Cool and Shady: Ecophysiological Preferences of Chrysophytes

Chrysophyceae (Stramenopiles) are a diverse group of protists widely distributed in various aquatic habitats, including alpine lakes, where they play key ecological roles. Their nutritional modes-phototrophy, heterotrophy, and mixotrophy-enable them to adapt to the unique light and temperature conditions found across altitudinal gradients in mountain environments. This study investigates the growth responses of three mixotrophic and three phototrophic chrysophyte strains, isolated from alpine and pre-alpine lakes, to varying light intensities and temperatures. Our results show that both temperature and light intensity exert strong, species-specific effects on the growth of phototrophic and mixotrophic Chrysophyceae. Despite their contrasting original habitats, most strains shared similar growth optima, with peak performance generally observed between 15°C and 19°C and reduced growth at both lower and higher temperatures. All strains exhibited negative growth in darkness, confirming their phototrophic dependence. Growth rates typically increased with light availability up to a species-specific optimum (12-35 μE m^-2 s^-1), beyond which either a plateau or a decline was observed. Despite environmental differences among their habitats of origin, temperature and light optima were remarkably consistent across strains.