Possible role of microbial polysaccharides in nontronite formation

Nontronite and microbes were detected in the surface layers of deep-sea sed iments from Iheya Basin, Okinawa Trough, Japan. Nontronite, an Fe-rich smec tite mineral, was embedded in acidic polysaccharides that were exuded by mi crobial cells and electron microscopy showed that the nontronite layers wer e apparently oriented in the polysaccharide materials. We propose that the formation of nontronite was induced by the accumulation of Si and Fe ions f rom the ambient seawater and that extracellular polymeric substances (EPS) served as a template for layer-silicate synthesis. Experimental evidence fo r this hypothesis was obtained by mixing a solution of polysaccharides (dex trin and pectin) with ferrosiliceous groundwater. After stirring the mixtur e in a sealed vessel for two days, and centrifuging, Fe-rich layer silicate s were identified within the precipitate of both the dextrin and pectin agg regates, whereas rod-shaped or spheroidal Si-bearing iron hydroxides were f ound in the external solution. Microbial polysaccharides would appear to ha ve affected layer-silicate formation.