Diversity and Impact of Prokaryotic Toxins on Aquatic Environments: A Review

Abstract

Microorganisms are ubiquitous in all habitats and are recognized by their metabolic versatility and ability to produce many bioactive compounds, including toxins. Some of the most common toxins present in water are produced by several cyanobacterial species. As a result, their blooms create major threats to animal and human health, tourism, recreation and aquaculture. Quite a few cyanobacterial toxins have been described, including hepatotoxins, neurotoxins, cytotoxins and dermatotoxins. These toxins are secondary metabolites, presenting a vast diversity of structures and variants. Most of cyanobacterial secondary metabolites are peptides or have peptidic substructures and are assumed to be synthesized by non-ribosomal peptide synthesis (NRPS), involving peptide synthetases, or NRPS/PKS, involving peptide synthetases and polyketide synthases hybrid pathways. Besides cyanobacteria, other bacteria associated with aquatic environments are recognized as significant toxin producers, representing important issues in food safety, public health, and human and animal well being. Vibrio species are one of the most representative groups of aquatic toxin producers, commonly associated with seafood-born infections. Some enterotoxins and hemolysins have been identified as fundamental for V. cholerae and V. vulnificus pathogenesis, but there is evidence for the existence of other potential toxins. Campylobacter spp. and Escherichia coli are also water contaminants and are able to produce important toxins after infecting their hosts. Other bacteria associated with aquatic environments are emerging as toxin producers, namely Legionella pneumophila and Aeromonas hydrophila, described as responsible for the synthesis of several exotoxins, enterotoxins and cytotoxins. Furthermore, several Clostridium species can produce potent neurotoxins. Although not considered aquatic microorganisms, they are ubiquitous in the environment and can easily contaminate drinking and irrigation water. Clostridium members are also spore-forming bacteria and can persist in hostile environmental conditions for long periods of time, contributing to their hazard grade. Similarly, Pseudomonas species are widespread in the environment. Since P. aeruginosa is an emergent opportunistic pathogen, its toxins may represent new hazards for humans and animals. This review presents an overview of the diversity of toxins produced by prokaryotic microorganisms associated with aquatic habitats and their impact on environment, life and health of humans and other animals. Moreover, important issues like the availability of these toxins in the environment, contamination sources and pathways, genes involved in their biosynthesis and molecular mechanisms of some representative toxins are also discussed.

Metagenomic Analyses of Drinking Water Receiving Different Disinfection Treatments

“…Cyanobacteria are one of the most important groups of toxin-producing aquatic bacteria. Species within the cyanobacterial genera identified in this study have been reported to produce various secondary metabolites, including microcystins, cytotoxins, and neurotoxins (52). Cyanobacterial populations have also been detected in 16S rRNA gene clone libraries of chlorine-treated DW (37).…”

Risk Assessment of Cyanobacteria and Cyanotoxins, the Particularities and Challenges of Planktothrix spp. Monitoring

“…Cyanotoxins detected and correspondent taxa from which have been isolated, as well as their primary target in mammals. Based on the information from Chorus et al, 2000;Charmichael, 2001;Codd et al, 2005;Stewart et al, 2006;van Apeldoom et al, 2007;Bláha et al, 2009;Valério et al, 2010;Mihali et al, 2009. * -the dose needed to kill 50% of exposed animals.…”

“…Most of cyanobacterial secondary metabolites are alkaloids, or possess peptidic substructures synthesised by NRPS (non-ribosomal peptide synthesis, involving peptide synthetases) or NRPS/PKS (involving peptide synthetases and polyketide synthases) hybrid pathways (Valério et al, 2010).…”

In vitro and in vivo toxicity evaluation of the freshwater cyanobacterium Heteroleiblenia kuetzingii

“…They are characterized by cosmopolitan distribution, species diversity and ability to produce several secondary metabolites with different biological activity. It is suggested that these secondary metabolites determine the high biological adaptability and cosmopolitanism of cyanobacteria and this is a part of their strategy for survival [1].…”