**2.2. Bioremediation of Te-polluted environments using bacterial pure cultures as planktonic cells**

Although Te does not have an essential biological role for living organisms [8], bacterial cells are able to uptake Te-oxyanions and to biomethylate and/or bioconvert them either as a decontamination strategy or during the anaerobic respiration [8, 19]. Particularly, TeO3 2− uptake within bacterial cells has been ascribed to the phosphate transporter in *E. coli* [97], *Lactococcus lactis* [98] and *R. capsulatus* [99, 100], considering that this Te-species is a strong competitive inhibitor of the phosphate group [19]. However, other carriers can be used to assist TeO3 2− uptake in microorganisms, such as the ActP monocarboxylate transporter of *R. capsulatus* [101], as well as an ATP-dependent efflux pump responsible for the arsenite/arsenate/antimonite resistance in *E. coli* [102]. Since Te shares several chemical properties with Se, microorganisms tolerant and/or resistant toward Te-oxyanions process them exploiting similar mechanisms to those described above for Se-species. In this regard, the biomethylation of Te-oxyanions to produce dimethyl telluride and dimethyl ditelluride [56] has been observed in several bacteria able to biomethylate Se-oxyanions as well, such as *R. rubrum* G9, *R. capsulatus* [59], *P. fluorescens* K27 [103] and *D. gigas* [57]. Moreover, *P. aeruginosa* ML4262 [104], *G. stearothermophilus* V [105] and *Mycobacterium tuberculosis* [106] showed their capability of biomethylating only Te-oxyanions.

can abiotically bioconvert SeO4

2−, accumulating Te0

able to abiotically reduce SeO3

**contaminated wastewaters**

cellular reduction [131].

**3.1. Microbial consortia**

will be discussed.

the EPS [86]. Further, biofilm formed by TeO<sup>3</sup>

SeO3

2− and/or SeO3

hallow-membrane biofilm reactor have been successfully used to remove SeO<sup>4</sup>

and Se0

bioremediation, resulting in the uptake and bioconversion of SeO4

[86]. Unlike SRB, *S. oneidensis* biofilms grown under anaerobic conditions can reduce TeO<sup>3</sup>

Since microorganisms grown as biofilms showed to play an important role in metal and chalcogen geochemistry [126], several biofilm-based reactors have been used to support the biosorption and the bioconversion of Se- and Te-oxyanions as detoxification strategy [8]. Indeed, *Burkholderia cepacia* biofilm grown on alumina surface [127], as well as a mixed species biofilm composed of *Dechloromonas* sp. and *Thauera* sp. [128] have been explored for Se-oxyanions

Similarly, biofilms-containing denitrifying and sulfate-reducing microorganisms grown on a

water [129, 130], while the pre-grown biofilm of the SRB *Desulfomicrobium norvegicum* resulted

synthetic bacteria showed their proficiency in bioconverting this Te-oxyanion through intra-

In the environment, microorganisms usually thrive as communities composed by multiple species, generally referred as microbial consortia [132]. The employment of these microbial consortia in the treatment of environmental matrices contaminated with different inorganic or organic pollutants is currently a field of great interest for researchers [133]. There are significant advantages for the utilization of microbial consortia over pure cultures, such as the larger volumes of wastewaters treatable, the ability of microbial communities to adapt to diverse conditions, the presence of synergic interactions among members within the consortium and the possibility to work in non-aseptic conditions [23]. This last aspect is particularly significant, since it facilitates process control and it reduces both maintenance and operational costs [134]. In the following section, the different biological systems based on processes of biosorption and bioconversion of Se- and Te-oxyanions from contaminated matrices by using microbial consortia

In recent years, the utilization of biological treatments based on the exploitation of microbial consortia has become the leading approach for the removal of toxic Se-species from environmental matrices, particularly from wastewaters (i.e., mine runoff, agricultural drainage, and flue gas desulfurization wastewater from plants) [23]. This decontamination strategy has

**3. Microbial consortia for the treatment of selenium and tellurium** 

**3.2. Microbial consortia for Se-removal from contaminated environments**

2−, leading to the precipitation of Se0

Microbial-Based Bioremediation of Selenium and Tellurium Compounds

http://dx.doi.org/10.5772/intechopen.72096

2− to Se0

2−-resistant isolates of non-sulfur marine photo-

2− extracellularly through its production of S-Se granules within

in both the cells and the EPS, respectively [125].

in the EPS

by the bacterial cells.

2− from waste-

2− and

123

Despite of TeO3 2− presence in lower amount in the environment compared to TeO4 2− [39], tellurite showed toxicity 10 times higher than tellurate [40, 41], leading the experimental research to focus on the study of TeO3 2−-tolerant/resistant microorganisms as ideal candidate for bioremediation purposes. Nevertheless, *B. beveridgei* [22], *B. selenitireducens*, *S. barnesii* [29] and *Shewanella frigidimarina* ER-Te-48 [28, 107] showed their ability under anaerobic growth conditions to use both TeO4 2− and TeO3 2− oxyanions as terminal electron acceptors in the respiratory chain to sustain their growth [8]. To date, the proposed mechanisms of Te-oxyanions bioconversion in microorganisms are similar to those described for Se-species [13, 56, 88, 104, 108]. Further, TeO3 2− processing in microorganisms have been ascribed to enzymatic reductions by periplasmic or cytoplasmic oxidoreductases [107, 109], such as nitrate reductases [109, 110], catalases [111] and thiol:disulfide oxidoreductase [112]. However, the function of all these enzymes for bioconverting Te-oxyanions appears to be not specific, leading to a low resistance level toward Te-species in these microorganisms. To date, only one specific TeO<sup>3</sup> 2− reductase has been identified as responsible for the anaerobic respiration of this Te-oxyanion in *Bacillus* sp. GT-83 [113].
