Physiological and biochemical responses of the marine dinoflagellate Prorocentrum minimum exposed to the oxidizing biocide chlorine

Two novel glutathione S-transferase (GST) genes in the toxic marine dinoflagellate Alexandrium pacificum and their transcriptional responses to environmental contaminants

The present study identified two novel glutathione S-transferase (GST) genes from the toxic dinoflagellate Alexandrium pacificum and examined their molecular characteristics and transcriptional responses to algicides and environmental contaminants. Bioinformatic analysis revealed that both ApGSTs are cytosolic, belonging to the chi-like class (ApGST1) and an undefined class (ApGST2). The overall expression of ApGSTs showed similar patterns depending on the exposed contaminants, while they were differently regulated by polychlorinated biphenyl (PCB). Copper treatments (CuCl2 and CuSO4) did not significantly induce the expression of ApGSTs. The highest up-regulations of ApGST1 and ApGST2 were under 6-h treatments of 0.10 and 0.50 mg L-1 NaOCl. Interestingly, only ApGST1 increased significantly after 0.10, 0.50, and 1.00 mg L-1 of PCB exposure (6 h). Intracellular reactive oxygen species (ROS) increased considerably under NaOCl; however, it was not significantly higher in the PCB-treated cells. GST activity was increased by NaOCl and PCB treatments, but only PCB caused apoptosis. These results suggest that GSTs are involved in the first line of phase II detoxification, protecting dinoflagellate cells against oxidative damage.

Chlorine toxicity to Navicula pelliculosa and Achnanthes spp. in a flow-through system: The use of immobilised microalgae and variable chlorophyll fluorescence

Chlorination is a widely used antifouling method for freshwater and marine applications. Chlorine added to seawater reacts to form oxidants that are toxic to biofouling organisms. Further, the oxidants that result are short-lived, but may nevertheless affect non-target species in waterbodies receiving the antifouling effluent. This study evaluated the toxicity of chlorinated seawater (e.g. following sodium hypochlorite addition) on two different species of marine benthic diatoms (Achnanthes spp., and Navicula pelliculosa), which are representative of microphytobenthos communities - an important component in coastal habitats that may be exposed to chlorinated seawater. To evaluate the growth inhibition over a 72 h period, algae were immobilised in alginate beads and exposed to different levels of chlorination in a flow through system. Growth rates and physiological condition of the microalgae were evaluated using a Fast Repetition Rate fluorometer (FRRf). To determine whether alginate influenced the sensitivity of algal response, studies were also conducted in a static test system (without renewal of test solutions) using both free cells and immobilised cells with initial chlorine added to achieve a similar range of concentrations as those used in the flow-through study. Within the first hour of the exposure period there was an indication that, for both species, the free algal cells in the static system were more sensitive to exposure to chlorinated seawater than were alginate-immobilised cells in the flow through system. Immobilised cells in a static system with a single addition of chlorine were also less sensitive to chlorination than free algal cells. However, for periods of 24 h or more due to decay of TRO in the static system the exposure of immobilised algae in the flow through system had a greater impact and hence lower effect concentrations. For the flow-through studies Achnanthes spp. was the most sensitive after 72 h exposure with a potential no effect concentration EC₁₀ value of 0.02 mg l^-1 as Cl₂ equivalents expressed as total residual oxidants (TRO) compared 0.04 mg l^-1 TRO for N. pelliculosa. Immobilisation of algal cells in alginate was found to be an effective means of determining the impact of chlorination and is likely to be effective for other non-persistent substances. Based on the data produced, the extent and significance of ecological effects of chlorination upon algal species typical of microphytobenthos are likely to be limited providing discharges comply with a maximum allowable concentration of 0.01 mg l^-1 TRO at the edge of an agreed mixing zone.

Long-term algal toxicity of oxidant treated ballast water

National and international regulations require that ships' ballast water is treated to minimize the risk of introducing potentially invasive species. A common approach employed by commercial ballast water management systems is chlorination. This study presents the algal toxicity findings for three chlorination-based BWMS and their implications to environmental safety of port waters receiving treated ballast water from ships. Discharged treated ballast water from all three BWMS was toxic to algae with IC25s (25% growth inhibition) ranging from 9.9% to 17.9%, despite having total residual oxidant concentrations below 0.02 mg/l, based on Whole Effluent Toxicity assays. When held at 4 °C, some of the ballast water samples continued to exhibit toxic effects with no observed effect concentrations as low as 18% after a 134 day holding time. Thirteen individual disinfection by-products were measured above the detected limit at the time of discharge. No correlation between DBPs and algal toxicity was observed.

6.0 K microarray reveals differential transcriptomic responses in the dinoflagellate Prorocentrum minimum exposed to polychlorinated biphenyl (PCB)

Endocrine disrupting chemicals (EDCs) have toxic effects on algae; however, their molecular genomic responses have not been sufficiently elucidated. Here, we evaluated genome-scaled responses of the dinoflagellate alga Prorocentrum minimum exposed to an EDC, polychlorinated biphenyl (PCB), using a 6.0 K microarray. Based on two-fold change cut-off, we identified that 609 genes (∼10.2%) responded to the PCB treatment. KEGG pathway analysis showed that differentially expressed genes (DEGs) were related to ribosomes, biosynthesis of amino acids, spliceosomes, and cellular processes. Many DEGs were involved in cell cycle progression, apoptosis, signal transduction, ion binding, and cellular transportation. In contrast, only a few genes related to photosynthesis and oxidative stress were expressed in response to PCB exposure. This was supported by that fact that there were no obvious changes in the photosynthetic efficiency and reactive oxygen species (ROS) production. These results suggest that PCB might not cause chloroplast and oxidative damage, but could lead to cell cycle arrest and apoptosis. In addition, various signal transduction and transport pathways might be disrupted in the cells, which could further contribute to cell death. These results expand the genomic understanding of the effects of EDCs on this dinoflagellate protist.

