Superoxide Production by the Red Tide-Producing Chattonella marina Complex (Raphidophyceae) Correlates with Toxicity to Aquacultured Fishes

Production of extracellular superoxide contributes to photosynthesis via elimination of reducing power and regeneration of NADP+ in the red-tide-forming raphidophyte Chattonella marina complex

The raphidophyte Chattonella marina complex (hereafter Chattonella) consists of noxious red-tide-forming algae that are damaging to fish farms. Chattonella produces and secretes large amounts of the superoxide anion (•O2-), and the production of extracellular •O2- has been associated with fish mortality. We reported previously that photosynthetic electron transport is correlated with the production of •O2- in the genus Chattonella. However, the physiological roles of the production of extracellular •O2- remain to be clarified. In the present study, we examined the effects of the production of extracellular •O2- on photosynthesis and cell proliferation in two strains of Chattonella, namely, Ago03, a highly toxic strain that produces large amounts of •O2- externally, and Ago04, a low-toxicity strain that produces very small amounts of •O2-. Both the growth rate and the net photosynthetic activity of Ago04 were higher than those of Ago03. In Ago04, levels of Rubisco and 3-phosphoglycerate, the product of the reaction catalyzed by Rubisco, were 4-fold higher than those in Ago03, suggesting the higher photosynthetic activity of Ago04. In the presence of glycolaldehyde, a specific inhibitor of the Calvin-Benson cycle, the levels of NADP+ and the photosynthetic parameter qP declined under strong light in Ago04. By contrast, levels of NADP+ and qP in Ago03 changed less significantly than those in Ago04. Given that •O2- is produced by a putative NADPH oxidase that converts O2 to •O2- in Chattonella, it seems likely that the production of •O2- might play a role not only in the elimination of excess reducing power of NADPH from the cell, via •O2-, but also in the regeneration of NADP+, as a result of the action of NADPH oxidase, which oxidizes NADPH, to maintain photosynthetic electron transport.

Strain, cell density, and nutrient condition affect patterns of diurnal vertical migration and superoxide production in a red-tide alga

A red tide occurs when cell densities of autotrophic microalgae and some heterotrophic protists increase dramatically and thereby change the color of the sea. Red tides sometimes have negative impacts on human activities, such as fisheries and tourism. Most red-tide flagellates display diurnal vertical migration (DVM) in which cells normally migrate upward during the day and downward at night. This behavior promotes active growth, due to the effective acquisition of nutrients and light, as well as population density increase and cell aggregation. However, the factors and their interactions influencing DVM remain to be clarified, such that no algorithm exists that can precisely simulate the DVM pattern and the development of a red tide in the field. Chattonella marina complex (hereafter Chattonella) is a representative microalga of harmful red tides and some previous studies has suggested that Chattonella's DVM plays important roles in development of a red tide. Chattonella can produce a large amount of superoxide (•O₂ ^-), which is responsible for the regulation of various physiological processes as well as its toxicity against microorganisms and animals. In the present study, we examined the effects of strain, growth phase, cell density, and nutrient deficiency on the pattern of DVM. In addition, we also measured the •O₂ ^- level in most experiments to assess the relationship between DVM and •O₂ ^- production. Some strains displayed clear DVM, whereas others aggregated at the surface all day in a fixed condition. Strains' DVM patterns did not show a relationship with •O₂ ^- production. Moreover, the DVM became less clear at high cell density and in nitrogen- or phosphorus-depleted conditions. Although a previous study reported that the •O₂ ^- production rate increased during the light period and decreased during the dark period, regardless of cell density, the diurnal pattern of •O₂ ^- became less clear at a higher cell density in a Chattonella strain used in the present study. Our findings indicate that DVM and •O₂ ^- production by a Chattonella population composed of various strains can change across developmental phases and environmental conditions. This characteristic may produce adaptability in species and increase the chances of a massive population increase.

Changes in Toxin Production, Morphology and Viability of Gymnodinium catenatum Associated with Allelopathy of Chattonella marina var. marina and Gymnodinium impudicum

Allelopathy between phytoplankton organisms is promoted by substances released into the marine environment that limit the presence of the dominating species. We evaluated the allelopathic effects and response of cell-free media of Chattonella marina var. marina and Gymnodinium impudicum in the toxic dinoflagellate Gymnodinium catenatum. Additionally, single- and four-cell chains of G. catenatum isolated from media with allelochemicals were cultured to evaluate the effects of post exposure on growth and cell viability. Cell diagnosis showed growth limitation and an increase in cell volume, which reduced mobility and led to cell lysis. When G. catenatum was exposed to cell-free media of C. marina and G. impudicum, temporary cysts and an increased concentration of paralytic shellfish toxins were observed. After exposure to allelochemicals, the toxin profile of G. catenatum cells in the allelopathy experiments was composed of gonyautoxins 2/3 (GTX2/3), decarcarbamoyl (dcSTX, dcGTX2/3), and the sulfocarbamoyl toxins (B1 and C1/2). A difference in toxicity (pg STXeq cell−1) was observed between G. catenatum cells in the control and those exposed to the filtrates of C. marina var. marina and G. impudicum. Single cells of G. catenatum had a lower growth rate, whereas chain-forming cells had a higher growth rate. We suggest that a low number of G. catenatum cells can survive the allelopathic effect. We hypothesize that the survival strategy of G. catenatum is migration through the chemical cloud, encystment, and increased toxicity.

Generation of Reactive Oxygen Species (ROS) by Harmful Algal Bloom (HAB)-Forming Phytoplankton and Their Potential Impact on Surrounding Living Organisms

Most marine phytoplankton with relatively high ROS generation rates are categorized as harmful algal bloom (HAB)-forming species, among which Chattonella genera is the highest ROS-producing phytoplankton. In this review, we examined marine microalgae with ROS-producing activities, with focus on Chattonella genera. Several studies suggest that Chattonella produces superoxide via the activities of an enzyme similar to NADPH oxidase located on glycocalyx, a cell surface structure, while hydrogen peroxide is generated inside the cell by different pathways. Additionally, hydroxyl radical has been detected in Chattonella cell suspension. By the physical stimulation, such as passing through between the gill lamellas of fish, the glycocalyx is easily discharged from the flagellate cells and attached on the gill surface, where ROS are continuously produced, which might cause gill tissue damage and fish death. Comparative studies using several strains of Chattonella showed that ROS production rate and ichthyotoxicity of Chattonella is well correlated. Furthermore, significant levels of ROS have been reported in other raphidophytes and dinoflagellates, such as Cochlodinium polykrikoides and Karenia mikimotoi. Chattonella is the most extensively studied phytoplankton in terms of ROS production and its biological functions. Therefore, this review examined the potential ecophysiological roles of extracellular ROS production by marine microalgae in aquatic environment.