Environmental Optima for Seven Strains of Pseudochattonella (Dictyochophyceae, Heterokonta)

Monitoring harmful microalgal species and their appearance in Tokyo Bay, Japan, using metabarcoding

During the recent decade, high-throughput sequencing (HTS) techniques, in particular, DNA metabarcoding, have facilitated increased detection of biodiversity, including harmful algal bloom (HAB) species. In this study, the presence of HAB species and their appearance patterns were investigated by employing molecular and light microscopy-based monitoring in Tokyo Bay, Japan. The potential co-appearance patterns between the HAB species, as well as with other eukaryotes and prokaryotes were investigated using correlation and association rule-based time-series analysis. In total, 40 unique HAB species were detected, including 12 toxin-producing HAB species previously not reported from the area. More than half of the HAB species were present throughout the sampling season (summer to autumn) and no structuring or succession patterns associated with the environmental conditions could be detected. Statistically significant (p < 0.05, rS ranging from −0.88 to 0.90) associations were found amongst the HAB species and other eukaryotic and prokaryotic species, including genera containing growth-limiting bacteria. However, significant correlations between species differed amongst the years, indicating that variability in environmental conditions between the years may have a stronger influence on the microalgal community structure and interspecies interactions than the variability during the sampling season. The association rule-based time-series analysis allowed the detection of a previously reported negative relationship between Synechococcus sp. and Skeletonema sp. in nature. Overall, the results support the applicability of metabarcoding and HTS-based microalgae monitoring, as it facilitates more precise species identification compared to light microscopy, as well as provides input for investigating potential interactions amongst different species/groups through simultaneous detection of multiple species/genera.

Interactive effects of temperature and salinity on the growth and cytotoxicity of the fish-killing microalgal species Heterosigma akashiwo and Pseudochattonella verruculosa

Fish-killing blooms of Heterosigma akashiwo and Pseudochattonella verruculosa have been devastating for the farmed salmon industry, but in Southern Chile the conditions that promote the growth and toxicity of these microalgae are poorly understood. This study examined the effects of different combinations of temperature (12, 15, 18 °C) and salinity (10, 20, 30 psu) on the growth of Chilean strains of these two species. The results showed that the optimal growth conditions for H. akashiwo and P. verruculosa differed, with a maximum rate of 0.99 day^-1 obtained at 15 °C and a salinity of 20 psu for H. akashiwo, and a maximum rate of 1.06 day^-1 obtained at 18 °C and a salinity of 30 psu for P. verruculosa. Cytotoxic assays (2 × 10¹ - 2 × 10⁵ cell mL^-1; cells, filtrates, and cell lysates) performed at salinities of 20 and 30 psu showed a 100% reduction in the viability of embryonic fish cells exposed to intact cells of H. akashiwo and a 39% reduction following exposure to culture filtrates of P. verruculosa. Differences in the fish-killing mechanisms (direct cell contact vs. extracellular substances) and physiological traits of H. akashiwo and P. verruculosa explain the recent occurrence of very large blooms under contrasting (cold-brackish vs. hot-salty) extreme climate conditions in Chile.

Interspecific Competition Study Between Pseudochattonella farcimen and P. verruculosa (Dictyochophyceae)-Two Ichthyotoxic Species that Co-occur in Scandinavian Waters

The genus Pseudochattonella has become a frequent component of late winter-early spring phytoplankton community in Scandinavian waters, causing extensive fish kills and substantial economic losses. One of currently two recognised species, P. farcimen, is often abundant prior to the diatom spring bloom. Recent field studies have revealed that P. farcimen and P. verruculosa have a period of overlap in their temperature ranges and thus their seasonal occurrences. Using laboratory cultures, we investigated the seasonal succession and growth of P. farcimen and P. verruculosa in both mono- and mixed-culture using the recently developed Pseudochattonella 'qPCR subtraction method', which for the first time allowed the simultaneous enumeration of these morphologically indistinguishable species in mixed assemblages. We examined how these species interacted over four different temperatures (5, 8, 11 and 15 °C). The observed growth rates and cell yields varied with temperature revealing their preferred temperature optima. P. farcimen was able to achieve positive net growth over all temperatures, while P. verruculosa failed to grow below 11 °C. Growth responses were statistically different between mono- and mixed-cultures with the outcome of these interactions being temperature-dependent. Nutrients (nitrate and phosphate) and pH levels were also measured throughout the growth experiments to better understand how these factors influenced growth of both species. P. verruculosa was shown to be less sensitive to high pH as growth ceased at pH 9.1, whereas P. farcimen stopped growing at pH 8.4. Understanding the influence of abiotic factors (e.g. temperature, pH and competition) on growth rates allows for a better understanding and prediction of phytoplankton community dynamics.

