Algal toxins a threat to aquatic organisms and humans
Microcystins are the most commonly found and poisonous algal toxins produced by several harmful cyanobacterial species. Microcystins are usually associated with freshwater environments, and can be accumulated by aquatic animals such as mussels, snails, zooplankton, shrimp, frogs and fish through ingestion of drinking water and foods with bioaccumulated toxicity.
Microcystins are monocyclic heptapeptides including several variants. As a family of potent liver toxins, they can cause hepatotoxicosis, gastroenteritis and allergic reactions, and are potentially hazardous in ecosystems and to human health.
Microcystins are associated with Microcystis, Anabaena, Oscillatoria, Nostoc, Hapalosiphon and Anabaenopsis algal species. M. aeruginosa is the primary microcystin-producing species. It often forms blooms in eutrophic freshwater ecosystems, representing a threat to aquatic organisms and human beings.
Since their molecular structure has cyclic and double bonds, microcystins have considerable physiochemical stability. Existing water treatment processes such as coagulation, sedimentation, filtration and chlorination are ineffective at removing the toxins. Therefore, techniques for eliminating the microcystin-producing algae in water bodies have been developed instead of direct microcystin removal during water treatment.
The most promising control strategy with respect to maximum effectiveness and minimal costs and environmental impacts is flocculation and sedimentation of harmful algal blooms with clay.
Over the past 25 years, clays have been investigated in several countries as a means of removing harmful algae from the water column.
In China, many freshwater bodies have been contaminated by organic compounds, and more than two-thirds are eutrophic. As a result, toxic cyanobacteria blooms occur frequently, and algal toxins, especially microcystins, are a serious concern. Developing safe and efficient techniques to control M. aeruginosa has become a top priority in China today.
The authors recently performed a study to investigate microcystin concentrations in Nile tilapia muscle tissue and surrounding pond water from a typical eutrophic fish pond in Hubei province, China (Table 1). Furthermore, it aimed to develop removal methods for eliminating the dominant microcystin-producing species, M. aeruginosa.
Biyu, Concentrations and distributions of microcystins, Table 1
|Sample Date||Cyanobacteria (%)||Dominant Cyanobacteria||Microcystin (%)||Microcystin Concentration (ng/g) in Fish Muscle|
|July||36.59||Microcystis||0.134 ± 0.041||0.84 ± 0.84
|August||27.13||Microcystis, Coelosphaerium species||0.052 ± 0.017||0.68 ± 0.49
Two types of clay were used to remove both laboratory-cultured and field-collected M. aeruginosa: chitosan- and polymeric aluminum chloride (PAC)-modified clay (kaolin).
Results showed that both clays removed cultured M. aeruginosa effectively. After treatment with clay, the algae settled to the pond bottom, and cell vitality decreased noticeably. The sedimented M. aeruginosa cells died within a month of flocculation with chitosan-modified clay. Maximum electron transport rates (ETRmax), a measure of photosynthetic activity, decreased by 76 percent after one month.
For algae treated with PAC-modified clay, algal cells became yellowish and decayed in a week. ETRmax declined 79 percent after one week. Of the two treatments, PAC-modified clay had a faster and slightly stronger effect. Table 2 shows optimal dosages of both clay treatments.
Biyu, Optimal doses of coagulants for maximum algal removal, Table 2
Chitosan-Modified Clay (ml)
PAC-Modified Clay (ml)
Chitosan-modified clay is more environmentally friendly because it does not contain aluminum chloride and is biodegradable. It can effectively remove field-reared M. aeruginosa. After chitosan-modified clay treatment, chlorophyll-α decreased 98.6 percent.
Editor’s Note: Co-author Yang Yi passed away on July 31, 2009. Yang Yi was an accomplished scholar, an excellent researcher and an innovator in applying simple aquaculture techniques to gain efficiency for small-scale farmers. He was on faculty at the Asian Institute of Technology in Bangkok and later at Shanghai Ocean University. He was also president of the Asian Fisheries Society from 2007 until 2009.
(Editor’s Note: This article was originally published in the November/December 2010 print edition of the Global Aquaculture Advocate.)
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