Cyanobacterial Toxins in Water Sources and Their Impacts on Human Health

Introduction

Cyanobacteria are ancient prokaryotic microorganisms that have been in existence for at least 2.7 billion years and the first organisms to have released oxygen into the primitive atmosphere (Zanchett & Oliveira-Filho, 2013). Cyanobacteria were formerly regarded as blue-green algae because of the presence of chlorophyll a and performing oxygen-evolving photosynthesis in a manner very similar to algae. They occupy a broad range of habitats including freshwater, marine, terrestrial and even extreme niches such as hot springs. In aquatic ecosystems, cyanobacteria constitute the most important compartment in the food web, as they are one of the main primary producers of organic matter and serve as a food source for aquatic organisms. However, cyanobacteria become a problem only when they increase to excessive numbers forming water blooms under eutrophication conditions (high nutrient concentrations) along with suitable light and temperature. These blooms can also consume all of the oxygen in the water when they die and decompose, leaving a dead zone (hypoxia) where fish and other animals cannot survive (Diaz & Rosenberg, 2008). The hypoxic dead zone is typically found in the central basin of Lake Erie for decades (Zhu, Michalak, Beletsk, Rao, & Richards, 2015). Additionally, because many species of cyanobacteria are capable of producing potent toxins called cyanotoxins, these blooms are harmful and may lead to many negative environmental impacts such as mortality of domestic and wild animals, and unsustainability of the overall ecosystem. Cyanotoxins vary in their chemical structure and mode of toxicity in human including hepatotoxins, neurotoxins, and irritants and gastrointestinal toxins (Codd, Morrison, & Metcalf, 2005). These cyanotoxins may lead to a wide array of biological impacts on aquatic organisms, including: allelopathic effects on other phytoplankton (Suikkanen, Fistarol, & Granéli, 2004); suppression of zooplankton grazing, leading to reduced growth and reproductive rates and changes in dominance, toxic effects on fish and accumulation of toxins in tissues of invertebrates (Lehtiniemi et al., 2002) and fish (Mohamed, Carmichael, & Hussein, 2003). Furthermore, cyanotoxins are known to affect a number of processes in plants, and their presence in water used for irrigation may have considerable impact on the growth and development of crop plants (Wiegand & Pflugmacher, 2005). The exposure of edible crop and vegetable plants to cyanotoxins is also a concern for human health, as the toxins can accumulate in plant tissues and might be transferred into the food chain (Mohamed & Al-Shehri, 2010). Cyanotoxins are not easily removed by conventional treatment techniques as they are resistant to heat and most physico-chemical factors. Consequently, the presence of such potent toxins in water sources poses a risk to human health upon exposure to them. Additionally, In vitro and in vivo studies conducted by Zhou, Yu, & Chen (2002) and Svircev, Krstic, Miladinov-Mikov, Baltic, & Vidovic (2009) provided evidences that chronic exposures to low concentrations of the cyanotoxins contribute to the increased the risk for primary liver in China and Serbia and colon cancer development (Zegura, Volcic, Lah, & Filipic, 2008), as microcystin is mutagenic tumor promoter and inhibit DNA repair (Zegura, Straser, & Filipic, 2011). This chapter provides information on cyanotoxins and their human health risks in water sources because many water suppliers may have little understanding of how to manage them, particularly in rural communities and developing countries. This chapter covers six parts: section 1 introduces the occurrence of toxin-producing cyanobacteria in different water sources; section 2 presents the environmental factors promoting cyanobacterial blooms; section3 describes the human health impacts of cyanotoxins; section 4 presents the exposure routes to cyanotoxins; section 5 highlights management strategies and guideline levels of cyanobacteria and their cyanotoxins in the water sources; and finally section 6 sheds some light on the sustainability of cyanotoxins in the aquatic environment.