THE ROLE OF EPS IN THE SILICIFICATION OF CALOTHRIX SPP. DOMINATED MICROBIAL MATS IN HOT SPRING ECOSYSTEMS

We provide evidence of the role of sheath exopolymers in the silicification of indigenous Calothrix spp. dominated microbial mats of silica-depositing hot springs in Yellowstone National Park. It has previously been demonstrated that silicification is initially localized on the outer surface of Calothrix sheaths (i.e. Konhauser et al. 2003), that Calothrix sheaths have a low density of reactive binding sites (Phoenix et al. 2002), and that cells remain viable after extensive silica encrustation (Phoenix et al. 2000). Laboratory silicification studies have shown that the presence of Calothrix spp. does not affect silica precipitation kinetics (i.e. Benning et al. 2005). However, the interactions between silica colloids and EPS sheath polymers have not been investigated at high spatial resolution.

Samples for our electron microscopy study of Calothrix biofacies mats were collected from Queen’s Laundry, a near-neutral silica-depositing hot spring at Yellowstone National Park. Transmission and Scanning electron microscope examination of these samples revealed that extracellular polymer fibrils controlled the silicification texture at the nanometer scale. We found that silica colloids aggregated directly on the surface of EPS fibrils, forming a three-dimensional, open network rather than a monolithic coating. This open, highly permeable network would allow for fluid exchange of nutrients and waste products, and is hypothesized to represent an adaptation of Calothrix spp. to the presence of mineralizing fluids in hot springs environments. Further, distinct silicification microstructures were observed in mats collected at different locations and times, consistent with environmental or genetic variables influencing the silicification process. We extend a model proposed by Benning et al. (2004) based on aggregation kinetics to test the role of environmental variables on silicification microstructures.

References:

Benning, L. G. et al. (2005). Biosilicification: the role of cyanobacteria in silica sinter deposition. SGM symposium 65, Keele University, Cambridge University Press.

Benning, L. G. et al. (2004). 68(4): 743-757

Konhauser, K. O. et al. (2003). Can. J. Earth Sci. 40: 1713-1724.

Phoenix, V. R. et al. (2000). Chem. Geo. 169: 329-338.

Phoenix, V. R. et al. (2002). AEM 68(10): 4827-4834.