Niche breadth specialization impacts ecological and evolutionary adaptation following environmental change.

Ecological theory predicts that organismal distribution and abundance depend on the ability to adapt to environmental change. It also predicts that eukaryotic specialists and generalists will dominate in extreme environments or following environmental change, respectively. This theory has attracted little attention in prokaryotes, especially in archaea, which drive major global biogeochemical cycles. We tested this concept in Thaumarchaeota using pH niche breadth as a specialization factor. Responses of archaeal growth and activity to pH disturbance were determined empirically in manipulated, long-term, pH-maintained soil plots. The distribution of specialists and generalists was uneven over the pH range, with specialists being more limited to the extreme range. Nonetheless, adaptation of generalists to environmental change was greater than that of specialists, except for environmental changes leading to more extreme conditions. The balance of generalism and specialism over longer timescales was further investigated across evolutionary history. Specialists and generalists diversified at similar rates, reflecting balanced benefits of each strategy, but a higher transition rate from generalists to specialists than the reverse was demonstrated, suggesting that metabolic specialism is more easily gained than metabolic versatility. This study provides evidence for a crucial ecological concept in prokaryotes, significantly extending our understanding of archaeal adaptation to environmental change.