Cyanobacteria: Natural Organisms with Toxic Effects
Recent awareness of exposure to cyanobacteria—also known as blue-green algae—and increasing reports of public health consequences serves to remind us of the delicate balance between man and the Earth’s resources. Increased pollution of and reliance on surface waters for drinking water sources have led to international concerns over cyanobacteria contamination and questions as to what treatment regimen is most practical and effective.
Cyanobacteria are common inhabitants of marine, brackish and fresh waters. With the ability to fix nitrogen, these organisms are known to be important contributors to the overall food chain. During conditions of ecological imbalance, such as erosion or excessive run-off from agricultural operations, natural waters are taxed with excessive nutrients, i.e., ideal conditions for algal growth. These periods of rapid and excessive growth are called algal blooms. In addition to depriving the general aquatic environment of vital oxygen concentrations, certain species of cyanobacteria produce toxins that are harmful, and even deadly, to fish, animals and humans. These harmful algal blooms appear to be occurring with greater frequency in the United States as concentrated agricultural practices and population increases alter ecological cycles.
Currently, most of the world’s population relies on surface water sources for drinking. Increasingly in the United States, groundwater supplies aren’t meeting drinking water needs and a greater reliance on surface water sources is widely predicted. An estimated 48 percent of lakes in North America are eutrophic, meaning they are high in nutrients and low in oxygen. Eutrophic conditions often coincide with the presence of cyanobacterial blooms and are considered an emerging water quality problem in the United States.
About 50 freshwater cyanobacteria genera exist, and 12 are capable of producing toxins. While blue-green algae have significant taste and odor constituents, representing a moldy smell, their toxic metabolites have no taste, odor or color. The risk of exposure to algal toxins may come from drinking water, recreational water, dietary supplements or residue on produce irrigated with contaminated water and consumption of animal tissue. Avoiding cyanobacteria toxins isn’t as easy as avoiding a harmful algal bloom as toxins may be present in fish, shellfish and water even after the bloom has dissipated. The three major classes of algal toxins are responsible for a variety of health effects such as skin irritations, respiratory ailments, neurological effects and carcinogenic effects. Below is a description of the most widely known cyanobacterial toxins:
1. Cyclic peptides (nodularins, microcystins)—Nodularia, a well-known cyanobacterium, produce nodularins and are primarily a concern in marine and brackish waters thus creating a risk to recreational swimmers. The 65 variants of microcystins, however, are isolated from freshwaters worldwide and are produced by Microcystis (the most commonly identified cyanobacteria in human and animal poisonings), Anabaena and other algae. They’re very stable in the environment and resistant to heat, hydrolysis and oxidation. Both toxins have an affinity for the liver. Other symptoms of exposure to microcystins may range from weakness, loss of appetite, vomiting, and diarrhea to cancer.
2. Alkaloids (anatoxin, saxitoxin)—Anatoxins may affect the nervous system, skin, liver or gastrointestinal tract. These neurotoxins can cause symptoms of diarrhea, shortness of breath, convulsions and death, in high doses, due to respiratory failure. Saxitoxins are the cause of paralytic shellfish poisonings in humans consuming contaminated shellfish. There are no reports of similar poisonings via the drinking water route.
3. Lipopolysaccharides (endotoxins)—A similar cell wall toxin as found in Salmonella bacteria, but appears to be less toxic. It was a suspect contaminant in a 1975 drinking water outbreak in Sewickley, Pa.
The physicochemical nature of the water source can have an effect on not only the growth but also the toxicity of the algal bloom. For example, some algae increase in toxicity when blooms are iron-deficient. In general, temperature, sunlight and nutrient loads have a substantial impact on the proliferation of the bloom.
