Ecological and molecular investigations of cyanotoxin production

Abstract

Cyanobacteria, broadly classified as oxygenic phototrophs containing chlorophyll-a and accessory pigments, are among the oldest life forms on earth. They may be unicellular, colonial or filamentous, with cell sizes varying from less than 2 μm to 40 μm in diameter. They may live as symbionts with plants and fungi, in the benthos or in the water column. Cyanobacteria have a cosmopolitan distribution and are common in all kinds of habitats, including Antarctic lakes, thermal springs, arid deserts and tropical acidic soils. Most commonly they are known for their existence as planktonic members of the water column in marine and freshwater environments. Cyanobacteria have special adaptations such as; nitrogen fixation, ability to regulate buoyancy, light harvesting pigments, and differentiated cell types for reproduction or resting, allowing them an advantage over many competitors. Anthropogenic factors such as increased nutrient loading in freshwater and coastal environments, have lead to the occurrence of cyanobacterial blooms world wide. While blooms are unpleasant both visually and due to released odour and taste factors, certain metabolites produced by some cyanobacterial species pose a more serious problem [1]. Due to their adverse effects on higher organisms, these compounds have been labelled as ‘toxins’ and from here on are referred to as such. The chemical structures of many cyanotoxins and their adverse effect on animals have been elucidated and recently reviewed [2]. However, the actual physiological function and ecological regulation of cyanobacterial toxins remain largely a mystery. Studies on regulation and function of cyanotoxins are interrelated and have focused on their effect on the local ecosystem, as well as the effect of environmental parameters on toxin production. Hypotheses regarding the ecophysiology of the cyanotoxins, stemming from such studies, are discussed in this review. With advances in molecular biology, it has more recently been possible to elucidate the genetics behind the biosynthetic pathways of some of these toxins [3] and begin to investigate the regulation of toxin synthesis on a molecular level [4]. While such studies on cyanotoxin gene regulation are limited, similar genetic systems have been discovered and studied in other bacteria [5]. An understanding of these systems will be helpful in guiding work and ideas in cyanotoxin research. This combination of genetic and ecological investigations, is part of a new era of research, in which ecosystem interactions are understood in terms of their molecular determinants. Cyanobacterial toxins can be classified according to their chemical structures as cyclic peptides (microcystin and nodularin), alkaloids (anatoxin-a, anatoxin-a(s), saxitoxin, cylindrospermopsin, aplysiatoxins, lyngbyatoxin-a) and lipopolysaccharides but are more commonly discussed in terms of their toxicity to animals. While there are several dermatotoxins (lyngbyatoxin and aplysiatoxins) produced primarily by benthic marine cyanobacteria (Table 1), most cyanotoxins are classified as either hepatotoxins or neurotoxins. Cyanotoxins and toxin producing cyanobacterial genera (see text for references to the relevant literature) Cyanotoxins and toxin producing cyanobacterial genera (see text for references to the relevant literature) Hepatotoxins are produced by many genera of cyanobacteria (Table 1). These toxins, which target the liver due to specific binding of the organic anion transport system in hepatocyte cell membranes, have been implicated in the deaths of birds, wild animals, agricultural livestock and fish [6], and have been responsible for human illness and death, reported from India, China, Australia and Brazil. Microcystins and nodularin inhibit eukaryotic protein phosphatases type 1 and type 2A resulting in excessive phosphorylation of cytoskeletal filaments, ultimately leading to liver failure [6]. They are cyclic hepta- and pentapeptides, respectively, both containing the same unusual C20 amino acid; 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid, abbreviated Adda [7]. Microcystins, cyclo (-D-Ala-X-D-MeAsp-Z-Adda-D-Glu-Mdha-), with a molecular mass (Mw) between 900 and 1100 Da are the largest and most diverse group of the cyanotoxins (Fig. 1). Over 60 different chemical forms, mostly variable in l-amino acids, have been reported for microcystin and are summarised elsewhere [2]. Although similar in structure, only four nodularins have been identified from Nodularia spumigena [7] and a fifth (trivially named motuporin) isolated from a sponge in Papua New Guinea, which is probably produced by a microbial symbiont [8] (Fig. 1). Microcystin, and tentatively nodularin, is the only cyanotoxin for which the biosynthetic pathway and gene cluster has been identified [3]. Chemical structures of the most common cyanobacterial hepatotoxins (a,b,c) and neurotoxins (d,e,f). A third hepatotoxin, cylindrospermopsin (C15H21N5O7S), belongs to a new