Mechanisms of extracellular photoelectron uptake by a Thiobacillus denitrificans-cadmium sulfide biosemiconductor system

Abstract

The excellent performance of biosemiconductors in solar energy conversion and environmental remediation has triggered an explosive research in recent years. However, there is still a lack of knowledge on the mechanisms of extracellular photoelectron uptake in biosemiconductors, especially for the denitrifier-based one. Herein, a model biosemiconductor, Thiobacillus denitrificans-cadmium sulfide (T. denitrificans-CdS), for photoelectrotrophic denitrification was used to investigate the mechanism. Electrochemical characterization demonstrated more redox species generation after exposing T. denitrificans-CdS to light than the controls. Photoelectron uptake process was in situ recorded by an ITO-bottom electrochemical reactor. UV–Vis spectroscopy assured more outer-membrane cytochrome c generation and its concentration was 3.35 times higher in the light than that at dark. A coupled technology of SDS–PAGE, heme staining and LC–MS/MS targeted periplasmic cytochrome c4 (Cyt c4) emerging in T. denitrificans-CdS (light). Moreover, the expression of the cyt c4 gene was significantly (p < 0.05) upregulated under light, and its abundance increased from 1.2 ± 0.3 to 4.7 ± 1.2 folds with light intensity changing from 0.5 to 2.0 mW cm−2, supporting its role in the photoelectron uptake process. This work revealed the photoelectron uptake mechanism in T. denitrificans-CdS, and the results will provide guidance for constructing an efficient biosemiconductor system for nitrate removal and conversion driven by light.