Freshwater cyanobacteria

Strongly hepatotoxic cyclic heptapeptides produced by some species of freshwater cyanobacteria (blue-green algae) (28). These microcystins represent a health risk to humans through drinking water, since they have been found to act as tumor promoters (29). Several chromatographic analytical procedures for microcystins have been... 

Acute poisoning of humans by freshwater cyanobacteria as occurs with paralytic shellfish poisoning, while reported, has never been confirmed. Humans are probably just as susceptible as pets, livestock, or wildlife but people naturally avoid contact with heavy waterblooms of cyanobacteria. In addition, there are no known vectors, like shellfish, to concentrate toxins from cyanobacteria into the human food chain. Susceptibility of humans to cyanobacteria toxins is supported mostly by indirect evidence. In many of these cases, however, if a more thorough epidemiological study had been possible these cases probably would have shown direct evidence for toxicity. 

Cyanobacterial neurotoxins are small ringed alkaloids and have dramatic effects on various components of vertebrate neurons. They are all water soluble and are synthesized by several cyanobacterial genera (Table 5.1). The most commonly isolated neurotoxins are the paralytic shellfish toxins, although several other potent neurotoxic alkaloids are synthesized by freshwater cyanobacteria (Table 5.1). 

Freshwater cyanobacteria blooms implicated in human and livestock intoxications have been extensively studied. And they have become a serious health problem in recent years. Blooms of marine cyanobacteria are also becoming an increasingly familiar occurrence within the tropical and subtropical regions of the world. Several systematic surveys within Europe and the USA have concluded that the two most commonly isolated groups of cyanotoxins are the alkaloids-neurotoxins and the cyclic peptide hepato-toxins, both of which are destructive to liver cells. Several genera... 

Peptide toxins. Of all the toxins produced by freshwater cyanobacteria, the peptide toxins of Microcystis aeruginosa have received the most attention. All research on these peptide toxins indicates they are small, possibly cyclic, with molecular weights estimated at 1200 to 2600 (10,11). Recent work has become more definitive in the estimation of molecular weight and amino acid composition. In 1978 Elleman et al. (12) reported that they had isolated and characterized the peptide toxin of an Australian strain Microcystis which was a pentapeptide with a minimum molecular weight of 654. It consisted of equimolar amounts of alanine, tyrosine, methionine, glutamic and 3-methyl aspartic acid and methylamine. 

Figure 1. Structure of anatoxin-a from Anabaena flos-aquae NRC-44-1, saxitoxin and neosaxitoxin. Saxitoxin and neosaxitoxin is produced by certain species of marine algae and by the freshwater cyanobacteria Aphanizomenon flos-aquae NH-5.

MAAs have not been studied as extensively in freshwater as in marine species, but these compounds have been reported in freshwater cyanobacteria, microalgae, invertebrates, and chordates (fish).106 116 130-133 There may be several MAAs unique to freshwater organisms. Aqueous extracts from the cyanobacterium Nostoc commune contain a mixture of UV-absorbing compounds with two distinct chromophores that have maximum absorbance at 312 and 335 nm.134 135 These compounds are comprised of the usual mycosporine cyclohexenone ring structure however, it is... 

Tab. 1.5.2. Natural /5-amino acids related to marine and freshwater cyanobacteria.

Saxitoxin (32) is listed in Schedule 1 of the CWC. It is a polar, cationic, relatively low molecular mass toxin and is one of 18 structurally related neurotoxins collectively known as paralytic shellfish poisoning (PSP) toxins. Analogues are formed by addition of sulfate, A-sulfo and A-hydroxyl groups, and by decarbamylation. They block neuronal sodium channels, and thereby neurotransmission, death resulting from respiratory paralysis. Saxitoxin is produced by dinoflagellate species (and by some freshwater cyanobacteria), and accumulates in shellfish. The cationic nature of saxitoxin makes capillary electrophoresis combined with... 

The anatoxins are neurotoxins produced by freshwater cyanobacteria that have caused periodic poisonings of wildlife, livestock, fowl, and fish in several countries. 