Documented outbreaks, risks
In March 2003, the U.S. House Science Subcommittee on Environment, Technology and Standards heard testimony on the problem of harmful algal blooms in the Great Lakes. Such events not only pose a risk to the drinking water supply but hamper states’ multi-billion dollar tourism and fishing industries. According to the sub-committee’s chair, harmful algal blooms cost the United States $50 million a year. Testifying at the meeting, a leading expert in the field stated that harmful algal blooms cause significant deaths in wild and domestic animals, farmed fish and shellfish as well as human health effects—all of which are admittedly not well understood.1
Previous reports indicate that death in mammals and birds in particular can occur within minutes to days. Animal studies have shown persistence of the toxins in edible mussels, harvested from a water bloom, following boiling. Animal studies have also shown tumor production, skin rashes and birth defects in exposed populations.
Evidence for human health effects is less clear. Seasonal gastrointestinal outbreak data correlated with algal bloom conditions suggest that algal blooms may play a role. Skin rashes, blisters, and deep lesions as well as diarrhea have been reported following recreational exposures to algal blooms. Pneumonia and other respiratory symptoms, including allergic reactions and asthma, have been documented following inhalation exposures.
Drinking water outbreaks are scarce but large: A 1931 report of 9,000 persons in Charleston, W.Va., with acute gastroenteritis; a 1976 report of 62 percent of the population of 8,000 in Sewickley, Pa., and a 1988 report of severe gastroenteritis in Brazil with 2,000 cases and 88 deaths, mostly children. A water-related exposure due to contaminated dialysis water, also in Brazil, led to 63 deaths.
Several drinking water outbreaks due to cyanobacteria have been reported in Australia, one leading to severe illness and kidney failure in 150 persons including 140 children. Australian researchers have shown elevated markers of liver injury in exposed populations. Increased liver cancer rates in China are suspected to be caused in part by cyanobacteria exposures. There may also be a link with traveler’s diarrhea and cholera infections as cyanobacteria are suspect carriers of the cholera-causing bacteria, Vibrio cholerae.
Children and immunocompromised populations are considered at a higher risk, as with many health-related contaminants. Neonates—newborns—have been shown to be 10 times more sensitive to saxitoxin than adults, suggesting the need for risk assessment studies on sensitive sub-populations.
Low-level chronic exposure to microcystins is known to increase the risk of cancer in the liver; however, no chronic effects are apparent with anatoxins. No data are available identifying what chronic exposure levels for which populations is a risk over what period of time, yet microcystin, cylindrosperm-opsin and anatoxin-a are listed as the highest priority cyanobacterial toxins by the U.S. Environmental Protection Agency (USEPA). In severe cases, treatment following exposure to cyanobac-terial toxins is generally supportive. Ingestion of activated carbon has been utilized to increase gut absorptions. The best defense is to avoid initial exposure to cyanotoxins.
Still, avoidance may not be that easy considering microcystin occurrence studies of 24 public water systems in the United States and Canada showed that 80 percent of the 677 samples tested positive, but only 4.3 percent exceeded the World Health Organization (WHO) guideline of 1 part per million (ppm). Seventeen percent of samples from Lake Champlain, a drinking water source in New York, were positive. Following the deaths of two dogs in 1999 and 2000, another study of Lake Champlain found 4 percent of samples positive for anatoxin. In Florida, concentrations were as high as 12 ppm with toxins detected in both raw and finished water. U.S. waters were positive for multiple algal toxins, and above WHO guideline levels in both raw and finished waters.2
Treatment and regulatory efforts around the world
Aside from the WHO guideline of 1 ppm of microcystin, regulatory agencies in Australia, New Zealand, Brazil and Canada have also set guideline values, ranging from 1 ppm to 10 ppm. The toxin is known to be retained in the body after prolonged periods of time judging from the presence of detectable levels in exhumed bodies. Other levels have been set for various cyanobacterial toxins. Yet, in the United States, no official guidelines have been set, except for an Oregon guideline of 1 ppm for health food.
