**2. Cyanotoxins and their producers**

expansion included increased nutrient inputs, transport of cells or cysts via anthropogenic activities and/or migratory birds, increased aquaculture production and/or overfishing, altering food webs and permitting harmful species to dominate algal communities [1, 2]. It has also been shown that an increase in surface water temperatures and CO2 concentrations due to changing global climate could play a role in the proliferation of cyanobacterial blooms [3–6] and may also affect the strain composition within a cyanobacterial community and consequently change the concentration of cyanotoxins, such as microcystins [7, 8]. The problems associated with cyanobacterial blooms in fresh waters are diverse, from the environment asphyxiation due to excessive consumption of oxygen to purely aesthetic problems in recreational areas when the blooms are a colourful and often smelly scum on the surface of the water [9]. To these problems possibly affecting the economic development of specific areas, productions of cyanotoxins as secondary metabolites can represent a human and animal health threat [10]. Humans can be indeed exposed to cyanotoxins through both direct routes, including contamination of drinking and recreational waters, and indirect routes, including food supplements made from cyanobacteria or through consumption of contaminated food after toxin accumulation in fish, shellfish and other aquatic organisms, as well as in vegetables after using contaminated water for irrigation [11]. In the case of use of surface waters contaminated by cyanotoxins for the supply of drinking water, the potential health risks are managed at the level of the treatment station. In general, a strengthening of clusters of treatment and a complete operation and correct of this station would avoid any risk of contamination of the drinking water [12–20]. By contrast, the raw water used in irrigation often comes from a natural water body or an artificial pond for agricultural

106 Soil Contamination - Current Consequences and Further Solutions

**Figure 1.** Schematic microcystins fate process in soil-plant systems and their impacts on human and animal health.

Some cyanobacteria species belong essentially to the genera *Microcystis*, *Anabaena*, *Aphanizo‐ menon*, *Planktothrix*, *Oscillatoria*, *Cylindrospermopsis* and less often *Gomphosphaeria*, *Coelosphae‐ rium*, *Gloeotrichia*, *Nodularia* and *Nostoc* are known to biosynthesize a diversity of alkaloid and peptide cyanotoxins that have been suggested to pose threats to human and environmental health worldwide [9, 21–24]. These cyanotoxins are essentially endotoxins that can be released in the environment following a cellular lyse during the senescence phase [25] or following treatment of cyanobacterial blooms with algaecides [26]. They can be classified into four families according to the organs on which they act: hepatotoxins (liver), neurotoxins (nervous system), cytotoxins (liver and kidneys) and dermatotoxins (irritant toxins). Hepatotoxins are divided into two groups: microcystins, cyclic heptapeptide hepatotoxins (MW 900–1200), that are regarded as the most frequently occurring and widespread of the cyanotoxins with more than 100 MC variants already reported [27, 28] and nodularins (MW 800–900) composed of five amino acids with only nine different natural analogues have been characterized [29–32]. Both microcystins and nodularins are water-soluble molecules and their cyclic structure provides them a high chemical stability [22]. Their toxicity resulted on a potent and specific inhibition of serine/threonine protein phosphatases [33]. They have also known to induce oxidative stress [34]. Cyanobacterial neurotoxins (for review, see Ref. [35]) are divided into three groups: anatoxins that are neuromuscular junction blocking agents [35], saxitoxins that block nerve cell voltage-gated sodium channels [36] and the unusual non-protein neurotoxic amino acid L-beta-*N*-methylamino-L-alanine (BMAA) that has been associated to the neurological disorder amyotrophic lateral sclerosis/Parkinsonium dementia complex (ALS/PDC) among the indigenous Chamorro people of Guam and other Marianas islands [37]. Its neurotoxicity may be mediated via glutamate regulation [38]. Anatoxins and the BMAA are specific of cyanobacteria, while saxitoxins are also synthesized by some marine dinoflagellates and associated with the human disease paralytic shellfish poisoning or PSP [39]. Cytotoxins are represented by the hydrophilic alkaloid cytotoxin, cylindrospermopsin (MW 415), that has been first isolated from the filamentous *Cylindrospermopsis raciborskii* [21], and further from other species *Aphanizomenon ovalisporum* [40, 41], *Anabaena bergii* [42], *Umezakia natans* [43] and *Raphidiopsis curvata* [44]. It inhibits the synthesis of protein, resulting in a wide spread necrosis of the tissues of many organs such as liver and kidneys [45–47]. Two structural variants of cylindrospermopsin (7-epicylindrospermopsin and deoxycylindrospermopsin) have been characterized so far from bloom samples and isolated strains of cyanobacteria [41, 44, 48]. The dermatotoxins, irritant toxins such as lipopolysaccharides (LPS) commonly known endotoxins, are major components of the cell wall in most Gram-negative bacteria including cyanobacteria. They can elicit irritant and allergenic responses in human and animal tissues with contact [49–51].

#### **3. Sources and occurrence of cyanotoxins in the soil**

The main source of contamination of soils by cyanotoxins is by using cyanotoxin-contamined water for agricultural purposes. Among the cyanobacterial toxins, microcystins are the most widespread group with microcystin-LR (MC-LR) the more toxic and the main congener detected in freshwaters [10]. Recently, concerns are also focused in the increasing occurrence of the cytotoxic cylindrospermopsin in temperate areas [52]. However, cyanobacterial neurotoxins are less reported in the literature and studies regarding their effects on organisms of soils and plants are relatively scarce. The concentrations of microcystins in the surface water are generally comprised between 1 and 100 µg L−1 [10] and the use of this microcystincontaminated water for agricultural purposes has already been reported in several countries such as Morocco [53], Finland [54], Spain [55], New Zealand [56], Algeria [57], Australia [58], Tunisia [59], Turkey [60], Saudi Arabia [61], India [62], China [63] and Guatemala [64]. In addition to the contamination of soils by dissolved cyanotoxins and with the strong occurrence of cyanobacterial blooms worldwide, a strong quantity of cyanobacterial biomass (from thousand to million tons) is removed from water and discharged directly into croplands and forest land without another treatment [65]. This alternative represents a possible source of soil pollution with cyanotoxins. Another source of soil contamination by cyanotoxins consists of direct application of cyanobacterial biomass as an organic fertilizer as in China [66, 67]. In fact, since 1970s, the cyanobacteria were known for their interest in rice culture, as a biofertilizer. In wetland rice and wheat crops, free living cyanobacteria allowed nitrogen fixation to supplement soil nitrogen [68–70]. Cyanobacterial and rhizobacterial associations are used with the objective to increase soil fertility and crop yields, but the cyanobacteria and their secondary metabolites represent also interesting properties and can be involved as natural biocide or biocontrol agents (see review in Ref. [71]). In a recent study Han et al. [72], they related the use of algae waste as an organic fertilizer after composting. This process can allow the degradation of 90–95% of the total microcystins containing in cyanobacteria between 1 and 35 days [73, 74]. The microbial degradation of cyanotoxins, during composting, may be due to the diversity of microorganisms present, the conditions of composting and the type of cyanotoxins present in the bloom, as observed for microcystins by Dawson [75] and Kormas and Lymperopoulou [76]. In addition, several studies reported that the presence of cyanotoxins in *biological soil crust* (biocrust) samples in arid soils can be considered as another source of cyanotoxin-contaminated soils [77–79].