Anatoxin-a is a naturally occurring homotropane alkaloid produced by freshwater cyanobacteria of the genera Anabama (A. flos-ctquae and A. circinalis), Aphanizome-non, Cylindrospermum, Planktothrix, Microcystis aeruginosa [5-7], and Phormidium favosum [8]. Fatal intoxications have typically included cattle and birds [9], and, more recently, dogs [8] and flamingos [10]. 

Homoanatoxin-a, obtained from various freshwater cyanobacteria, is a relatively rare natural analog of anatoxin-a for which the C-11 side chain is extended by one methylene unit (Figure 6.4). It was originally isolated from Planktothrix sp. (formerly Oscillatoria) in 1992 [51]. It has recently been isolated from Raphidiopsis mediterranea Skuja from Japan [52] and Planktothrix (formerly Oscillatoria) formosa blooms in Ireland [53]. 

Carmichael, W., and Mahamood, N. 1984. Toxins from freshwater cyanobacteria. In Seafood Toxins ACS Symposium Series 262. Washington, DC American Chemical Society, 377. 

Hamill, K.D. 2001. Toxicity in benthic freshwater cyanobacteria (blue-green algae) first observations in New Zealand. New Zealand J Mar Freshwat Res 35, 1057-1059. 

An ELISA test using monoclonal antibodies against microcystin-LR has been used by Ueno (1996b) to analyze the microcy stin concentration in environmental samples from ponds, lakes, reservoirs, and rivers in Japan, Thailand, Germany, and Portugal. Although microcystins are mainly associated with freshwater cyanobacteria, these results are relevant to the analysis of microcystins and particularly nodularins in brackish water environments. 

The neurotoxins known to be produced by freshwater cyanobacteria include anatoxin-a and homoanatoxin-a, ana-toxin-a(s), and saxitoxins. Their target is the neuromuscular system, and they can paralyze peripheral, skeletal muscle, including respiratory muscles. Death ensues as a result of respiratory arrest within a few minutes to a few hours (see reviews by Duy et al, 2000 Kuiper-Goodman et al, 1999). 

The phosphono form of phosphatidylethanolamine, 1,2-diacyl- n-glycerol-3-(2 -aminoethyl)phosphonate (Fig. 5), is present in several species of protozoa as well as in bovine tissues and in human aorta. Its alkylacyl and alkenylacyl forms have also been found. From the freshwater cyanobacteria Aphani-zomenon flos-aquae, 2-acyloxyethylphosphonate (with unusual biosurfactant properties) has been isolated. Phosphonolipids have been reviewed in References 36 and 37. 

The best known carbon concentrating mechanism (CCM), is that of freshwater cyanobacteria... 

With various modifications, the CCM of freshwater cyanobacteria is also generally accepted as the underlying scheme for the CCM of marine cyanobacteria and eukaryotic phyto-plankters. In particular, the CCM of the model... 

Krishnamarty, T. Carmichael, W.W. Sarver, E.W. Investigations of freshwater cyanobacteria (blue-green algae) toxic peptides I. Isolation, purification and characterization of peptides from Microcystis aeruginosa and Ana-baena flos-aquae. Toxicon 1986, 24, 865-872. 

Freshwater cyanobacteria Microcystis, Oscillatoria, Ana-baena, and Nostoc produce several types of toxins, among which the most commonly detected are the hepatotoxic peptides microcystins. The general structure of the microcystins is cyclo-(D-Ala -X -D-MeAsp -Z -Adda -D-Glu -Mdha ), in which X and Z represent variable L-amino acids, D-MeAsp is D-eryf/iro-p-methylaspartic acid, Mdha is A-methyldehydroalanine, and Adda is the unusual C20 amino acid, (25,35,85,95)-3-amino-9-meth-oxy-2,6,8-trimethyl-10-phenyldeca-4( ),6( )-dienoic acid (Fig. 1). The structural differences in the microcystins mainly depend on the variability of the two L-amino acids (denoted X and Z), and secondarily on the methylation or demethylation of D-MeAsp and/or Mdha. More than 60 microcystins have been isolated from bloom samples and isolated strains of cyanobacteria. 

Pentecost, A. Spiro, B. (1990) Stable carbon and oxygen isotope composition of calcites associated with modern freshwater cyanobacteria and algae. Geomicro-biological Journal 8, 17-26. 

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