson clone, cells responded quickly to hypoosmotic stress by increasing their brevetoxin cell quota from ~10 to 160 pg of brevetoxin per cell, while cell volume remained stable. In contrast, the K. brevis SP1 clone, which has a consistently low brevetoxin cell quota (<1 pg per cell), was unable to balance the hypoosmotic stress, and although brevetoxin production remained low, average cell volume increased. Our findings close a critical gap in knowledge regarding mechanisms for toxin production in K. brevis by providing an explanation for toxin production in this harmful dinoflagellate. [ABSTRACT FROM AUTHOR]

Harmful algal blooms caused by Karenia brevis result in large fish kills, human respiratory irritation, and shellfishing closures in affected areas. Most previous work on bloom formation in the Gulf of Mexico has focused on the west coast of Florida. To investigate the origin of bloom-forming cells along the coast of Texas, potential distributions of cells during two bloom years (2009, 2011) and one non-bloom year (2010) were examined using a spatially explicit, individual-based model of K. brevis . The model incorporates a previously developed model of dinoflagellate vertical migration and utilizes observed data (field samples of cell concentrations, photosynthetically active radiation) and modeled environmental output (salinity, temperature, current velocities) from a hydrodynamic model. Running the model in reverse showed that cells near the coast of Texas during early fall originate from the southern Gulf of Mexico in bloom years and from the northern Gulf of Mexico in the non-bloom year for the three years studied. Identification of a southern origin for bloom-forming cells provides a target area for increased sampling in order to provide early warning of potentially harmful algal blooms of K. brevis . [ABSTRACT FROM AUTHOR]

• New isolates of T. amphioxeia and D. ovum established from the Gulf of Mexico. • Dinophysis ovum and Dinophysis acuminata growth was optimal at 18 – 24 °C and salinities 22 – 26. • D. acuminata and D. ovum have unique toxin profiles for North American region. • Dinophysis blooms in the Gulf of Mexico are likely initiated by temperature increase in spring. Mixotrophic Dinophysis species threaten human health and coastal economies through the production of toxins which cause diarrhetic shellfish poisoning (DSP) in humans. Novel blooms of Dinophysis acuminata and Dinophysis ovum have occurred in North American waters in recent decades, resulting in the closure of shellfish harvesting. Understanding the ecology of Dinophysis species and their prey is essential to predicting and mitigating the impact of blooms of these dinoflagellates. The growth response of two new isolates of Dinophysis species, one isolate of Mesodinium rubrum , and two strains of Teleaulax amphioxeia were evaluated at a range of temperature, salinity, and irradiance treatments to identify possible environmental drivers of Dinophysis blooms in the Gulf of Mexico. Results showed optimal growth of T. amphioxeia and M. rubrum at 24 °C, salinity 30 – 34, and irradiances between 300 and 400 µmol quanta m −2 s −1. Optimal Dinophysis growth was observed at salinity 22 and temperatures between 18 and 24 °C. Mesodinium and both Dinophysis responded differently to experimental treatments, which may be due to the suitability of prey and different handling of kleptochloroplasts. Dinophysis bloom onset may be initiated by warming surface waters between winter and spring in the Gulf of Mexico. Toxin profiles for these two North American isolates were distinct; Dinophysis acuminata produced okadaic acid, dinophysistoxin-1, and pectenotoxin-2 while D. ovum produced only okadaic acid. Toxin per cell for D. ovum was two orders of magnitude greater than D. acuminata. Phylogenies based on the cox1 and cob genes did not distinguish these two Dinophysis species within the D. acuminata complex. [ABSTRACT FROM AUTHOR]

