Microcystin

NOAA captured this view of Lake Erie in October 2011, during the worst cyanobacteria bloom the lake experienced in decades. Torrential rains increased fertilizer runoff, which promoted the growth of microcystin-producing bacteria.

Microcystins (or cyanoginosins) are a class of toxins produced by certain freshwater cyanobacteria; primarily Microcystis aeruginosa but also other Microcystis, as well as members of the Planktothrix, Anabaena, Oscillatoria and Nostoc genera. 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.

Microcyctins can be produced in large quantities during algal blooms and pose a major threat to drinking and irrigation water supplies, as well as the environment at large.

Characteristics

File:Microcystin-LR-structure.png

Chemical structure of microcystin-LR

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.

Microcystins contain several uncommon non-proteinogenic amino acids such as dehydroalanine derivatives and the uncommon β-amino acid ADDA. Microcystins covalently bond to and inhibit protein phosphatases PP1 and PP2A and can thus cause pansteatitis.

Exposure

Microcystins are hepatotoxic (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. 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 of exposure to microcystin by patients with a compromised immune system is not yet fully known, but is starting to raise some concern.

Formation

File:Microcystis aeruginosa.jpeg

A culture of microcystis aeruginosa, a photosynthesizing bacteria

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). An Ohio state task force found that Lake Erie received phosphorus and more recently reactive phosphorus from crop land, due to the farming practices, and evidence suggests that in particular dissolved reactive phosphorus (from fertilizer) promotes additional growth.

Pathways

Microcystin-producing bacteria blooms can overwhelm the filter capacities of water treatment plants. Some evidence shows the toxin can be transported by irrigation into the food chain, Microcystins are chemically stable over a wide range of temperature and pH, possibly as a result of their cyclic structure. Due to their cyclic nature they cannot be broken down by standard proteases like pepsin, trypsin, collagenase, and chymotrypsin.

Cyanobacteria blooms

See also: Algal bloom and Water treatment

A study concluded in 2009 that climate change can act as a catalyst for global expansion of harmful cyanobacterial blooms. The EPA reported in 2013, that changing environmental conditions such as harmful algae growth is associated with current climate change, and may negatively impact the environment, human health, and the economy for communities across the US and around the world.

Lake Erie Blooms

A record outbreak of blooming microcystis occurred in Lake Erie 2011, 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 that generates more than $10 billion in revenue annually.

On 2 August 2014, the City of Toledo, Ohio detected higher levels of microcystin than deemed safe in its water supply due to harmful algal blooms (HABs) in Lake Erie, the shallowest of the Great Lakes. They issued a DO NOT DRINK OR BOIL water advisory to approximately 500,000 people. Algal blooms have been occurring more frequently, and scientists had predicted this significant bloom of blue-green algae, and they projected it to peak in early September.

See also

• Microcystis
• Nodularin-R

Further reading

• National Center for Environmental Assessment. Toxicological Reviews of Cyanobacterial Toxins: Microcystins LR, RR, YR, and LA (NCEA-C-1765)

External links

• Harmful Algal Blooms (EPA)
• Blue-Green Algae (Cyanobacteria) and their Toxins (Health Canada)
• Toxic cyanobacteria in water: A guide to their public health consequences, monitoring, and management (WHO)
• Cyanobacteria and Cyanotoxins: Information for Drinking Water Systems (EPA)
• Cyanobacteria, and microcystin production (The New York Times)