Adaptation to metal(loid)s in strain Mucilaginibacter rubeus P2 involves novel arsenic resistance genes and mechanisms

Arsenic is a ubiquitous environmental toxi substance that affects human health. Compared to inorganic arsenicals, reduced organoarsenicals are more toxic, and some of them are recognized as antibiotics, such as methylarsenite [MAs(III)] and arsinothricin (2-amino-4-(hydroxymethylarsinoyl)butanoate, or AST). To date, organoarsenicals such as MAs(V) and roxarsone [Rox(V)] are still used in agriculture and animal husbandry. How bacteria deal with both inorganic and organoarsenic species is unclear. Recently, we identified an environmental isolate Mucilaginibacter rubeus P2 that has adapted to high arsenic and antinomy levels by triplicating an arsR-mrarsUBact-arsN-arsC-(arsRhp)-hp-acr3-mrme1Bact-mrme2Bactgene cluster. Heterologous expression of mrarsMBact, mrarsUBact, mrme1Bact and mrme2Bact, encoding putative arsenic resistance determinants, in the arsenic hypersensitive strain Escherichia coli AW3110 conferred resistance to As(III), As(V), MAs(III) or Rox(III). Our data suggest that metalloid exposure promotes plasticity in arsenic resistance systems, enhancing host organism adaptation to metalloid stress.

Magnetic Biochar Obtained by Chemical Coprecipitation and Pyrolysis of Corn Cob Residues: Characterization and Methylene Blue Adsorption

Biochar is a carbonaceous and porous material with limited adsorption capacity, which increases by modifying its surface. Many of the biochars modified with magnetic nanoparticles reported previously were obtained in two steps: first, the biomass was pyrolyzed, and then the modification was performed. In this research, a biochar with Fe₃O₄ particles was obtained during the pyrolysis process. Corn cob residues were used to obtain the biochar (i.e., BCM) and the magnetic one (i.e., BCM_Fe). The BCM_Fe biochar was synthesized by a chemical coprecipitation technique prior to the pyrolysis process. The biochars obtained were characterized to determine their physicochemical, surface, and structural properties. The characterization revealed a porous surface with a 1013.52 m²/g area for BCM and 903.67 m²/g for BCM_Fe. The pores were uniformly distributed, as observed in SEM images. BCM_Fe showed Fe₃O₄ particles on the surface with a spherical shape and a uniform distribution. According to FTIR analysis, the functional groups formed on the surface were aliphatic and carbonyl functional groups. Ash content in the biochar was 4.0% in BCM and 8.0% in BCM_Fe; the difference corresponded to the presence of inorganic elements. The TGA showed that BCM lost 93.8 wt% while BCM_Fe was more thermally stable due to the inorganic species on the biochar surface, with a weight loss of 78.6%. Both biochars were tested as adsorbent materials for methylene blue. BCM and BCM_Fe obtained a maximum adsorption capacity (q_m) of 23.17 mg/g and 39.66 mg/g, respectively. The obtained biochars are promising materials for the efficient removal of organic pollutants.

Biocatalyst developed with recovered iron-rich minerals enhances the biotransformation of SARS-CoV-2 antiviral drugs in anaerobic bioreactors

The biotransformation of the SARS-CoV-2 antiviral drugs, ribavirin and tenofovir, was studied in methanogenic bioreactors. The role of iron-rich minerals, recovered from a metallurgic effluent, on the biotransformation process was also assessed. Enrichment of anaerobic sludge with recovered minerals promoted superior removal efficiency for both antivirals (97.4 % and 94.7 % for ribavirin and tenofovir, respectively) as compared to the control bioreactor lacking minerals, which achieved 58.5 % and 37.9 % removal for the same drugs, respectively. Further analysis conducted by liquid chromatography coupled to mass spectroscopy revealed several metabolites derived from the biotransformation of both antivirals. Interestingly, tracer analysis with ¹³CH₄ revealed that anaerobic methane oxidation coupled to Fe(III) reduction occurred in the enriched bioreactor, which was reflected in a lower content of methane in the biogas produced from this system, as compared to the control bioreactor. This treatment proposal is suitable within the circular economy concept, in which recovered metals from an industrial wastewater are applied in bioreactors to create a biocatalyst for promoting the biotransformation of emerging pollutants. This strategy may be appropriate for the anaerobic treatment of wastewaters originated from hospitals, as well as from the pharmaceutical and chemical sectors.