Due to the increasing prevalence of Dinophysis spp. and their toxins on every US coast in recent years, the need to identify and monitor for problematic Dinophysis populations has become apparent. Here, we present morphological analyses, using light and scanning electron microscopy, and rDNA sequence analysis, using a ~2‐kb sequence of ribosomal ITS1, 5.8S, ITS2, and LSU DNA, of Dinophysis collected in mid‐Atlantic estuarine and coastal waters from Virginia to New Jersey to better characterize local populations. In addition, we analyzed for diarrhetic shellfish poisoning (DSP) toxins in water and shellfish samples collected during blooms using liquid‐chromatography tandem mass spectrometry and an in vitro protein phosphatase inhibition assay and compared this data to a toxin profile generated from a mid‐Atlantic Dinophysis culture. Three distinct morphospecies were documented in mid‐Atlantic surface waters: D. acuminata, D. norvegica, and a "small Dinophysis sp." that was morphologically distinct based on multivariate analysis of morphometric data but was genetically consistent with D. acuminata. While mid‐Atlantic D. acuminata could not be distinguished from the other species in the D. acuminata‐complex (D. ovum from the Gulf of Mexico and D. sacculus from the western Mediterranean Sea) using the molecular markers chosen, it could be distinguished based on morphometrics. Okadaic acid, dinophysistoxin 1, and pectenotoxin 2 were found in filtered water and shellfish samples during Dinophysis blooms in the mid‐Atlantic region, as well as in a locally isolated D. acuminata culture. However, DSP toxins exceeded regulatory guidance concentrations only a few times during the study period and only in noncommercial shellfish samples. [ABSTRACT FROM AUTHOR]

• Interaction between upwelling and river discharge influenced phytoplankton community. • Stronger upwelling in 2013 led to a diatom-dominated phytoplankton community. • Coastal upwelling recognized as a strong temporal forcing factor in Gulf of Mexico. • Spatial variations in assemblages were determined by upwelling and river discharge. • Dinoflagellates were more prevalent than previously documented in the region. The regional circulation in the northwestern Gulf of Mexico during late spring-summer is modulated by upwelling-favorable winds that can cause coastal upwelling in the western region and by freshwater inputs from the Mississippi-Atchafalaya Rivers. Spatial variability and temporal dynamics of phytoplankton community composition were examined during two upwelling-favorable periods using data obtained with an Imaging FlowCytobot (IFCB) from two cruises on the Texas-Louisiana shelf in June 2013 and 2014 and from the Texas Observatory for Algal Succession Time series (TOAST) at Port Aransas (Texas). Phytoplankton spatial distributions were determined by the influence of upwelling and river discharged waters. In the 2013 cruise, upwelling was detected in a large portion of the western region and the phytoplankton assemblages were dominated by diatoms, mostly chain-forming taxa. As revealed by the TOAST time series, the upwelling onset caused a dramatic increase in diatom carbon biomass. In the areas not affected by upwelling, variation in the river plume distribution that resulted from the circulation and the different discharge magnitudes for each year influenced the spatial distributions of the phytoplankton community composition. Dinoflagellates and other flagellated taxa were notably dominant during the 2013 cruise, whereas both diatoms and flagellated groups dominated the assemblages during the 2014 cruise. High stratification promoted by freshwater input, notably higher during 2013 than 2014, likely favored the dominance of flagellated groups in 2013. This study provides evidence of the influence of coastal upwelling in the phytoplankton community of the northwestern Gulf of Mexico and contributes to the knowledge of the drivers of community composition in this high-productivity area. [ABSTRACT FROM AUTHOR]

Blooms of Karenia brevis, the major bloom-forming dinoflagellate in the Gulf of Mexico, are thought to originate in the eastern Gulf. Single-cell polymerase chain reaction and five microsatellites were used to obtain genotypes for >1800 cells from 38 samples collected from six bloom events. A consistent pattern of genetic divergence between blooms from Florida and Texas was not detected, which supports the hypothesis of a common origin for blooms of K. brevis in the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

