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Cyanobacterial Toxins -- Microcystin-LR
Guideline
The maximum acceptable concentration (MAC) for the cyanobacterial toxin microcystin-LR in drinking water is 0.0015 mg/L (1.5 µg/L). This guideline is believed to be protective of human health against exposure to other microcystins (total microcystins) that may also be present.
Identity of Cyanobacterial Toxins
Cyanobacterial toxins are toxins produced by cyanobacteria, or blue-green algae. They include neurotoxins (e.g., anatoxins), hepatotoxins (e.g., microcystins), skin irritants and other toxins. Both hepatotoxins and neurotoxins are produced by cyanobacteria commonly found in surface water supplies and therefore appear to be of most relevance to water supplies at present.1-3 However, the neurotoxins are relatively unstable and, as such, are not considered to be as widespread as hepatotoxins in water supplies; in addition, they do not appear to pose the same degree of risk from chronic toxicity.3 It should be noted, however, that, due to limited analytical capabilities, there are only limited quantitative data available on the levels of neuro-toxins in water supplies. Cyanobacterial toxins were detected during a survey in the summer (July/August) of 2000 in Onondaga Lake and Oneida Lake in upstate New York, USA.4Microcystins were detected in only one of 13 samples from Onondaga Lake, as was anatoxin-a. However, 50% of the 22 samples from Oneida Lake tested positive for microcystins (seven or eight tested >1.0 µg/L), and two samples were positive for anatoxin-a. Anatoxin-a was less common than microcystins, with levels -0.85 µg/L. It may be possible that neurotoxins are more widespread than is currently believed, particularly since many of the neurotoxin-producing algae have been linked to deaths of both livestock and domestic animals.
Most of the hepatotoxins are collectively referred to as microcystins, because the first hepatotoxin was isolated from Microcystis aeruginosa. About 50 different microcystins have been isolated, and several of these may be produced during a bloom. Structurally, the microcystins are monocyclic heptapeptides that contain two variable L-amino acids and two novel D-amino acids. Microcystins are named according to their variable L-amino acids -- for example, microcystin-LR contains leucine (L) and arginine (R), whereas microcystin-YA contains tyrosine (Y) and alanine (A).5 The two novel D-amino acids in microcystins are N-methyldehydro-alanine (Mdha), which hydrolyses to methylamine, and a unique non-polar-linked amino acid 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid, also known as ADDA.6 The key component for biological activity appears to be linked with the ADDA side chain, as cleavage of the ADDA side chain from the cyclic peptide renders both components non-toxic.1,5,6 Nodularins, hepatotoxins that were first isolated from the blue-green alga Nodularia, are cyclic pentapeptides2 found mostly in saltwater and hence have not been a focus in drinking water.
Microcystin-LR, produced as a secondary metabo-lite by Microcystis aeruginosa and other blue-green algal species, appears to be the most commonly occurring microcystin.1 To date, most of the work on microcystins has been conducted using this variant because of its presence in most countries reporting toxic episodes.3 Microcystin-LR has a molecular weight of about 1000 daltons. Its toxicity does not appear to vary greatly from that of the other microcystins.5 There have been over 65 variant microcystins reported, the LD50s of which range from 50 to 300 µg/kg bw. Microcystin-LR is one of the 7 most toxic, with an LD50 of 50 µg/kg bw.
Calculated octanol-water partition coefficients (which measure solubility and interaction with water molecules) for the majority of microcystin variants suggest that their adsorption by activated carbon would be similar to, or greater than, that of microcystin-LR.7
Occurrence of Cyanobacteria
The cyanobacteria, or blue-green algae, owe their name to the presence of photosynthetic pigments. Freshwater cyanobacteria are known to occur throughout the world. Their classification and identification have been reviewed,8 and some 40 toxigenic species have been identified. Freshwater cyanobacteria may accumulate in surface water supplies as "blooms" and may concentrate on the surface as blue-green "scum."
Cyanobacterial genera known to occur in Canada and most often associated with poisonings are Anabaena, Aphanizomenon, Microcystis, Oscillatoria and Nodularia.5 Cultures of toxic Microcystis were first isolated from the Rideau River in Ottawa in the 1950s by P.R. Gorham and his associates at the National Research Council. A high degree of polymorphism of Microcystis aeruginosa has been reported, with variability between species in the northern and southern hemispheres.9
It has generally been found that 50-75% of bloom isolates are capable of producing toxins, with often more than one toxin being present. More than 70% of over 380 bloom biomass samples from 19 lakes in Alberta between 1990 and 1992 showed detectable levels of toxin (>1 µg of microcystin-LR per gram of dry biomass).10 The overall toxicity of a bloom can be uncertain because of variations in toxin concentration temporally and spatially within a water body experiencing a bloom.10
The growth of cyanobacteria and the formation of blooms are influenced by a variety of physical, chemical and biological factors; these were reviewed by the NRA Toxic Algae Task Group11 and Ressom et al.6 and are discussed below. As a result of the interplay of these factors, there may be large year-to-year fluctuations in the levels of cyanobacteria and their toxins.12 There is also seasonal variation with regard to which species predominate. Timing and duration of the cyanobacterial bloom season are dependent upon climatic conditions; in temperate zones such as Canada, cyanobacterial occurrences are most prominent during the late summer and early autumn and may last 2-4 months. However, blooms of some species of cyanobacteria have been found in winter under ice in Scandinavian and German lakes7 and early spring and summer in the Midwest, which can lead to a year-round problem.