Part of the difficulty in setting a standard is implementing the best action plan for when that standard is exceeded. With cyanobacteria, utilizing an appropriate action plan is critical. By nature, microcystin toxins are typically bound inside the algal cell until death. Once the cell begins to die, toxins are released into the surrounding water. Therefore, simply killing the algae with disinfectants may actually worsen the contamination problem, elevating the public health risk of toxin exposure. Other algal toxins, such as cylindrospermopsin, are released by healthy cells as well. As a result, effective treatment often requires the application of multiple technologies such as coagulation, flocculation, sedimentation, filtration (i.e., slow sand filtration, granular or biologically activated carbon), microfiltration and oxidants. Coagulation, sedimentation and filtration together can remove 90-99.9 percent of algae but is unable to remove dissolved toxins.
Carbon, ozone, and chlorine are effective for reduction of anatoxins, microcystins, nodularin and cylindro-spermopsin. All but chlorine are useful for saxitoxin, which is also sensitive to boiling. If using granular activated carbon (GAC), the presence of dissolved organic carbon in the source water must be monitored as it reduces the efficacy of the GAC. Although chemical disinfectants may inactivate biotoxins, little is known about the potential creation of more potent toxins during the process. Nanofiltration and reverse osmosis filtration are expected to be effective but with the production of a more concentrated toxic waste stream.
The frequency and duration of freshwater algal blooms is on the rise due to climatic, environmental and human behavioral factors including runoff from agricultural, urban and industrial activities that impact surface waters. In particular, the nutrient quality of waters, i.e., the nitrogen/phosphate ratio, is critical to preserve the balance of the plant, animal and microbial populations. Nutrient overload leads to eutrophication, allowing algae to bloom. Compounded by factors of stagnation, temperature and sunlight, harmful algal blooms may occur. The problem with monitoring cyanobacterial contamination is that it’s often transient, representative of periodic nutrient overloads on an otherwise healthy water source. In addition, toxins may remain stable in the water long after the bloom has disappeared.
In 1996, the USEPA amended the Safe Drinking Water Act (SDWA) requiring regulatory decisions on a minimum of five contaminants from the 1998 list of known or suspected drinking water contaminants, i.e., the Contaminant Candidate List, or CCL. Cyanobacteria and their toxins were listed on the 1998 CCL. Like many CCL contaminants, data are lacking on health effects, treatment and occurrence—information needed to make regulatory decisions. Therefore, algal toxins are slated to be monitored under the Unregulated Contaminant Monitoring Regulation (UCMR) program’s pre-screen testing component. This means new methods will be tested and aimed at targeting vulnerable water systems that may be used to collect information on occurrence of algal toxins.
Future research is focused on development of rapid, sensitive and standardized analytical testing methods so a better occurrence and monitoring database can be developed for algal toxins. Studies are also needed to evaluate acute and chronic health effects in healthy and sensitive populations and the efficacy of various treatment options for raw, finished and storage water reservoirs. Research must consider new drinking water management practices, such as groundwater recharge, since algal toxins and other contaminants survive longer in the absence of sunlight.
Overall, the key to minimizing risks of algal toxins is to reduce nutrient overload in surface waters. Effective watershed management, agricultural practices and wastewater treatment and disposal may prove to be more cost effective than the implementation of post-contamination treatment technologies and monitoring protocols.
- Ely, S.J., “Toxic blue-green algae threaten Lake Erie,” Wright State University, News Releases, www.wright.educgibinnews_ item.cgi?446, March 28, 2003.
- USEPA, “Creating a Cyanotoxin Target List for the Unregulated Contaminant Monitoring Rule,” Meeting Summary, USEPA Technical Service Center, Cincinnati, Ohio, May 17-18, 2001.
About the author
Dr. Kelly A. Reynolds is a research scientist at the University of Arizona with a focus on development of rapid methods for detecting human pathogenic viruses in drinking water. She holds a master of science degree in public health (MSPH) from the University of South Florida and doctorate in microbiology from the University of Arizona. Reynolds has also been a member of the WC&P Technical Review Committee since 1997.