<p>Papillae are small siliceous protuberances occurring on the scales and cysts of many chrysophyte algae, yet the environmental factors shaping their distribution remain poorly understood. We combined pan-European and local datasets to identify the key drivers of papillae formation on the scales of the chrysophyte genus <i>Mallomonas</i> Perty. At the continental scale, warmer temperatures and low pH were associated with a higher proportion of papillae-forming species. At the local scale, low pH was again confirmed as the strongest predictor, along with elevated dissolved silica concentrations. The observed temperature effect is consistent with the idea that papillae help maintain hydrodynamic performance in warmer, less viscous water by modifying microscale flow and supporting nutrient exchange. The strong negative relationship with pH suggests that papillae may also mitigate chemical stress, for example by stabilizing the silica scale case or influencing interactions with metal ions. Together, these results show that both broad climatic gradients and fine-scale water chemistry shape the occurrence of papillae-forming species. By linking environmental conditions to variation in surface ornamentation, this study provides new insight into the ecological significance of papillae and the environmental pressures that may have driven their repeated evolution in chrysophyte algae.</p>