Detection of Metal-reducing Enzyme Complexes by Scanning Electrochemical Microscopy

The in-situ detection of dissolved redox-active metals and metalloids such as iron and manganese is attracting increasing interest for understanding the complex network of enzymatic and geochemical reactions occurring at the microbe-mineral interface. A versatile and promising tool for the investigation of redox-active metal distributions at a microscopic scale is scanning electrochemical microscopy (SECM), specifically in combination with square wave anodic stripping voltammetry (SWASV). Here, we report the application of platinum/mercury (Pt/Hg) microelectrodes for the direct detection of chemical species with negative reduction potentials including Mn2+, Fe2+, O-2, and sulfur species. The combination of Pt/Hg microelectrodes with SWASV-SECM was evaluated as a potential read-out tool in gel electrophoresis for identifying Fe(III)- and Mn(IV)-reducing proteins isolated from the outer membrane of metal-respiring bacteria (here, Shewanella oneidensis) prior to separation in native polyacrylamide gels. PMA-SWASV-SECM facilitated direct detection of metal reductase activity above protein complexes separated in native gel buffered at neutral pH without the aid of traditional colorimetric stains. Control experiments at inactive reductase proteins confirmed the involvement of sulfur species within this microbial respiratory redox process.