60 MARCH 2018 • WORLD AQUACULTURE • WWW.WAS.ORG Prymnesiophyte blooms have increased in occurrence and now are seen in low salinity and marine waters. Prymnesium parvum, the most frequent causative organism, is a small flagellate that produces ichthyotoxins (fatty acid amides, prymnesins) that affects gill function. There is some controversy about the bioactive metabolite formed by prymnesiophytes, and it is clear that more research is needed to confirm what toxin is causing observed mortality. Raphidophyte blooms occur mainly in marine waters and toxic blooms typically involve two genera, Chatonella and Heterosigma. These flagellates can produce high concentrations of several bioactive molecules: reactive oxygen (similar to hydrogen peroxide), a neurotoxin similar to that produced by dinoflagellates, as well as physically clog gills, leading to mortality. There is some evidence in the literature of benthic invertebrate mortalities — including shrimp, clams, and oysters — of exposure to toxins. One toxin has been reported in diatoms (Bacillariophyceae), however mechanical effects from diatoms can also cause malaise and even death in fish. Diatoms have an exterior silica wall that can affect gill function by irritation and clogging, affecting water movement and oxygen exchange. The toxin produced is domoic acid, which causes amnesic shellfish poisoning if contaminated fish or shellfish are eaten. This toxin has been identified only in marine planktonic diatoms to date. Climate Change Effects Climate changes may increase occurrence of algal bloom events. The concentration of dissolved carbon dioxide in water is increasing, thereby making it possible for microalgae to grow in aquatic systems to greater biomass/density than possible when 60 species are known to produce bioactive metabolites in estuarine areas. Toxins includes paralytic shellfish poisons (largely saxitoxin and its 21 structural variants), neurotoxic shellfish poisons (brevetoxin that includes two structural classes), maitotoxin responsible for ciguatera formation after the dinoflagellate Gambierdiscus toxicus are ingested by fish and invertebrates, azaspiracid shellfish poisoning from mussels, yessotoxin and its three structural variants that are found in benthic invertebrates such as mussels and scallops. We recently identified a saxitoxinlike compound present in a freshwater dinoflagellate Peridinium c.f. inconspicuum, results that are intriguing given that previous work was unsuccessful identifying a causative compound in another freshwater species. Recent evidence for karlotoxin and pfeisteria toxin impacts on fish is well documented and point to increased occurrence in tidal estuaries with water exchange. Both dinoflagellates have a multi-stage life cycle, making identification of certain stages difficult. Commercial harvesting of oysters and clams can be halted if brevetoxin is identified in coastal waters. Euglenophycin toxin was identified in 2010 from a North Carolina striped bass rearing facility. Since then toxin production has been documented in algal isolates from four continents for E. sanguinea and an additional five species of euglenoids were shown to produce the toxin. Toxic blooms have resulted in cattle deaths in Georgia and Michigan as well as fish in 12 other states in the United States, as well as South America. Active euglenoids producing euglenophycin have been observed in hypersaline (to 70 ppt salinity) to freshwater creeks, shallow ponds, and lakes. Recognition of blooms can be hampered by the loss of flagella and subsequent formation of ameboid stages when water is turbulent, causing visible blooms to completely disappear. TABLE 1. Algal toxin producing divisions, toxin class produced, impact on aquacultured species, and pertinent literature. Algal divisions Number Habitat Toxin Impact on Reference of toxic classes aquaculture species species Cyanobacteria >65 Freshwater, marine >50, including Liver, heart damage, Zimba et al. 2001 hepatotoxin, death, reduced food neurotoxin, cytotoxin conversion Dinophyceae 35 Freshwater, marine Neurotoxins, None (storage and Deeds et al. 2002 diarrheic food chain transfer) to death Euglenophyceae 7 Freshwater, estuarine Neurotoxin, Fish mortality Zimba et al. 2001 cytotoxin? Raphidophyceae Freshwater, marine Neurotoxin, None to death Haque and cytotoxin Onoue 2002 Haptophyceae 1 Freshwater, marine Ichthyotoxin Fish gill hemorrhaging Bertin et al. 2012 Bacillariophyceae 1 marine Neurotoxin None (storage and Bates et al. 1989 transfer to higher trophic levels)
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