A new planktonic species of Prorocentrum is described from the Gulf of Mexico. First observed with the Imaging FlowCytobot, Prorocentrum texanum sp. nov. was characterized using LM, SEM, and TEM along with sequencing of the SSU, LSU, and ITS ribosomal regions and the mitochondrial cob and cox1 regions. P. texanum sp. nov. is a round to oval bivalvate dinoflagellate, with a prominent anterior, serrated solid flange on periflagellar a platelet and an opposing short, flat flange on the h platelet. The periflagellar area consists of 10 platelets. Both left and right valves have shallow round depressions and two-sized valve pores. The anterior ejectosome pore pattern differs between the left and right valve in relation to the periflagellar area and margins. Ten to eleven rows of tangential ejectosome pores are present on each valve. P. texanum sp. nov. has two varieties which exhibit distinct morphotypes, one round to oval (var. texanum) and the other pointed (var. cuspidatum). P. texanum var. cuspidatum is morphologically similar to P. micans in surface markings, but is smaller, and has a serrated periflagellar flange, and is genetically distinct from P. micans. Cytologically, P. texanum has two parietal chlo-roplasts, each with a compound, interlamellar pyrenoid, trichocysts, fibrous vesicles that resemble mucocysts, pusules, V- to U-shaped posterior nucleus, golgi, and tubular mitochondria. No genetic difference was found between the two varieties in the five genes examined. Phylogenetic analysis of the SSU, LSU, and ITS ribosomal regions place P. texanum sp. nov. as a sister group to P. micans. One isolate of P. texanum var . texanum produces okadaic acid. [ABSTRACT FROM AUTHOR]

Brachidinium capitatum F. J. R. Taylor, typically considered a rare oceanic dinoflagellate, and one which has not been cultured, was observed at elevated abundances (up to 65 cells · mL) at a coastal station in the western Gulf of Mexico in the fall of 2007. Continuous data from the Imaging FlowCytobot (IFCB) provided cell images that documented the bloom during 3 weeks in early November. Guided by IFCB observations, field collection permitted phylogenetic analysis and evaluation of the relationship between Brachidinium and Karenia. Sequences from SSU, LSU, internal transcribed spacer (ITS), and cox1 regions for B. capitatum were compared with five other species of Karenia; all B. capitatum sequences were unique but supported its placement within the Kareniaceae . From a total of 71,487 images, data on the timing and frequency of dividing cells was also obtained for B. capitatum, allowing the rate of division for B. capitatum to be estimated. The maximum daily growth rate estimate was 0.22 d. Images showed a range in morphological variability, with the position of the four major processes highly variable. The combination of morphological features similar to the genus Karenia and a phylogenetic analysis placing B. capitatum in the Karenia clade leads us to propose moving the genus Brachidinium into the Kareniaceae. However, the lack of agreement among individual gene phylogenies suggests that the inclusion of different genes and more members of the genus Karenia are necessary before a final determination regarding the validity of the genus Brachidinium can be made. [ABSTRACT FROM AUTHOR]

• Geographically distant Pseudo-nitzschia multistriata isolates are sexually compatible. • Microsatellites analyses show that distant isolates belong to different populations. • Three new alleles for the sex locus are found in P. multistriata Atlantic strains. • The gene MRP3 is expressed on the new allele A2, specific for mating type +. The cosmopolitan, species-rich diatom genus Pseudo-nitzschia represents a good system for the study of speciation, evolution and diversity. Understanding elements linked to population dynamics and life cycle regulation for these species is of particular importance in view of their ability to produce the toxin domoic acid and cause harmful blooms. Pseudo-nitzschia multistriata , one of the toxic species that represents a model for the study of life cycle related questions, is the only diatom for which a sex determination mechanism has been described. Populations in the Gulf of Naples (Mediterranean Sea), can share four different allelic variants (A, M, B, N) of the mating type determination region, and one of them (A) is responsible for the determination of the mating type + (MT+), defined by the MT+ restricted expression of the gene MRP3. Here, we analysed the sex determination genomic region in three new strains isolated from the Gulf of Mexico and compared it to the alleles previously described in the Mediterranean strains. We first show that these geographically distant strains of P. multistriata belong to different populations but can interbreed. Next, we show that the two populations share an overall similar structure of the genomic locus although differences can be seen in the polymorphic regions upstream of MRP3. In strain P4-C1, we amplified and sequenced an allele (M) identical to one of those previously characterized in the Mediterranean strains. In the other two strains, P4-C2 and P4-C5, we identified three new alleles, which we named A2, B2 and N2. P4-C2 and P4-C5 are heterozygous and share the common allele A2 linked to the monoallelic expression of the MT+ specific sex determining gene MRP3. Our results expand information on the global distribution of P. multistriata and on the level of conservation of the sex determination region in different populations. The definition of the extent of intra- and inter-specific conservation of this region would be a relevant addition to our understanding of Pseudo-nitzschia diversity and evolution. [ABSTRACT FROM AUTHOR]