Our understanding of microbial life is greatly biased by our narrow focus on microbes as they grow in the laboratory. Yet, as discussed previously in this blog, microbes can persist in various dormant forms for extended periods of time. Sporulation (from the Greek “spora” or seed) is perhaps the best-known strategy for dormancy, a tactic used by some bacteria to transcend unfavorable conditions. When a brighter day dawns, the spores germinate and the lineage continues. (For more information about the mechanism and regulation of sporulation in the model bacterium Bacillus subtilis, check some earlier STC blogs here and here.) This is not the only ‘resting state’ that enables microbes to persist and then revive with the return of favorable conditions. Some bacteria, such as species of Azotobacter, form cysts to withstand hard times and also to facilitate their dispersal to new environments. Vegetative cells transition into cysts by thickening their cell wall, contracting their cytoplasm, inhibiting their motility, and slowing down their metabolism. Cysts and spores thus represent an example of convergent evolution in which disparate processes arise to serve a similar purpose: long-term survival in an adverse environment. This is a topic of interest to many scientific fields, most especially astrobiology. It raises the intriguing possibility that some planets that provided hospitable conditions for life in the past might still house persistent forms of microbial life. But, more specifically, what types of life forms would we predict these planets to harbor in their inhospitable depths?
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