The ichthyotoxic genus Pseudochattonella (Dictyochophyceae): Distribution, toxicity, enumeration, ecological impact, succession and life history - A review

The marine genus Pseudochattonella is a recent addition to the list of fish killing microalgae. Currently two species are recognised (viz. P. verruculosa and P. farcimen) which both form recurrent coastal blooms sometimes overlapping in space and time. These events and their ecological and economic consequences have resulted in great interest and concern from marine biologists and the aquaculture industry. Since the first recorded blooms in Japanese (late 1980s), Scandinavian (1993) and Chilean (2004) waters numerous studies have focused on understanding the causative means of the fish killing. Mortality is probably due to Pseudochattonella discharging mucocysts that cause gill irritation and damage to the fish fills. Here, a review is provided of the literature on Pseudochattonella that covers the last ca. 25 years and focus on a number of topics relevant to understanding the general biology of the genus including ways to distinguish the two species. The literature addressing biogeography and known harmful events is evaluated and based on these findings an updated distribution map is proposed. P. farcimen is presently restricted to North European waters. Despite being very difficult to delineate based on morphology alone the two Pseudochattonella species seem to have separate growth optima. In laboratory experiments P. verruculosa consistently has higher temperature growth optima compared to P. farcimen though periods of overlap have been noted in the field. The review ends by proposing five areas with knowledge gaps and each of these could form the basis of future studies.

Genetic diversity affects ecological performance and stress response of marine diatom populations

Genetic diversity is considered an important factor, stabilizing ecological functions when organisms are faced with changing environmental conditions. Although well known from terrestrial systems, documentations of this relationship from marine organisms, and particularly planktonic microorganisms, are still limited. Here we experimentally tested the effects of genotypic diversity on ecologically relevant cellular parameters (growth, primary production, particulate organic carbon, particulate organic nitrogen, particulate organic phosphorus and biogenic silica) at optimal and suboptimal salinity conditions in a marine phytoplankton species. Multiple clonal genotyped and phenotypically characterized isolates of the diatom Skeletonema marinoi from the Baltic Sea were grown in monocultures and mixes of 5 and 20 clones at native (5 psu) and reduced (3 psu) salinities and respective parameters were compared. Re-genotyping of 30 individuals from each population at five microsatellite loci at the end of the experiment confirmed maintenance of genotypic richness. Although a diversity effect on growth was not detected, primary production and particulate organic nutrients were positively affected by increased diversity independent of salinity condition. Under salinity stress, highest values of primary production and particulate organic nitrogen content were measured at the high diversity level. The observed diversity effects emphasize the importance of genetic diversity of phytoplankton populations for ecological functions.

A quantitative real-time PCR assay for identification and enumeration of the occasionally co-occurring ichthyotoxic Pseudochattonella farcimen and P. verruculosa (Dictyochophyceae) and analysis of variation in gene copy numbers during the growth phase of single and mixed cultures

The ichthyotoxic genus Pseudochattonella forms recurrent extensive blooms in coastal waters in Japan, New Zealand and Northern Europe. It comprises of two morphologically similar species, P. verruculosa and P. farcimen, which complicates visual species identification and enumeration of live and fixed material. Primers designed previously could not quantitatively distinguish species in mixed assemblages. To address this issue we developed two primer sets: one revealed itself to be genus specific for Pseudochattonella and the other species-specific for P. verruculosa. By subtracting cell estimates for P. verruculosa from combined results we could calculate cell numbers for P. farcimen. This approach has overcome the challenges posed by the very limited sequence availability and low gene variability between the two species. The qPCR assay was extensively tested for specificity, efficiency and sensitivity over an entire growth cycle in both single and mixed assemblages. Comparison of cell abundance estimates obtained by qPCR assay and microscopy showed no statistically significant difference until stationary and death phases. The assay was also tested on environmental samples collected during a small Pseudochattonella bloom in Denmark in March-April 2015. It was impossible to distinguish P. farcimen and P. verruculosa by light microscopy but qPCR showed both species were present. The two methods provided nearly identical cell numbers but the assay provided discrimination and enumeration of both species.