Physical Factors
As water temperatures increase in the spring, there is a natural succession of algal groups from diatoms and green algae to cyanobacteria. Different genera of cyanobacteria have different minimum temperature tolerances. For example, Microcystis is less tolerant of cooler temperatures than Oscillatoria. The length of daylight required to optimize growth is species dependent. For example, Microcystis is more adapted to shorter days than Anabaena. This is perhaps one of the reasons why Microcystis species are the dominant species in North America in late summer, when day length is shorter. Some cyanobacteria, such as
Cylindrospermopsis, can tolerate low light levels and can therefore compete more efficiently with other planktonic algae for available light, owing primarily to the presence of photosynthetic pigments. These pigments also allow photosynthesis in coloured waters.
In addition, some cyanobacteria, such as Microcystis aeruginosa, can optimize their position in the water column in response to available light by actively regulating their buoyancy. This characteristic also allows cyanobacteria to migrate through thermal gradients and utilize nutrients confined to the cooler water below. The main control operates through photosynthesis (through the production of carbohydrates) and breaks down if there is too little carbon dioxide. Buoyancy cannot be adjusted during the night.
Increased turbidity favours cyanobacteria over other algae. As noted above, cyanobacteria can use a wide spectrum of light for photosynthesis and are able to migrate to the surface to maximize light intensity. However, very high turbidity can reduce the availability of phosphate and thus limit their growth. Turbulence and high water flows, on the other hand, are unfavourable to the growth of cyanobacteria, as these interfere with their ability to maintain a position in the water column.
Heavy rain storms can increase runoff and nutrient levels in the water and thus encourage the formation of blooms.84
Chemical Factors
As is the case for other photosynthetic organisms, the availability of the macronutrients phosphorus and, to a lesser degree, nitrogen controls the growth of cyano-bacteria. In general, cyanobacteria do not have as high a demand for phosphorus as do other phytoplankton but are efficient at phosphorus storage. As phosphorus readily adheres to reactive surfaces (organic and inorganic), much of the phosphorus in a water body will be associated with sediments. Unnaturally high levels of phosphorus in waters are indicative of disturbances in the watershed. Important point and non-point sources of phosphorus include raw and treated sewage, detergents and urban and agricultural runoff. Mass occurrences of toxic cyanobacterial blooms are not always associated with human activities causing pollution. For example, toxic blooms have been reported in Australian reservoirs with pristine or near-pristine watersheds.
Iron and molybdenum are particularly important micronutrients for cyanobacteria because of their direct involvement in nitrogen fixation and photosynthesis (iron) as well as carbon fixation and nitrogen uptake (molybdenum). Microcystis is not a nitrogen-fixing cyanobacterial genus.
Alkalinity and pH determine the chemical speciation of inorganic carbon, such as carbonate, bicarbonate and carbon dioxide. Low carbon dioxide concentrations favour the growth of several cyanobacterial species. Hence, water conditions such as low alkalinity and hardness and the consumption of carbon dioxide during photosynthesis by algae, increasing the pH, give cyanobacteria a competitive advantage.
Biological Factors
The role of cyanobacteria in aquatic food webs is very complex. In general, phytoplankton are grazed upon by zooplankton, which in turn are consumed by fish. Cyanobacteria are not easily digested by zooplankton; therefore, their populations may increase in relation to other, more easily digestible algae. Macrophytes compete with cyanobacteria and other phytoplankton for nutrients and light and may also suppress phytoplankton by releasing inhibitory compounds. Other aquatic bacteria can also compete with cyanobacteria for nutrients.
Formation of Surface Scum
The formation of surface scum requires a sudden change in weather conditions. Initially, there may be high barometric pressure and light to moderate winds, accompanied by constant circulation in a water body in which a substantial population of cyanobacteria may have optimally positioned itself in the water column to take advantage of those conditions. If the wind stops and circulation also stops, the cyanobacteria may suddenly become "overbuoyant." If they cannot adjust their buoyancy fast enough or at all (if it is night), then they will float to the surface and form a surface scum. Thus, scums may often be formed overnight. The scum may drift downwind and may settle at lee shores and quiet bays, where the cyanobacteria may eventually die, releasing their toxins.
