Reducing capacity of terrestrial humic acids

The role of terrestrial humic substances as electron shuttles in bioreduction processes has gained broad acceptance as recognition has grown that the ability to transfer electrons to humic materials is distributed widely among microorganisms in nature. A fundamental property of humic substances pertinent to their mediation of reductive transformations is the maximum moles of electron charge they can transfer to an added oxidant, a parameter for which the name Reducing Capacity has been suggested. A number of different operational definitions of this important parameter have appeared in the literature recently, leading to conflicting terminology that has not heretofore been rationalized. In this paper, we present a consistent set of independent definitions of Reducing Capacity and develop laboratory methodologies for applying them, illustrating our concepts and methods with representative International Humic Substances Society (IHSS) humic acids that have been widely studied in connection with electron shuttling. Our principal results are: (1) non-negligible Reducing Capacity for humic acid (HA) maintained under oxic conditions, indicating that important reductant functional groups persist in humic substances, and (2) Reducing Capacity of chemically-reduced HA equal, within experimental precision, to that for microbially-reduced HA, indicating that chemical reduction can be used as a convenient laboratory method to assess the capacity of HA to be reduced by microorganisms. Our results also demonstrate that complexed Fe contributes negligibly to the Reducing Capacity of HA. We further illustrate our proposed definitions by applying them to interpret some published field data on the Reducing Capacity profile of HA measured in a freshwater lake sediment exhibiting a pronounced redox zonation.