Blågrønalgetoksiner i bade- og drikkevand

English Summary

The phytoplankton in several Danish lakes used for recreational purposes and as drinking water reservoirs is seasonally dominated by blue-green algae that may produce toxins causing e.g. headache, abdominal pains, and pneumonia-like symptoms in humans. In severe cases, the toxins may have lethal effects. In addition to acute toxic effects, some blue-green algal toxins (hepatotoxins) show tumour-promoting potential or carcinogenic effects.

Blue-green algae produce four types of toxins: hepatotoxins, neurotoxins, lipopolysaccharide endotoxins, and cytotoxins. The most common hepatotoxins are microcystins, a family of cyclic heptapeptides of which more that 50 different variants have been chemically characterised. In Danish coastal waters, the hepatotoxic pentapeptide nodularin is produced by Nodularia spumigena. Microcystins and nodularin exert their toxic effects by inhibition of protein phosphatases 1 and 2A. In mammals, these toxins target the liver due to an active uptake by the hepatocytes. Once taken up, the inhibition of protein phosphatases leads to cytoskeletal disintegration, cell deformation and liver haemorrhage.

The most common blue-green algal neurotoxin is anatoxin-a. In addition, an anatoxin-a derivative (homoanatoxin-a), paralytic shellfish poisons (PSP), and anatoxin-a(s) have been found in a few species from a small number of localities world wide. More than 20 different PSP-toxins, all derivatives of tetrahydropurine, have been characterised. Though the neurotoxins have different modes of action at the cellular level, they all interfere with the transmission of signals in neurones or across the neuro-muscular junction, leading to muscular paralysis and in severe cases death due to respiratory failure.

The occurrences of blue-green algal endotoxins and cytotoxins are not well studied. The endotoxic lipopolysaccharides are similar to those found in other gram-negative bacteria. These toxins are suspected of having caused influenza-like symptoms in humans in Sweden and Finland.

Toxins from blue-green algae may be detected by unspecific bioassays, specific enzyme-assays, and through chemical analysis. Mouse-bioassay was the first commonly employed test, applicable for the discrimination of neurotoxins from hepatotoxins and toxin quantification. An advantage of the mouse-bioassay is the measurable response to toxic compounds that may be chemically uncharacterised and therefore undetectable using standard chemical toxin analysis. More recently, ELISA (Enzyme-Linked ImmunoSorbent Assay) for microcystins and PSP-toxins, and protein phosphatase inhibition assays for microcystins/nodularin have been developed. Methods for chemical detection and quantification by HPLC (high-performance liquid chromatography) have been improved and the number of commercially available standard has increased, making HPLC useful for routine screening of several blue-green algal toxins. Though HPLC is much more sensitive than the mouse-bioassay, several different runs may be necessary to cover the range of hitherto described toxins.

In Denmark there have been no reports on human intoxication by toxic blue-green algae. However, due to the general lack of awareness of the effects and symptoms of blue-green algal toxicosis, such cases may have been mistaken as bacterial or viral infections. Toxic effects on domestic and wild animals are frequently observed. Several dogs died after drinking water from a coastal water bloom of Nodularia spumigena in 1975. At Lake Arresø, miscarriages in cattle in 1984 were connected to a bloom of blue-green algae, and since 1981 fish-, bird-, and dog-kills have recurred in Lake Knud Sø during blooms of Anabaena flos-aquae/lemmermannii.

The present study represents the first survey on the distribution of, and the toxins produced by, toxic blue-green algae in Danish fresh waters. In 1994, a total of 102 phytoplankton net-samples from 96 lakes in Jutland, Funen, and Zealand were screened for toxicity by the mouse-bioassay and, in addition, all samples were analysed by HPLC for the presence of microcystins, anatoxin-a, and PSP-toxins. Samples from 89 lakes (93% of the localities) were toxic; 59 (61%) hepatotoxic, 14 (15%) neurotoxic, and samples from 21 lakes (22%) gave a protracted toxic response in the mouse-bioassay. The toxin(s) causing the protracted toxic response is unknown. More than one type of toxin/toxicity was found in 13% of the samples. No regional differences in toxicity of samples were found between the three geographical areas. Most hepatotoxic samples were dominated by Microcystis, Planktothrix, Anabaena, or Aphanizomenon, and Anabaena lemmermannii was the most common species in neurotoxic samples.

