Glyphosate-based herbicide exposure triggers genetic adaptation but not diversity collapse within bacterioplankton species.

Bacterial populations evolve rapidly in the lab when faced with experimentally applied selective pressures. Yet how bacteria evolve in nature, in more complex multi-species communities, is both challenging to study and essential to our understanding of ecosystem responses to rapid anthropogenic change. To track bacterial evolution in a semi-natural context, we applied Roundup, a glyphosate-based herbicide (GBH) that interferes with aromatic amino acid synthesis, as a selective pressure to 1000 L ponds containing bacterioplankton communities from a pristine lake. We show that both ecological and evolutionary changes can occur on short timescales after a strong selective pressure. Using metagenome-assembled genomes as a proxy for species, we found that GBH treatment substantially affected community diversity but did not purge within-species genetic diversity over the 4 weeks of the experiment. We identified several functional categories of genes consistently targeted by GBH selection across seven different species of bacteria. Genes involved in amino acid transport and metabolism were more likely to experience GBH-driven changes in allele frequency, including the gene aroA targeted by glyphosate, along with other potentially novel targets of selection. Together, these results show how environmental change can rapidly affect bacterial community structure and select for specific genetic targets without purging genetic diversity genome-wide.