Evolution of stress tolerance
I am doing two long-term (hundreds to thousands of generations) evolution experiments using a well-studied bacterium, Escherichia coli, specifically strain B. This strain is the organism used for an ongoing 20-year-long (> 40,000 generations) Long-Term Evolution Experiment established by Richard Lenski in 1988. In one experiment, I am studying how microbes evolve to tolerate desiccation and radiation, characteristics epitomized by the bacterium Deinococcus radiodurans. I have found that E. coli B can survive, albeit with high mortality (> 99 % die), short periods of desiccation (at least a few minutes) and irradiation (at least one minute of ultraviolet B). Hence, it is sensitive to these stresses, it barely survives brief exposure to them, but it is not tolerant of them, it does not survive prolonged exposure to them. This makes E. coli B ideally suited for this experiment because the opportunity exist for it to become, via de novo evolution, at least slightly tolerant of desiccation and radiation (compared with D. radiodurans). Beyond understanding the evolutionary mechanisms underlying each tolerance, a further aim of this experiment, prompted by the fact that D. radiodurans is highly tolerant of these two stresses, is to see if co-tolerance arises in the stressed E. coli B populations. I will do this by exposing populations that evolved under desiccation stress to radiation and then determine if they have become more tolerant of radiation without having been evolved under radiation stress, and vice versa. If co-tolerance occurs, I will look for genetic and phenotypic changes that accompany it. My overall goal of this experiment is to gain evolutionary insight into characteristics that permit the survival of microbes in low water, high insolation environments such as equatorial and high-elevation deserts.
Evolutionary biological stoichiometry
In the other experiment, I am studying the effects of long-term static and dynamic nutrient limitation on phenotypic characteristics, primarily the cellular stoichiometry of carbon, nitrogen, and phosphorus. Populations are propagated in a nutritionally static or dynamic medium. This medium, a modified Davis Minimal Medium (buffered with Tris instead of mono- and dibasic phosphate), varies by whether carbon, nitrogen, or phosphorus constantly or alternately limits growth. For example, the growth of some populations is always limited by phosphorus while others experience a cycle in which their growth is limited by phosphorus one day and is unlimited by it the other day. The principal aim of this experiment is to determine if a means for storing nitrogen and / or phosphorus evolves and, if so, what the consequences are for cellular carbon : nitrogen : phosphorus stoichiometry. My broad goal of this experiment is to further knowledge of a process, stoichiometric homeostasis, that influences the ecology and evolution of populations, communities, and ecosystems.