Efficient nitrate and Cr(VI) removal by denitrifier: The mechanism of S. oneidensis MR-1 promoting electron production, transportation and consumption

When Cr(VI) and nitrate coexist, the efficiency of both bio-denitrification and Cr(VI) bio-reduction is poor because chromate hinders bacterial normal functions (i.e., electron production, transportation and consumption). Moreover, under anaerobic condition, the method about efficient nitrate and Cr(VI) removal remained unclear. In this paper, the addition of Shewanella oneidensis MR-1 to promote the electron production, transportation and consumption of denitrifier and cause an increase in the removal of nitrate and Cr(VI). The efficiency of nitrate and Cr(VI) removal accomplished by P. denitrificans as a used model denitrifier increased respectively from 51.3% to 96.1% and 34.3% to 99.8% after S. oneidensis MR-1 addition. The mechanism investigations revealed that P. denitrificans provided S. oneidensis MR-1 with lactate, which was utilized to secreted riboflavin and phenazine by S. oneidensis MR-1. The riboflavin served as coenzymes of cellular reductants (i.e., thioredoxin and glutathione) in P. denitrificans, which created favorable intracellular microenvironment conditions for electron generation. Meanwhile, phenazine promoted biofilm formation, which increased the adsorption of Cr(VI) on the cell surface and accelerated the Cr(VI) reduction by membrane bound chromate reductases thereby reducing damage to other enzymes respectively. Overall, this strategy reduced the negative effect of chromate, thus improved the generation, transportation, and consumption of electrons. SYNOPSIS: The presence of S. oneidensis MR-1 facilitated nitrate and Cr(VI) removal by P. denitrificans through decreasing the negative effect of chromate due to the metabolites' secretion.

Synergy of cyano groups and cobalt single atoms in graphitic carbon nitride for enhanced bio-denitrification

Bio-denitrification plays a crucial role in the purification of nitrogen contaminated water, yet the low efficiency of the pure biological system often leads to the accumulation of harmful intermediates. Semi-biological catalysis provides an effective approach to improving the reaction efficiency through hybridizing artificial nanomaterials with natural organisms, yet the application of this strategy in bio-denitrification is limited. In this study, the effect of surface engineered carbon nitride on the denitrification capability of denitrifying bacteria was investigated. We found that cyano groups availed the biotic-abiotic interactions, while immobilized cobalt single atoms attenuated the local electrostatic repulsion. This synergistic effect endowed carbon nitride modified with cobalt atoms and cyano groups (Co/C₃N₄-C) with the unexpected acceleration of bio-denitrification reaction, without the accumulation of harmful intermediates. According to the metabolomics analysis, this improvement was attributed to the moderate metabolic adaptation caused by nanoelicitor, which induced dramatically boosted electron transfer and energy supply for extracellular polymeric substance (EPS) secretion. The elevation of intracellular iron level increased the activities of denitrification reductase, which was evidenced by metatranscriptomic analysis. Our results not only demonstrate the great potential of carbon nitride as an artificial elicitor for biological regulation, but also shed light on comprehending the complicated biotic-abiotic interactions for versatile application.