Detected total microcystin concentrations ranged from 10 to 1695 µg microcystin (per g dry weight). Eighty percent of the hepatotoxic samples contained < 500 µg microcystin (per g dry weight). The world-wide most common blue-green algal neurotoxin, anatoxin-a, was not detected in any samples, whereas PSP-toxins were detected in 11 localities (11% of the samples) with toxin contents ranging from 4 to 160 µg saxitoxin (STX) equivalents (per g dry weight). Eighty-two percent of these samples contained < 40 µg STX equivalents (per g dry weight). PSP-toxins in blue-green algae have previously been reported from North America and Australia. Samples from three lakes showed a neurotoxic response in the mouse-bioassay but neither anatoxin-a nor PSP-toxins were detected by HPLC. Salivation and lachrymation in some test mice indicated the presence of anatoxin-a(s) which has subsequently been confirmed by chemical analysis. Anatoxin-a(s) has previously been found only in a few localities in North America.

Spatial and temporal variations in toxicity were observed. The seasonal variation in species composition, abundance, and microcystin content was examined during May-September 1995 in Lake Bryrup Langsø that is used for recreational purposes. Blue-green algal biomass, blue-green algal contribution to the total phytoplankton biomass, and microcystin concentrations increased throughout the summer. Maxima of 3.5 mm3 per litre (46% of the total phytoplankton biomass) and 737 µg microcystin (per g dry weight) were found on 23 August. The microcystin content increased five-fold within two weeks (9-23 August). The dominant species were Microcystis aeruginosa and Woronichinia naegeliana.

During a similar seasonal study in Lake Ravn Sø from 8 June to 16 August 1995, no toxins could be detected by mouse-bioassays and HPLC. Blue-green algae, dominated by Anabaena lemmermannii and Woronichinia naegeliana, constituted < 13% of the total phytoplankton biomass during the study period. In 1994, Anabaena lemmermannii was the dominant species in an anatoxin-a(s) containing neurotoxic sample collected in Lake Ravn Sø.

Surface water serves as a major drinking water resource world wide, and contamination by blue-green algae has on several occasions in different parts of the world been linked to outbreaks of gastro-enteritis. In China, a high incidence of primary liver cancer in populations drinking water from ditches or rivers has been ascribed to the presence of microcystin producing blue-green algae in these water supplies. In Denmark, drinking water mainly originates from groundwater. Only two waterworks (Sjælsø waterwork at Lake Sjælsø and Regnemark at Lake Haraldsted Sø) in the vicinity of Copenhagen supplement groundwater with surface water. The contribution of surface water to the final drinking water never exceeds 50%, and on an annual basis approximately 4-8% (during 1995-1997) of the water supplied is surface water.

Blue-green algae are common in Lake Sjælsø and Lake Haraldsted Sø supplying surface water to the greater Copenhagen area, and microcystin production was verified from net-samples collected in these lakes during summer-autumn 1995. Examination of the cleaning processes in the water works showed retention of particulate matter, and thereby intracellular toxins, by filtration and sedimentation. Efficiency of subsequent chlorination, as employed by the Regnemark water work, in toxin removal could not be established since no dissolved toxins were detected in the lake water taken in. Australian laboratory experiments have shown removal of 95-100% of the dissolved microcystins by the chlorination process in use. At Sjælsø waterwork, pilot-scale experiments revealed 100% removal of dissolved microcystins by the used ozone treatment and subsequent activated carbon filtering at initial concentration of 47 µg microcystin per litre. Toxins were not detected in the treated water from the two water works.

Blooms of blue-green algae are most common in summer and autumn. Environmental data from more than 300 Danish lakes show a general proliferation of blue-green algae at high water temperatures. In addition, total phosphorous (TP) concentration is a key parameter with a strong correlation between TP and blue-green algal abundance as well as contribution to the total phytoplankton biomass. Shallow lakes are characterised by blue-green algal dominance at summer-mean TP concentrations between 0.1 and 0.5-1.0 mg P per litre, whereas blue-green algae dominate in deep lakes with summer-mean TP concentrations > 0.01 mg P per litre. The effects of nitrogen concentrations and wind on blue-green algal abundance are ambiguous and less important than the effects of phosphorous and water temperature.

Further studies on the different distribution patterns of potentially toxic species and genera may elucidate the ecology of blue-green algae in general and the environmental and biological conditions favouring toxic bloom formations in particular. It should be considered whether the existing monitoring program is sufficient in lakes used for drinking water or recreational purposes, or whether necessary to implement separate monitoring programs with emphasis on toxic blue-green algae.