Stabilization of potentially toxic elements contained in mine waste: A microbiological approach for the environmental management of mine tailings

Metals are key materials extensively employed in several industries to produce technological and daily-life products. The mining industry that produces such commodities generates Tons of waste that if not remediated can be transferred to the surrounding environment, thus representing a water, air, and soil pollution threat. In this work, we evaluated the feasibility of microbial sulfate reduction (SR) as a management strategy for this waste. Mine tailings were sampled from two abandoned mining sites located in Sonora (northwestern Mexico) and treated in anaerobic microcosms under SR conditions using anaerobic sludge as the inoculum at two different tailing:inoculum ratios (TIR). Major TIR's were found to be the triggering factor for the highest SR activities observed (73.6 ± 8.8 mg SO₄^2- L^-1 day^-1). This stimulation was linked to the dissolution of sulfate bearing minerals (anglesite, jarosite, and gypsum) which provided additional sulfate for microbial activity. However, under this condition, longer lag phases for SR were observed, which was potentially due to pH inhibition at early incubation stages (pH ~3.7). Despite this, all biologically SR performing treatments presented important sulfide precipitation which was associated to changes in the mineralogy of the mine tailings. Metals of environmental concern such as As, Cd, Co, Cr and, Pb were detected to have shifted from the aqueous extractable phase to the bound to Fe and Mn oxides and residual phases. This finding was in accordance with the non-detectable concentrations of these metals in the aqueous phase by the end of the biological treatment which proved the effectiveness of this approach. This study provides insights into the promising potential of anaerobic microbes for the environmental management of mine tailings.

Immobilization of metal-humic acid complexes in anaerobic granular sludge for their application as solid-phase redox mediators in the biotransformation of iopromide in UASB reactors

Metal-humic acid complexes were synthesized and immobilized by a granulation process in anaerobic sludge for their application as solid-phase redox mediators (RM) in the biotransformation of iopromide. Characterization of Ca- and Fe-humic acid complexes revealed electron accepting capacities of 0.472 and 0.556milli-equivalentsg(-1), respectively. Once immobilized, metal-humic acid complexes significantly increased the biotransformation of iopromide in upflow anaerobic sludge blanket (UASB) reactors. Control UASB reactor (without humic material) achieved 31.6% of iopromide removal, while 80% was removed in UASB reactors supplied with each metal-humic acid complex. Further analyses indicated multiple transformation reactions taking place in iopromide including deiodination, N-dealkylation, decarboxylation and deacetylation. This is the first successful application of immobilized RM, which does not require a supporting material to maintain the solid-phase RM in long term operation of bioreactors. The proposed redox catalyst could be suitable for enhancing the redox conversion of different recalcitrant pollutants present in industrial effluents.

Recovery of palladium(II) by methanogenic granular sludge

This is the first report that demonstrates the ability of anaerobic methanogenic granular sludge to reduce Pd(II) to Pd(0). Different electron donors were evaluated for their effectiveness in promoting Pd reduction. Formate and H2 fostered both chemically and biologically mediated Pd reduction. Ethanol only promoted the reduction of Pd(II) under biotic conditions and the reduction was likely mediated by H2 released from ethanol fermentation. No reduction was observed in biotic or abiotic assays with all other substrates tested (acetate, lactate and pyruvate) although a large fraction of the total Pd was removed from the liquid medium likely due to biosorption. Pd(II) displayed severe inhibition towards acetoclastic and hydrogenotrophic methanogens, as indicated by 50% inhibiting concentrations as low as 0.96 and 2.7 mg/L, respectively. The results obtained indicate the potential of utilizing anaerobic granular sludge bioreactor technology as a practical and promising option for Pd(II) reduction and recovery offering advantages over pure cultures.

Direct and quinone-mediated palladium reduction by Geobacter sulfurreducens: mechanisms and modeling

Palladium(II) reduction to Pd(0) nanoparticles by Geobacter sulfurreducens was explored under conditions of neutral pH, 30 °C and concentrations of 25, 50, and 100 mg of Pd(II)/L aiming to investigate the effect of solid species of palladium on their microbial reduction. The influence of anthraquinone-2,6-disulfonate was reported to enhance the palladium reaction rate in an average of 1.7-fold and its addition is determining to achieve the reduction of solid species of palladium. Based on the obtained results two mechanisms are proposed: (1) direct, which is fully described considering interactions of amide, sulfur, and phosphoryl groups associated to proteins from bacteria on palladium reduction reaction, and (2) quinone-mediated, which implies multiheme c-type cytochromes participation. Speciation analysis and kinetic results were considered and integrated into a model to fit the experimental data that explain both mechanisms. This work provides elements for a better understanding of direct and quinone-mediated palladium reduction by G. sulfurreducens, which could facilitate metal recovery with concomitant formation of valuable palladium nanoparticles in industrial processes.