**2.3. Bioremediation of chalcogen-polluted environments based on bacterial biofilms**

The majority of the investigations regarding the bioremediation of Se- and Te-contaminated environments have been focused on the exploitation of bacterial species grown as free planktonic cells [8]. However, in natural settings microorganisms are most often found in close association with surfaces and interfaces as complex communities, which are indicated as biofilms [114–116]. In bacterial biofilms, the cells are embedded and protected from the surrounding environments by the presence of a matrix defined as Extracellular Polymeric Substance, containing a high amount of water, polysaccharides, proteins, extracellular-DNA (e-DNA) and lipids [117, 118]. The communal life of bacterial cells in the form of biofilm offers them several advantages [114, 117, 119], resulting in their innate ability to populate a vast array of environments [119], including those contaminated by chalcogen-oxyanions. Thus, peculiar features of bacterial biofilms (i.e., quorum sensing signaling process, different cellular physiology, presence of the EPS and colony morphology variants) [120–124] confer them tolerance and/or resistance toward either Se- or Te-oxyanions without having specific Se- and Te- genetic resistant determinants [19]. In this regard, sulfate-reducing bacteria (SRB) within a biofilm produce sulfide (S<sup>2</sup> ), which can abiotically bioconvert SeO4 2− and/or SeO3 2−, leading to the precipitation of Se0 in the EPS [86]. Unlike SRB, *S. oneidensis* biofilms grown under anaerobic conditions can reduce TeO<sup>3</sup> 2− and SeO3 2−, accumulating Te0 and Se0 in both the cells and the EPS, respectively [125].

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 bioremediation, resulting in the uptake and bioconversion of SeO4 2− to Se0 by the bacterial cells. Similarly, biofilms-containing denitrifying and sulfate-reducing microorganisms grown on a hallow-membrane biofilm reactor have been successfully used to remove SeO<sup>4</sup> 2− from wastewater [129, 130], while the pre-grown biofilm of the SRB *Desulfomicrobium norvegicum* resulted able to abiotically reduce SeO3 2− extracellularly through its production of S-Se granules within the EPS [86]. Further, biofilm formed by TeO<sup>3</sup> 2−-resistant isolates of non-sulfur marine photosynthetic bacteria showed their proficiency in bioconverting this Te-oxyanion through intracellular reduction [131].
