Microcystins — or cyanoginosins — are a class of toxins produced by certain freshwater blue-green algae. Over 50 different microcystins have been discovered so far, of which microcystin-LR is the most common. Chemically they are cyclic heptapeptides produced through nonribosomal peptide synthases.
Cyanobacteria can produce microcystins in large quantities during algal blooms which then pose a major threat to drinking and irrigation water supplies, and the environment at large.
Microcystin-LR is the most toxic form of over 80 known toxic variants, and is also the most studied by chemists, pharmacologists, biologists, and ecologists. Microcystin-containing 'blooms' are a problem worldwide, including China, Brazil, Australia, South Africa, the United States and much of Europe. Hartebeespoort Dam in South Africa is one of the most contaminated sites in Africa, and possibly in the world.
The microcystin-producing Microcystis is a genus of freshwater cyanobacteria and is projected to thrive with warmer climate conditions, such as the rise of water temperatures or in stagnant waters, and through the process of eutrophication (oversupply of nutrients). A study concluded in 2009 that climate change can catalyze the global expansion of cyanobacterial blooms. The EPA reported in 2013, that climate change and changing environmental conditions are associated with harmful algae growth and may negatively impact human health, and the economy for communities across the US and around the world.
Evidence developed at the University of Heidelberg suggests that in particular dissolved reactive phosphorus promotes additional growth.
Humans are exposed by swallowing, skin contact with or inhaling contaminated water.
Microcystins are chemically stable over a wide range of temperature and pH, possibly as a result of their cyclic structure.
Microcystin-producing bacteria algal blooms can overwhelm the filter capacities of water treatment plants. Some evidence shows the toxin can be transported by irrigation into the food chain,
Lake Erie blooms
In 2011, a record outbreak of blooming microcystis occurred in Lake Erie, in part related to the wettest spring on record, and expanded lake bottom dead zones, reduced fish populations, fouled beaches, and the local tourism industry which generates more than $10 billion in revenue annually.
In August 2014, the City of Toledo, Ohio detected unsafe levels of microcystin in its water supply due to harmful algal blooms in Lake Erie, the shallowest of the Great Lakes. They issued a DO NOT DRINK OR BOIL water advisory to approximately 500,000 people.
An Ohio state task force found that Lake Erie received more phosphorus than any other Great Lake, both from crop land, due to the farming practices, and from urban water-treatment centres
Algal blooms have been occurring more frequently, and scientists had predicted this significant bloom of blue-green algae to peak in early September 2014.
San Francisco Bay Area
In 2016, microcystin had been found in San Francisco Bay Area shellfish in seawater, apparently from freshwater runoff, exacerbated by drought.
In 2018, the Iowa Department of Natural Resources found microcystins at levels of .3 µg/L, or micrograms per liter, which is equivalent to .3 parts per billion in the raw water supplies of 15 out of 26 public water systems tested.
Human health effects upon exposure
Microcystins cannot be broken down by standard proteases like pepsin, trypsin, collagenase, and chymotrypsindue to their cyclic chemical nature.
They are hepatotoxic, i.e. able to cause serious damage to the liver. Once ingested, microcystin travels to the liver via the bile acid transport system, where most is stored, though some remains in the blood stream and may contaminate tissue.
Acute health effects of Microcystin-LR are abdominal pain, vomiting and nausea, diarrhea, headache, blistering around the mouth, and after inhalation sore throat, dry cough, and pneumonia.
There appears to be inadequate information to assess the carcinogenic potential of microcystins by applying EPA Guidelines for Carcinogen Risk Assessment. A few studies suggest a relationship may exist between liver and colorectral cancers and the occurrence of cyanobacteria in drinking water in China. Evidence is, however, limited due to limited ability to accurately assess and measure exposure.
The impact on patients with a compromised immune system is not yet fully known, but as of 1991 it was starting to raise some concern.
In mice, high dose green tea might have a protective effect against microcystin induced toxicity
In the US, the EPA issued a health advisory in 2015. A ten
day Health Advisory was calculated for different ages which is considered protective of non-carcinogenic adverse health effects over a ten-day exposure to microcystins in drinking water: 0.3 μg/L for bottle-fed infants and young children of pre-school age and 1.6 μg/L for children of school age through adults.: 28–29
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