Mechanisms of Hexavalent Chromium Resistance and Removal by Microorganisms

Abstract

Chromium has been and is extensively used worldwide in multiple industrial processes and is routinely discharged to the environment from such processes. Therefore, this heavy metal is a potential threat to the environment and to public health, primarily because it is non-biodegradable and environmentally persistent. Chromium exists in several oxidation states, the most stable of which are trivalent Cr(Ill) and hexavalent Cr(VI) species. Each species possesses its own individual chemical characteristics and produces its own biological effects. For example, Cr (Ill) is an essential oligoelement for humans, whereas Cr(VI) is carcinogenic and mutagenic. Several chemical methods are used to remove Cr(VI) from contaminated sites. Each of these methods has advantages and disadvantages. Currently, bioremediation is often the preferred method to deal with Cr contaminated sites, because it is eco-friendly, cost-effective and is a "natural" technology. Many yeast, bacterial and fungal species have been assessed for their suitability to reduce or remove Cr(VI) contamination. The mechanisms by which these microorganisms resist and reduce Cr(VI) are variable and are species dependent. There are several Cr-resistance mechanisms that are displayed by microorganisms. These include active efflux of Cr compounds, metabolic reduction of Cr(VI) to Cr (ill), and either intercellular or extracellular prec1p1tation. Microbial Cr (VI) removal typically involves three stages: binding of chromium to the cell surface, translocation of chromium into the cell, and reduction of Cr(VI) to Cr (ill). Cr(VI) reduction by microorganisms may proceed on the cell surface, outside the cell, or intracellularly, either directly via chromate reductase enzymes, or indirectly via metabolite reduction of Cr(VI). The uptake of chromium ions is a biphasic process. The primary step is known as biosorption, a metabolic energyindependent process. Thereafter, bioaccumulation occurs, but is much slower, and is dependent on cell metabolic activity. Choosing an appropriate bioremediation strategy for Cr is extremely important and must involve investigating and understanding the key mechanisms that are involved in microbial resistance to and removal of Cr(VI).

Chromium immobilization by extra- and intraradical fungal structures of arbuscular mycorrhizal symbioses

“…Cr(VI) was first bound to the positively charged complexing groups on fungal surfaces, and then reduced to Cr(III) by adjacent electron-donor compounds (such as glutathione, cysteine, amino acid etc. ), or by chromate reductase excreted by the fungi [49][50][51]. In addition, it is essential to point out that a small proportion of Cr might also be absorbed by AMF as our previous study has confirmed the direct uptake and transport of Cr by ERM [20,21].…”

“…6E), which indicates a main strategy for mycorrhizal roots to tolerate Cr(VI). Similar to extraradical mycelium mentioned above, Cr(VI) was possibly reduced by electron-donor compounds or chromate reductase in both fungal structures or plant cells [27,51,61]. Cr(III) is easily bound to phosphate, carboxyl or thiol groups, resulting in lower mobility and toxicity [27].…”

Transformation and Immobilization of Chromium by Arbuscular Mycorrhizal Fungi as Revealed by SEM–EDS, TEM–EDS, and XAFS

“…48 ERM Adsorption, Accumulation, and Active Transport of Cr to Mycorrhizal Roots. Figure 2A shows that the ERM treated with Cr(VI) can accumulate more than 1000 mg kg −1 Cr.…”

“…Just as mentioned above, Cr(VI) was probably reduced to Cr(III) in the medium, in the fungi or on the fungal surface by chromate reductase or compounds that can serve as electron donors (e.g., dissolved organic carbons, cysteine, glutathione, or sulfite). 48 The Cr concentrations in the inactivated hyphae were likely associated with Cr adsorption by the fungal biomass because the inactivated hyphae exhibited a higher adsorption capacity than living hyphae. 30 Similar to previous studies on Serratia spp.…”

Several Functions of Rhizopus sp on Increasing Nutritional Value of Feed Ingredient

“…Rhizopus mampu mengubah Cr VI yang toksik dan mudah mengalami oksidasi menjadi bentuk Cr III yang lebih aman karena lebih stabil (Liu et al 2012). Cr VI berubah menjadi Cr III karena adanya kontak dengan elektron donor pada biomasa Rhizopus (Joutey et al 2015). Pada penambahan Cr dalam diet, sebaiknya dalam bentuk organik karena tidak toksik dan dapat dapat diabsorbsi dengan baik oleh tubuh (25-30%).…”