Chlorination induced damage and recovery in marine diatoms: Assay by SYTOX® Green staining

Phytoplankton entrained into cooling water systems of coastal power stations are subjected to acute chemical stress due to biocides (chlorine) used for biofouling control. They are subsequently released into the environment, where they may survive/recover or succumb. Experiments were conducted to evaluate the susceptibility of a centric (Chaetoceros lorenzianus) and pennate (Navicula sp.) diatom to in-plant administered concentrations of chlorine (0.2-0.5mg/L, TRO). Viability of cells exposed to chlorine was assessed by SYTOX® Green fluorimetry and was compared with other conventional end points like total cell counts, chlorophyll a content and cellular autofluorescence. Results showed a concentration-dependant reduction in viability, chlorophyll a and autofluorescence. C. lorenzianus cells were more susceptible to chlorine compared to Navicula sp. SYTOX® Green staining appears to be a sensitive method to assess chlorine-induced damages. The data show that in-use levels of chlorination can potentially impact entrained organisms; however, they can recover when returned to coastal waters.

Physiological and biochemical responses of the freshwater green algae Closterium ehrenbergii to the common disinfectant chlorine

Chlorine (Cl2) is widely used as a disinfectant in water treatment plants and for cleaning swimming pools; it is finally discharged into aquatic environments, possibly causing damage to the non-target organisms in the receiving water bodies. Present study evaluated the effects of the biocide Cl2 to the green alga Closterium ehrenbergii (C. ehrenbergii). Growth rate, chlorophyll a levels, carotenoids, chlorophyll autofluorescence, and antioxidant enzymes were monitored up to 72-h after Cl2 exposure. C. ehrenbergii showed dose-dependent decrease in growth rate and cell division after exposure to Cl2. By using cell counts, the median effective concentration (EC50)-72-h was calculated to be 0.071mgL(-1). Cl2 significantly decreased the pigment levels and chlorophyll autofluorescence intensity, indicating that the photosystem was damaged in C. ehrenbergii. In addition, it increased the production of reactive oxygen species (ROS) in the cells. This stressor significantly increased the activities of antioxidant enzymes, including superoxide dismutase (SOD), catalase, and glutathione, and affected the physiology of the cells. These results indicate that Cl2 induces oxidative stress in the cellular metabolic process and leads to physiological and biochemical damages in the green algae. Cl2 discharged in industrial effluents and from water treatment plants may cause harmful effects to the C. ehrenbergii a common freshwater microalgae and other non-target organisms.

Genome-wide analysis of transcription and photosynthesis inhibition in the harmful dinoflagellate Prorocentrum minimum in response to the biocide copper sulfate

Copper is an essential trace metal for organisms; however, excess copper may damage cellular processes. Their efficiency and physiological effects of biocides have been well documented; however, molecular transcriptome responses to biocides are insufficiently studied. In the present study, a 6.0K oligonucleotide chip was developed to investigate the molecular responses of the harmful dinoflagellate Prorocentrum minimum to copper sulfate (CuSO₄) treatment. The results revealed that 515 genes (approximately 8.6%) responded to CuSO₄, defined as being within a 2-fold change. Further, KEGG pathway analysis showed that differentially expressed genes (DEGs) were involved in ribosomal function, RNA transport, carbon metabolism, biosynthesis of amino acids, photosystem maintenance, and other cellular processes. Among the DEGs, 49 genes were related to chloroplasts and mitochondria. Furthermore, the genes involved in the RAS signaling pathway, MAPK signaling pathway, and transport pathways were identified. An additional experiment showed that the photosynthesis efficiency decreased considerably, and reactive oxygen species (ROS) production increased in P. minimum after CuSO₄ exposure. These results suggest that CuSO₄ caused cellular oxidative stress in P. minimum, affecting the ribosome and mitochondria, and severely damaged the photosystem. These effects may potentially lead to cell death, although the dinoflagellate has developed a complex signal transduction process to combat copper toxicity.

ROI-scavenging enzyme activities as toxicity biomarkers in three species of marine microalgae exposed to model contaminants (copper, Irgarol and atrazine)

There is a need to develop efficient tools to prevent damage to marine ecosystems due to pollution. Since microalgae play a key role in marine ecosystems, they are considered potentially useful for quick and sensitive toxicity bioassays. In this study an integrative analysis has been carried out of the anti-oxidant enzyme activities of marine microalgae species. Three marine microalgae species (Cylindrotheca closterium, a benthic diatom; Phaeodactylum tricornutum, a diatom which has been used as model organism in toxicity bioassays; and Rhodomonas salina, a cryptophyceae which is considered to present a certain level of heterotrophy) were exposed to selected concentrations of three model pollutants: copper (5 and 10µg L(-1)), atrazine (25 and 50µg L(-1)) and Irgarol (0.5 and 1.0µg L(-1)). These pollutant concentrations are environmentally relevant for coastal ecosystems, and have been selected for checking the efficiency of the reactive oxygen intermediate (ROI) scavenging enzyme system of these organisms. Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APx) and glutathione peroxidase (GPx) activities were measured at the end of 24h exposure. The integrated biomarker response (IBR) index - in our case for oxidative stress - has been employed to evaluate the ROI-scavenging enzyme system for each species and each treatment. In general, the SOD and CAT enzyme activities measured were higher in exposed populations than in controls, whereas APx and GPx activities showed the opposite trend. These microalgae showed significant responses of oxidative stress biomarkers at environmentally relevant concentrations for the assayed pollutants and short exposure periods, conditions that most other model organisms cannot match. Therefore microalgae present clear advantages over other species for their prospective employment in an "early warning system".