Physiological mechanism of the response to Cr(VI) in the aerobic denitrifying ectomycorrhizal fungus Pisolithus sp.1

Enhancement of bio-promoters on hexavalent chromium inhibited sulfur-driven denitrification: repairing damage, accelerating electron transfer, and reshaping microbial collaboration

Proposing recovery strategies to recover heavy-metal-inhibited sulfur-driven denitrification, as well as disclosing recovery mechanisms, can provide technical support for the stable operation of bio-systems. This study proposed an effective bio-promoter (mediator-promoter composed of L-cysteine, biotin, cytokinin, and anthraquinone-2,6-disulfonate) to recover Cr(VI) inhibited sulfur-driven denitrification, which effectively reduced the recovery time of NO3--N reduction (18-21 cycles) and NO2--N reduction (27-42 cycles) compared with self-recovery. The mediator-promoter repaired microbial damage by promoting intracellular chromium efflux. Moreover, the mediator-promoter reduced the accumulated reactive oxygen species by stimulating the secretion of antioxidant enzymes, reaching equilibrium in the oxidative-antioxidant system. To improve electron transmission, the mediator-promoter restored S2O32- oxidation to provide adequate electron donors and increased electron transfer rate by increasing cytochrome c levels. Mediator-promoter boosted the abundance of Thiobacillus (sulfur-oxidizing bacterium) and Simplicispira (denitrifying bacterium), which were positively correlated, facilitating the rapid denitrification recovery and the long-term stable operation of recovered systems.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

Performance and mechanism of chromium reduction in denitrification biofilm system with different carbon sources

In the process of biological reduction of Cr(VI), the type of carbon sources affects the rate and effect of Cr(VI) reduction, but its specific performance and influencing mechanism have not yet been explored. In this study, four denitrification biofilm reactors were operated under four common carbon sources (C6H12O6, CH3COONa, CH3OH, CH3COONa:C6H12O6 1:1) to reveal the impact of carbon sources on Cr(VI) reduction. Through preliminary experimental concentration research, 75 mg/L Cr(VI) was selected as the dosing concentration. In long-term operation, the composite carbon sources of CH3COONa and C6H12O6 demonstrated excellent stability and achieved an impressive Cr(VI) removal efficiency of 99.5 %. The following sequence was C6H12O6, CH3COONa, and CH3OH. Among them, CH3OH was less competitive and the system was severely unbalanced with lowest Cr(VI) reduction efficiency. The toxicity reactions, changes in EPS and its functional groups, and electron transfer revealed the reduction and fixation mechanism of chromium on denitrification biofilm. The changes in microbial communities indicated that microbial communities in composite carbon sources can quickly adapt to the high toxic environment. The proportion of Trichococcus reached 43.6 %, which played an important role in denitrification and Cr(VI) reduction. Meanwhile, the prediction of microbial COG function reflected its excellent metabolic ability and defense mechanism.

Cr(VI) Reduction and Fe(II) Regeneration by Penicillium oxalicum SL2-Enhanced Nanoscale Zero-Valent Iron

Nanoscale zero-valent iron (nZVI) faces significant challenges in Cr(VI) remediation through aggregation and passivation. This study identified a Cr(VI)-resistant filamentous fungus (Penicillium oxalicum SL2) for nZVI activation and elucidated the synergistic mechanism in chromium remediation. P. oxalicum SL2 and nZVI synergistically and effectively removed Cr(VI), mainly by extracellular nonenzymatic reduction (89.1%). P. oxalicum SL2 exhibited marked iron precipitate solubilization and Fe(II) regeneration capabilities. The existence of the Fe(II)-Cr(V)-oxalate complex (HCrFeC₄O₉) indicated that in addition to directly reducing Cr(VI), iron ions generated by nZVI stimulated Cr(VI) reduction by organic acids secreted by P. oxalicum SL2. RNA sequencing and bioinformatics analysis revealed that P. oxalicum SL2 inhibited phosphate transport channels to suppress Cr(VI) transport, facilitated iron and siderophore transport to store Fe, activated the glyoxylate cycle to survive harsh environments, and enhanced organic acid and riboflavin secretion to reduce Cr(VI). Cr(VI) exposure also stimulated the antioxidative system, promoting catalase activity and maintaining the intracellular thiol/disulfide balance. Cr(VI)/Fe(III) reductases played crucial roles in the intracellular reduction of chromium and iron, while nZVI decreased cellular oxidative stress and alleviated Cr(VI) toxicity to P. oxalicum SL2. Overall, the P. oxalicum SL2-nZVI synergistic system is a promising approach for regenerating Fe(II) while reducing Cr(VI).

