I have blogged a number of times about Harmful Algal Blooms (HABs) that have caused major environmental and health problems in many countries this last year. One group of microorganisms that can cause these blooms are the Cyanobacteria, sometimes referred to as blue-green algae.
These blooms of Cyanobacteria can cause problems with human health such as liver damage. They can contaminate drinking water, severely damage fisheries, kill livestock and pets, and reduce tourism. People swimming, waterskiing or boating in contaminated water can absorb toxins produced by the Cyanobacteria. The toxins can accumulate in fish that are the eaten.
This blog takes a look at some of the toxins produce by the Cyanobacteria – the microcystins – including which species produce them, their chemistry, and what mitigation steps are available.
First, let’s take a look at the Cyanobacteria.
Cyanobacteria are single cell algae that belong to the phylum Cyanobacteria. They proliferate in bodies of water including ponds, lakes, reservoirs and slow-moving streams. They produce toxic blooms when the water is warm and when nutrients are available. Some cyanobacteria produce a group of toxins called microcystins. When the cyanobacteria cells die, the cell walls disintegrate and the microcystins are released into the water.
Microcystins are stable, and at typical ambient conditions the half-life is c. 10 weeks. In darker and cooler bodies of water the microcystins can last months or years. Microcystins can also survive if water is boiled, so cooking contaminated fish may not destroy them.
Not all of the Cyanobacteria produce microcystins. Those that do fall into a number of genera. These include:
The way these genera are related is shown below.
The commonest species of Cyanobacteria producing microcystins is Microcystis aeruginosa.
Chemistry of microcystins
Microcystins are cyclic heptapeptides – molecules that contain seven amino acids. The typical structure of microcystins is shown below.
Eighty types of microcystins have been identified. These variants are caused by differences in the amino acids present – X and Z in the above Figure. In microcystin-LA the amino acids are leucine and alanine, in microcystin-YR they are tyrosine and arginine, in microcystin-RR they are both arginine, and in microcystin-LR they are leucine and arginine. In the above Figure, R1 and R2 are both methyl groups.
Microcystin-LR (below) is the commonest form produced by cyanobacteria and is the best studied.
All microcystins have the same side chain called ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca;4,6-dienoic acid). It is the ADDA that analytical methods use to quantify the presence of microcystins in contaminated water.
Prevention and control of HABs
As the EPA states, the best way to avoid HABs, including those caused by Cyanobacteria, is to prevent increases in the nutrient content of the water, for example from run-off from sewage treatment plants, or from agricultural sources such as applied manures and fertilizers.
Methods to control and remove HABs from contaminated water are receiving intense scrutiny at the moment. For example, Barry Cohen of the National Algae Association is running an Algae Bloom Remediation Workshop on 21-22 May 2019 in Lauderdale, Florida. Florida was particularly badly hit with HABs during 2018.
Attempted treatment of contaminated water can be divided into two activities:
- Removal of intact cells – using coagulation, sedimentation, filtration, floatation
- Removal of dissolved microcystins – using nanofiltration, ozone, chlorination, activated carbon, bacteria-impregnated membranes (see earlier blog.)
The efficiency of these methods varies considerably, and none can be said to be the complete answer. Much work remains to be done.
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