Saxitoxin-group neurotoxins (paralytic shellfish poisons) have been identified in a cultured strain of Anabaena circinalis and in natural bloom samples in which this species was the dominant organism collected from widely distributed sites in the Murray-Darling Basin of Australia. These toxins have hitherto been isolated almost exclusively from 'red tide' dinoflagellates and contaminated shellfish. Two 'aphantoxins', which appear to be identical to two of the paralytic shellfish poisons, have been identified in a cyanobacterium from a small number of sites in New Hampshire, USA. The conclusions are supported by electrophysiological studies and by high-performance liquid chromatographic (HPLC) and fast atom bombardment-mass spectrometric (FAB-MS) analyses.
Surveys of cyanobacterial blooms were made over four consecutive summer seasons (1990-93) in surface waters of the Murray-Darling Basin in south-eastern Australia to determine the incidence and geographic distribution of toxicity associated with a range of recognized taxa.<P. In all, 231 field samples and 143 cultured isolates, representing 13 genera, were tested for toxicity by intra-peritoneal mouse bioassay. Toxicity was recorded in 42% of all field samples and was expressed quantitatively on the basis of both dry weight and cell number. Anabaena was the most abundant genus in blooms occurring in riverine and wetland habitats, and Anabaena circinalis was prominent in all field samples that were neurotoxic. Neurotoxicity was not demonstrated in any other species of Anabaena, or in any other genus, in both field and cultured material. Assays for anatoxin-a were negative, and symptoms of neurotoxicity in mice were not consistent with those reported elsewhere for anatoxin-a(s). Hepatotoxic blooms occurred predominantly in standing waters and were invariably caused by Microcystis aeruginosa f. aeruginosa throughout the greater part of the Basin. Toxic blooms of Nodularia spumigena were recorded only in Lake Aiexandrina and Lake Albert at the mouth of the River Murray. Hepatotoxicity was also demonstrated in strains of Cylindrospermopsis raciborskii, which was previously recognized in Australia only as a subtropical cyanobacterium.
The occurrence of a severe cyanobacterial bloom is described. This bloom affected almost 1000 km of the Barwon-Darling River, New South Wales, Australia, in November and December 1991 and was dominated by Anabaena circinalis Rabenhorst. This cyanobacterium was present in concentrations of around half a million cells per millilitre at some localities during its peak in mid November. Moderate to very high toxicity was demonstrated by mouse bioassay at many localities during this time. The bloom was attributed to very low flow conditions and high nutrient concentrations, especially of total phosphorus. However, warm water temperatures, elevated pH, reduced turbidity, and improved water transparency would also have been contributing factors. Very high ammonia concentrations were also observed during the bloom. The bloom declined during December and was eventually flushed from the river by increased flows following heavy catchment rainfall between mid December and early January.
Toxic freshwater cyanobacteria can contaminate water supplies and adversely effect humans, agricultural livestock, and wildlife. Toxicity is strain-specific so morphological observations alone cannot predict the hazard level. Two microtiter plate based bioassays have emerged for measuring saxitoxin (STX) and its derivatives, commonly found in the freshwater cyanobacteria Anabaena and Aphanizomenon. They use radioactively labeled STX binding by sodium channels, STX's pharmacological target, or an unrelated protein, saxiphilin. These bioassays were challenged with extracts of toxic and nontoxic strains of Anabaena circinalis, and the results were compared with HPLC analysis. Both radioreceptor assays had detection limits of 2 microg STX equivalents (STXeq)/L, which is belowthe concentration proposed for a health alert, namely 3 microg STXeq/L. In all cases, statistically significant correlations existed between all toxicity measurements of the same extracts with the methods used herein. Sodium channel and saxiphilin assays however predicted less toxicity relative to HPLC analysis. The only exception to this was the equivalency observed between saxiphilin measurement and HPLC quantitation corrected for mammalian toxicity. Saxiphilin assay predicted toxicity in one strain was 3 orders of magnitude more than by sodium channel assay, and no STX was detected by HPLC. Lack of acetylcholinesterase inhibition showed this bioactivity was not anatoxin-a(S), a toxin also produced by this A. circinalis with some resemblance to the region of STX bound by saxiphilin. Presence of anatoxin-a(S) was predicted for another strain by this same acetylcholinesterase assay that, if confirmed by chemical analysis, would be the first report of anatoxin-a(S) in an Australian cyanobacterium.
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