Natale, Alessandra Pia (2010) Genotypic and phenotypic flexibility of microbial communities. PhD thesis, University of Warwick.

Abstract

Paracoccus denitrificans is a facultative anaerobic Gram-negative α- proteobacterium able to shift to denitrification under anaerobic conditions (John & Whatley, 1978; Zumft, 1997). Because of its metabolic versatility, P. denitrificans has been used in this study as a model organism to investigate the role of environmental heterogeneity in maintaining metabolic flexibility in bacterial communities. The hypotheses underlying this study are: - Metabolic flexibility is maintained in situ by environmental heterogeneity and, specifically: - Under constant environmental conditions the metabolic flexibility of the generalist P. denitrificans will be lost by accumulation of mutations in unused genes. Chemostat cultures under constant aerobic conditions revealed how after ~150 generations genetic loci not in use under aerobic conditions (in particular nirS and nosZ) are subjected to a lower selective pressure that leads to a higher genetic polymorphism in the population. The phenotypic analysis of the population resulting from the same chemostat culture showed a lower specific growth rate and a higher yield compared to the ancestor population, suggesting a deactivation of concomitant denitrification and aerobic respiration (Robertson et al., 1988). Furthermore, the resulting population shows a down-regulation of expression of all three denitrification genes tested and a lower production of nitrous oxide (N2O). When cultured in batch cultures for a long period of time under aerobic conditions, P. denitrificans shows a similar adaptative response. Four parallel populations, originated from a common ancestor and propagated aerobically for more than 500 generations undertook some important communal genotypic and phenotypic changes that suggested that P. denitrificans repetitively adapts to constant environmental conditions by losing its characteristic metabolic flexibility. By following the first steps of loss of metabolic flexibility as an adaptative response to novel environmental conditions in a generalist model as P. denitrificans we could empirically witness the important role of the environment on bacterial evolution and speciation.

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