Thursday, November 4, 2010

The Ability of Nostoc commune to Resist Dessication

Nostoc commune
The cosmopolitan cyanobacteria Nostoc commune is found on all continents, in all conditions, from tropical rain forests to the poles (Whitton and Potts, 2000 Ecology of Cyanobacteria: their Diversity in Time and Space). It is a common pest of gardens, associated with areas of poor drainage and/or poor nutrients, often forming masses of gelatinous, globular, brown-green colonies. These are composed of filaments of cells surrounded by a sheath. In the past, these colonies were believed to have fallen from the sky and were referred to as, among other things, “Sternschnuppen” (shooting stars) (Potts, 1997 International journal of Systemic Bacteriology p. 584).
Nostoc commune in a dessicated state

N. commune shows a remarkable capacity to resist desiccation, being able to survive storage at -400 MPa (0% relative humidity) for centuries (Potts, 1994 Microbiology Reviews 58 pp. 755–805). Prior to rehydration, N. commune appears as a brittle crust which increase in size rapidly upon the addition of water.

This remarkable ability is due to the presence of a viscous extracellular polysaccharide that is excreted by the cells. This glycan consist of a 1-4-linked xylogalactoglucan backbone with D-ribofuranose and 3-O-[(R)-1-carboxyethyl]-D-glucuronic acid (nosturonic acid) pendant groups (Helm et al., 2000 Journal of Bacteriology 182 pp. 974-982). Prevention of dessication is achieved by a number of processes. The glycan inhibits fusion of membrane vesicels during dessication (Hill et al., 1997 Journal of Applied Phycology 9 pp. 237–248). It also provides a structural and/or molecular scaffold with rheological properties which can accommodate the rapid biophysical and physiological changes in the community upon rehydration and during recovery from desiccation. The glycan matrix contains both lipid- and watersoluble UV radiation-absorbing pigments which protect the cell from photodegradation (Hill et al., 1994 Protoplasma 182 pp. 126–148). Finally although epiphytes colonize the surfaces of N. commune colonies, there is no penetration of the glycan due in part to a silicon- and calcium-rich pellicle and inherent resistance of the glycan to enzymatic breakdown.

Production of such an extracellular polysaccharide as this by N. commune may explain why microfossils of cyanobacteria are preserved so well from more than 3.5 billion years ago (Schopf and Klein, 1992 The Proterozoic Biosphere p. 185–193).

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