One of the most conspicuous examples of evolution in action is the awesome spectacle of how environmental microorganisms manage to face and eventually metabolize xenobiotic compounds with synthetic chemical structures that have not been in the Biosphere before we release them—in many cases with bad long-term consequences. Along with the onset of antibiotic resistances, emergence of new metabolic capabilities is one of the exemplary cases of real-time appearance of new activities in the biological world. Just to raise an example: when artificial sweetener acesulfame (ACE) was first introduced it was considered to be altogether recalcitrant to any biological action. Yet, years later, bacterial biodegradation was reported and a just a bit later, ACE-degrading strains were found all over the world bearing a defined pathway for its metabolization (Castronovo et al., 2017). In reality, the factual concentrations of ACE in waste-water treatment plants are very low and thus the selective pressure to leverage the as a nutrient is generally mild. Under such circumstances, what drove the emergence of a new pathway? What were the intermediate steps from no-degradation to full degradation?
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