New insights into hexavalent chromium exposure in electron donor limited denitrification: bio-electron behavior

The bio-electron behavior (electron production, transmission, and consumption) response to a typical heavy metal, hexavalent chromium, was unraveled in the electron donor limited system (EDLS) and electron donor sufficient system (EDSS). Nicotinamide adenine dinucleotide and adenosine triphosphate production were reduced by 44% and 47%, respectively, due to glucose metabolism inhibition, leading to NO₃^--N declining to 31% in EDLS. The decreased electron carrier contents and denitrifying enzymes activity inhibited electron transmission and consumption in both EDLS and EDSS. Additionally, electron transfer and antioxidant stress abilities were reduced, further hindering the survival of denitrifiers in EDLS. The lack of dominant genera (Comamonas, Thermomonas, and Microbacterium) in EDLS was the primary reason for poor biofilm formation and chromium adaptability. The decreased expression of enzymes related to glucose metabolism caused the imbalance of electron supply, transport, and consumption in EDLS, adversely impacting nitrogen metabolism and inhibiting denitrification performance.

Novel insights into aerobic denitrifying bacterial communities augmented denitrification capacity and mechanisms in lake waters

Scanty attention has been paid to augmenting the denitrification performance of polluted lake water by adding mix-cultured aerobic denitrifying bacterial communities (Mix-CADBCs). In this study, to solve the serious problem of nitrogen pollution in lake water bodies, aerobic denitrifying bacteria were added to lake water to enhance the nitrogen and carbon removal ability. Three Mix-CADBCs were isolated from lake water and they could remove >94 % of total nitrogen and dissolved organic carbon, respectively. The balance of nitrogen analysis shown that >70 % of the initial nitrogen was converted to gaseous nitrogen, and <11 % of the initial nitrogen was converted into microbial biomass. The batch experiments indicated that three Mix-CADBCs could perform denitrification under various conditions. According to the results of nirS-type sequencing, the Hydrogenophaga sp., Prosthecomicrobium sp., and Pseudomonas sp. were dominated genera of three Mix-CADBCs. The analysis of network indicated Pseudomonas I.Bh25.14 and Vogsella LIG4 were correlated with the removal of total nitrogen (TN) and dissolved organic carbon in the Mix-CADBCs. Compared with lake raw water, the addition of three Mix-CADBCs could promote the denitrification capacity (the removal efficiencies of TN > 78.72 %), microbial growth (optical density increased by 0.015-0.138 and the total cell count increased by 2 times), and organic degradation ability (the removal efficiency chemical oxygen demand >38 %) of lake water. In general, the findings of this study demonstrated that Mix-CADBCs could provide a new perspective for biological treatment lake water body.

Selenium regulates the mitogen-activated protein kinase pathway to protect broilers from hexavalent chromium-induced kidney dysfunction and apoptosis

Hexavalent chromium [Cr (VI)] is a common environmental pollutant. Although selenium (Se) can antagonize the toxicity of Cr (VI), the specific underlying mechanism has not been identified. To investigate this mechanism, we used potassium dichromate (K₂Cr₂O₇) and selenium-rich yeast (SeY) to construct single Cr (VI)- and combined Se/Cr (VI)-exposed broiler models during a 42-day period. Broilers were randomly assigned to the control (C), SeY (Se), SeY + Cr (VI) (Se/Cr), and Cr (VI) (Cr) groups. The antagonistic mechanisms of Se and Cr (VI) were evaluated using histopathological evaluation, serum and tissue biochemical tests, real-time fluorescence quantitative polymerase chain reaction, and western blotting. The results suggested that Se alleviated the morphological and structural damage to renal tubules and glomeruli, while reducing the organ index, creatinine levels, and blood urea nitrogen levels in the kidneys of Cr (VI)-exposed broilers. Furthermore, Cr (VI) reduced the levels of superoxide dismutase and glutathione, and increased the levels of malondialdehyde, in broiler kidney tissues. However, Se alleviated Cr (VI)-induced oxidative stress by increasing the levels of superoxide dismutase and glutathione, and decreasing the levels of malondialdehyde, within a certain range. Compared to the C group, the levels of p38, JNK, p-p38, p-JNK, p-p38/p38, and p-JNK/JNK significantly increased, whereas those of ERK, p-ERK, and p-ERK/ERK decreased, in the Cr group. Compared to the Cr group, the levels of p38, JNK, p-p38, p-JNK, p-p38/p38, and p-JNK/JNK significantly decreased, whereas those of ERK, p-ERK, and p-ERK/ERK increased, in the Se/Cr group. Furthermore, the levels of p53, c-Myc, Bax, Cyt-c, caspase-9, and caspase-3 significantly increased, and those of Bcl-2 and Bcl-2/Bax significantly decreased, following Cr (VI) exposure, while Se restored the expression of these genes. In conclusion, our findings suggest that SeY can protect against Cr (VI)-induced dysfunction and apoptosis by regulating the mitogen-activated protein kinase pathway activated by oxidative stress in broiler kidney tissues.