Reduction of quinone and non-quinone redox functional groups in different humic acid samples by Geobacter sulfurreducens

Humic substances constitute the most abundant organic fraction in the biosphere, which have extensively been studied for more than 200 years; however, much remains unknown regarding their structure and properties. In the present study, 12 different samples of humic acids (HA) were extracted from a wide diversity of organic rich environments, including soil from different forests and plantations, and were characterized based on their elemental composition, carboxyl and phenolic content, FTIR spectra and their electron transferring capacities (ETCs). The total ETCs determined in a H2/Pd reaction system greatly varied (112−392 μmol g− 1) depending on the origin of the organic bulk material. Further analysis indicated that non-quinone (NQ) redox functional groups in HA accounted for an important fraction (25−44%) of the total quantified ETCs. Results derived from the present study constitute the first demonstration that humus-reducing microorganisms are capable of reducing both quinone and NQ redox functional groups in different HA samples and expand our understanding on the microbial reduction of humic substances.