Cyanobacterial Toxins in Florida’s Freshwater Center for Risk Analysis and Management USF College of Public Health

Introduction Cyanobacteria (blue-green algae) are found in freshwater sources, such as ponds, lakes, rivers and streams. Discharge of nutrients into the water can cause eutrophication, and promote cyanobacterial growth. During their growth phases some species of cyanobacteria can produce toxins.

Introduction The toxic effects and the target organs vary greatly between the toxins. Livestock and wildlife may become ill, as can recreational water users. Agents used to kill cyanobacterial blooms can result in the release of toxins into the water, due to cell lysis.

Introduction Surface water will be an increasing source of Florida’s drinking water in the future. Filtering the cyanobacteria from drinking water does not remove free toxins, but is necessary to prevent cell lysis from later treatments. Water suppliers can employ UV light, ozone or chlorine to break down toxins.

Introduction Boiling drinking water does not appear to significantly break down toxins, and could lyse any cyanobacteria from unfiltered water. Metabolites and breakdown products of these cyanobacterial toxins have not been studied extensively.

Cylindrospermopsin

Cylindrospermopsis raciborskii, Umezakia natans and Aphanizomenon ovalisporum produce this alkaloid during growth phases. It is a hepatotoxin, with a five day median lethal dose (LD 50 ) in mice (intraperitoneal injections) of 200  g/kg and a one day LD 50 of 2 mg/kg.

Cylindrospermopsin Cylindrospermopsin has a tricyclic guanidine moiety bridged to hydroxymethyl uracil. A deoxy- analog of cylindrospermopsin, which is also frequently found, is much less toxic. The mechanism of toxicity involves the inhibition of protein synthesis.

Cylindrospermopsin A study in mice suggests that oxidation by cytochrome P450 is a necessary step in the bioactivation of cylindrospermopsin. The heightened cytochrome P450 activity in mice relative to humans may explain the severe hepatotoxicity in mice.

Cylindrospermopsin C. raciborskii is found at depths of six to eight feet, as are most water intakes. It is recommended that alternative water sources be used for up to a week after blooms subside, if algaecides are used. Treatment of a C. raciborskii bloom in Australia with copper sulfate lysed cells and poisoned 149 people.

Cylindrospermopsin A number of cases of acute liver failure occurred in a Brazilian dialysis center, where the water source contained cylindrospermopsin and microcystins. Pending further review, the World Health Organization recommended a range of 1-13  g/L for cylindrospermopsin.

Cylindrospermopsin Relatively low chlorine concentrations (< 1 mg/L) have been found sufficient to degrade cylindrospermopsin. Organic matter tends to consume chlorine, requiring more chlorine to effectively degrade cylindrospermopsin.

Cylindrospermopsin Titanium dioxide has been found to catalyze the UV-mediated degradation of cylindrospermopsin.

Microcystins Microcystin-LR

Microcystins Microcystins are a grouping of over 50 hepatotoxins with highly similar monocyclic heptapeptide structures. There are a number cyanobacterial genera that produce microcystins, including Microcystis, Oscillatoria, Anabaena and Nostoc. Microcystis aeruginosa is the best studied microcystin producing species.

Microcystins The most commonly studied and most prevalent member of this group is microcystin-LR (leucine and arginine are the variable amino acids). Microcystin-LR has LD50 values in mice and rats ranging from 36 to 122  g/kg by a number of routes, including inhalation of aerosols.

Microcystins Microcystins can inhibit protein phosphatases, with a reported 50% inhibition at 3  g/L for microcystin-LR. The only direct associations between microcystins and human deaths involved the 43 hepatitis fatalities at a dialysis clinic in Brazil, and possibly a similar incident in Portuguese clinic.

Microcystins Symptoms of microcystin poisoning include diarrhea, vomiting, piloerection, pallor and weakness. The liver is the target organ of microcystins, with cytoskeletal damage, necrosis and blood pooling. In vitro blebbing of hepatocytes was reported with microcystins.

Microcystins Primary degradation of microcystin-LR at low levels (10  g/L) in water occurs in less than one week. Sunlight was observed to decompose the toxin in roughly ten days. Microcystins –LR and –RR were degraded in minutes by UV light with wavelengths of nm.

Microcystins Chorine or calcium hypochlorite levels of 1 mg/L degrade 95% of microcystins. The World Health Organization established a recommended limit of 1  g/L for microcystin-LR. Higher concentrations of other microcystins may be acceptable, depending on toxicities relative to microcystin-LR.

Other Toxins Anatoxin-a is a neurotoxin in some species of Anabaena, Cylindrospermopsis, Plankthorix, Aphanizomanon and Microcystis. Anatoxin-a is a muscle depolarizer with LD50 values of 1 to 10 mg/kg in different species. No human deaths have been associated with anatoxin-a. High doses caused hydrocephaly in rats.

Other Toxins Anatoxin-s, found in Anabaena flos-aquae and Anabaena lemmermanii, is the only known naturally occurring organophosphate. Intraperitoneal dosages at or above 4 mg/kg of A. flos-aquae were toxic in rats. Rats displayed salivation, lacrimation, incontinence, fasciculation, convulsion, paralysis and death within 2 to 20 minutes. A. flos-aquae is used as a dietary supplement.

Other Toxins Saxitoxins are carbamate alkaloids found in marine dinoflagellates and in Anabaena circinalis, Cylindrospermopsis raciborskii and Lyngba wollei. Sodium channel blocking can be used in assays that can detect as low as 2  g/L. The main concern with saxitoxins is that concentration in marine fish can cause paralytic shellfish poisoning.

Conclusions There are a variety of toxins that can originate from cyanobacteria. The primary toxins of concern, based on human and animal data are cylindrospermopsin and microcystins. These toxins may potentially cause adverse health effects depending upon the amount, frequency and duration of exposure.