**1.1. Environmental toxicity of selenium and tellurium compounds**

due to their intermediate chemical–physical properties between metal and non-metals [1]. Te estimated average amount in the environment is around 0.027 ppm [2], while Se is unevenly distributed on the Earth's surface with a concentration ranging from 0.01 to 1200 ppm [3, 4]. These elements can be found in natural rocks and ores, soils, sediments or in association with

, sylvanite AgAuTe4

2−), Tellurate (TeO4

of reactive oxygen species (ROS), such as hydrogen peroxide (H2

(O2−) [17], therefore causing cell death [18–20]. An additional target of TeO3

[4–6]. Moreover, Se is an essential micronutrient for living systems, being part of the structure of several important enzymes, (i.e., glutathione peroxidases and thioredoxin reductases), as the 21st amino acid seleno-cysteine, in at least 25 human selenoproteins [7], while, to date, any biological function has been ascribed to Te [8]. Both these chalcogens exist in four different valence states in the environment (i.e., +VI, +IV, 0 and –II), and among them the oxyanion

The wide spread use of Se- and Te-compounds by anthropogenic activities related to oil refining, phosphate and metal ore mining, electronics and industrial glasses, have led to an increase in the presence of these chemicals in the environment [6, 11]. In this regard, although Se is an essential micronutrient, it is toxic at concentrations higher than the human dietary requirement (25–30 μg day−1) [10], while the toxicity exerted by Te is even more dramatic, negatively affecting both prokaryotes and eukaryotes at concentration as low as 1 μg mL−1 [6]. Particularly, Se- and Te- oxyanions are recognized as harsh toxicants of public health and environmental concern due to their association with oxygen, which makes them highly bioavailable, enabling the mobilization of Se- and Te-compounds through water and soil [12, 13]. On the contrary, Se and Te organic forms (e.g., dimethyl selenide, trimethyl selenonium, selenomethionine, selenocysteine,

Se-methilselenocysteine, dimethyl telluride), as well as their zero-valence states (Se0

showed lower toxicity levels [2, 12, 14]. Considering the shared physical–chemical features of Se and Te, the suggested mechanism of toxicity exerted by the chalcogen-oxyanions is based on their interaction with glutathione molecules (GSHs) and related molecules, which are likely responsible for their reduction [8, 13, 15]. This bioconversion mechanism leads to the generation

of the heme metabolism in *E. coli* K-12 cells, by which this oxyanion is responsible for the accumulation of the heme precursor protoporphyrin IX, causing iron depletion and, subsequently,

Despite the toxic effects of Se- and Te-oxyanions, in the last 20 years several microorganisms able to sequester, bioconvert or biomethylate these chalcogen-ions have been isolated from extreme environments, such as ocean hydrothermal vents and the highly alkaline water Monolake (California), to name a few [22]. Mainly anaerobic or facultative-anaerobic bacteria capable of growing phototrophycally or chemotrophycally under oxic and anoxic conditions have been described for their metabolic potential in bioconverting these species, while much less is known about strictly aerobic microorganisms [23]. In this regard, anaerobic microorganisms have been described for their use of chalcogen-oxyanions as terminal electron acceptors to sustain their growth [19, 22, 24–29]. Although the exact biochemical mechanisms behind Se and Te metabolism and bioconversion in these microorganisms have not been fully elucidated,

2−), Selenite (SeO3

, crooksite CuTlSe, calusthalite PbSe)

2−) and Tellurite (TeO3

O2

2−) are the

and Te0

) [16] or superoxide ions

2− is the impairment

)

rare minerals (e.g., calaverite AuTe2

most abundant in soils and waters [9, 10].

forms of Selenate (SeO4

118 Biosorption

cell death [21].

Annually, the total average amount of either Se or Te produced worldwide is 2500–2800 or 220 tons, respectively, with USA, Japan, Russia, Canada, Germany, Belgium and Sweden as main manufacturers [7, 30]. The accumulation of Se- or Te-compounds in the environment mainly relies on their anthropogenic use in several application fields, causing therefore their emission in the atmosphere [31, 32]. Se-accumulation derives from metallurgic industries, glass manufactures, pigments production, electronics and agriculture applications [33], while Te-containing compounds are used in copper refining [19], tarnishing metals [34], vulcanization of rubber [8], production of color glass or ceramics [19], photovoltaic cells and solar panels [8], as well as catalysis of several reactions [19]. Recently, the possibility to develop new Te-based nanomaterials such as fluorescent quantum dots (QDs) has been extensively investigated to create new high-tech probes in biological detection [8, 35], exasperating the already dramatic waste disposal circumstances.

Among the different Se-species present in the environment, the inorganic forms of Se2−, SeO4 2−, or SeO3 2− are generally found in surface and ground waters as pollutants [36], while the organic and volatile ones (i.e., methylselenides, trimethylselenonium ions and selenoamino acids) occur in air and soils [37]. Similarly, Te-compounds result to be highly concentrated either in soils [38, 39] or waters [34] mainly in the form of TeO4 2− and TeO3 2−, being the latter highly soluble and toxic [35, 40, 41].

The presence of Se- and Te-compounds in water reservoirs has become a problem for both human health and ecological wildlife [42–45], which led to the development of several strategies aimed to protect aquatic and human life [46], as Se-poisoning events have occurred in the last few years worldwide, such as in the Kesterson Wildlife reservoir (California) [47], the uranium mine in Saskatchewan (Canada), and the Lake Sutton (USA) [48], causing physical deformities and mutations [46]. The major areas of the world affected by water contamination due to the presence of SeO4 2− and SeO3 2− are North America, Australia and New Zealand [23], while higher level of Te-oxyanions has been detected in the surface waters of Te-contaminated basins in Angola and Panama as compared to the deep ones, indicating a difference in behavior between Te and Se, which, as nutrient, is usually highly concentrated in the deep ocean [49]. Finally, Te-compounds emission in the atmosphere is now investigating, even if the implication related to the presence of Te-species in the air has not been established yet [19].