Diverse seed banks favour adaptation of microalgal populations to future climate conditions

Selection of suitable genotypes from diverse seed banks may help phytoplankton populations to cope with environmental changes. This study examines whether the high genotypic diversity found in the Baltic cyst pool of the toxic dinoflagellate Alexandrium ostenfeldii is coupled to phenotypic variability that could aid short-term adaptation. Growth rates, cellular toxicities and bioluminescence of 34 genetically different clones isolated from cyst beds of four Baltic bloom sites were determined in batch culture experiments along temperature and salinity gradients covering present and future conditions in the Baltic Sea. For all parameters a significant effect of genotype on the response to temperature and salinity changes was identified. General or site-specific effects of the two factors remained minor. Clones thriving at future conditions were different from the best performing at present conditions, suggesting that genotypic shifts may be expected in the future. Increased proportions of highly potent saxitoxin were observed as a plastic response to temperature increase, indicating a potential for higher toxicity of future blooms. The observed standing variation in Baltic seed banks of A. ostenfeldii suggests that the population is likely to persist under environmental change.

Ichthyotoxicity of the microalga Pseudochattonella farcimen under laboratory and field conditions in Danish waters

Blooms of the marine dictyochophyte Pseudochattonella farcimen have been associated with fish kills, but attempts to verify ichthyotoxicity of this microalga under experimental conditions have not been successful. In the early spring of 2009 and 2011, P. farcimen bloomed in the inner Danish waters. The blooms occurred at a seawater temperature of ~2°C and correlated with extensive kills of farmed salmonid fish (2009) and wild populations (2011). Several strains of P. farcimen were isolated from the 2009 bloom. However, exposure of rainbow trout Oncorhynchus mykiss to laboratory-grown P. farcimen cultures did not reveal any toxic effects. During the 2011 bloom, fish were exposed to bloom water under both laboratory and field conditions. While no clinical effect was observed on fish incubated in bloom water in the laboratory trial, a remarkable difference was seen in the field trial between rainbow trout kept in tanks supplied with a continuous flow of filtered versus non-filtered bloom water. Histological examination of the gill tissue revealed karyorrhexis and epithelial loosening in the affected fish. Microscopy analysis of algal cell morphology suggested that mucocysts detected on the cell surface only in freshly sampled bloom water might be associated with ichtyotoxicity.

Seasonal dynamics of harmful algae in outer Oslofjorden monitored by microarray, qPCR, and microscopy

Monitoring of marine microalgae is important to predict and manage harmful algal blooms. Microarray Detection of Toxic ALgae (MIDTAL) is an FP7-funded EU project aiming to establish a multi-species microarray as a tool to aid monitoring agencies. We tested the suitability of different prototype versions of the MIDTAL microarray for the monthly monitoring of a sampling station in outer Oslofjorden during a 1-year period. Microarray data from two different versions of the MIDTAL chip were compared to results from cell counts (several species) and quantitative real-time PCR (qPCR; only Pseudochattonella spp.). While results from generation 2.5 microarrays exhibited a high number of false positive signals, generation 3.3 microarray data generally correlated with microscopy and qPCR data, with three important limitations: (1) Pseudo-nitzschia cells were not reliably detected, possibly because cells were not sufficiently retained during filtration or lysed during the extraction, and because of low sensitivity of the probes; (2) in the case of samples with high concentrations of non-target species, the sensitivity of the arrays was decreased; (3) one occurrence of Alexandrium pseudogonyaulax was not detected due to a 1-bp mismatch with the genus probe represented on the microarray. In spite of these shortcomings our data demonstrate the overall progress made and the potential of the MIDTAL array. The case of Pseudochattonella - where two morphologically similar species impossible to separate by light microscopy were distinguished - in particular, underlines the added value of molecular methods such as microarrays in routine phytoplankton monitoring.