Iron reduction and gley formation by nitrogen-fixing clostridia

Abstract

Studies on iron reduction and the mechanism of gley formation by nitrogen-fixing clostridia are reported. Up to 10⁶ cells/g soil of anaerobic, nitrogen-fixing clostridia, capable of reducing iron (III)-oxide, were counted in samples taken from various top soils. In a gleyed subsoil as many as 10⁵ bacteria per g soil, capable of reducing and fixing nitrogen, were enumerated using the most probable number technique. In general, the ratio of the auxotrophic iron reducing clostridia (glucose+yeast extract fermenters) to the prototrophic iron reducing flora (glucose fermenters) was found much larger in the top soil samples than in those derived from various gleyed subsoils. An enrichment method for the isolation of nitrogen-fixing, iron reducing clostridia of the butyric-butyl type is described. The iron reducing capacity of this type of clostridia as well as of Clostridium pasteurianum was determined quantitatively. Generally, the presence of soil or soil extract enhanced the amount of dissolved ferrous iron, both with butyric acid fermenters and with Cl. pasteurianum. When enriched iron reducing clostridia were incubated anaerobically under N₂-atmosphere in a sterile, red-colored, lateritic type of soil with glucose, intense gleying occurred within a few days. Microscopic observations indicated the presence of sporeforming bacteria of the Clostridium butyricum type or related species. The biological and chemical mechanism of gley formation is discussed.