Electron shuttling via humic acids in microbial iron(III) reduction in a freshwater sediment
2004, Kappler, Andreas, Benz, Marcus, Schink, Bernhard, Brune, Andreas
The biological and chemical potential for electron shuttling via humic acids was evaluated by analyzing the depth distribution of humicacid-reducing and iron-reducing bacteria in a freshwater sediment, and correlating it to the redox characteristics of humic acids and iron. Physicochemical analysis of profundal sediments of Lake Constance revealed a distinct stratification, with oxygen respiration, microbial iron and sulfate reduction, and methanogenesis allocatable to defined layers. Among the acid-extractable iron in the surface layer, ferric iron (Fe(III)) was dominant, whereas ferrous iron (Fe(II)) prevailed below 2 cm depth. Humic acids showed a higher electron-accepting (oxidizing) capacity in the surface layer and a higher reducing capacity in deeper layers. The more reduced redox state of humic acids in deeper layers was probably due to reduction by humic-acid-reducing microorganisms. Most-probable-number analysis revealed that the sediments contained populations of humic-acid-reducing bacteria that (i) were substantially larger than those of the iron-reducing bacteria in the respective sediment layers and (ii) were in the same range as those of the fermenting bacteria. Our results suggest that microbial reduction of humic acids and subsequent chemical reduction of poorly soluble iron(III) minerals by the reduced humic acids represents an important path of electron flow in anoxic natural environments such as freshwater sediments.
Dynamics in composition and size-class distribution of humic substances in profundal sediments of Lake Constance
2001, Kappler, Andreas, Ji, Rong, Schink, Bernhard, Brune, Andreas
Organic matter from profundal lake sediments was fractionated and analyzed with high depth resolution, and the size-class distribution of humic substances was determined by gel-permeation chromatography. Together with a depth- dependent increase of the thermal stability of the organic matter and an increasing C/N atomic ratio of the humic acids, the size-distribution patterns of di erent fractions indicated an increasing degree of humification with depth. The main stabilization processes occurred in the surface layer (0±2 cm depth), which is clearly distinguished from the deeper sediment. The depth profiles also indicated that humification in lake sediments consists of simultaneous degradation and polymerization processes.