Optimal growth temperature is not an adequate proxy of microbial cold adaptation

Microbial thermal adaptation is commonly characterized using the minimum (Tmin), optimum (Topt), and maximum (Tmax) temperatures that define the thermal growth range of microorganisms; with Topt been frequently used to compare microbial thermal characteristics. Although previous studies have shown that Tmin and Topt reflect distinct aspects of microbial thermal physiology, the generality and quantitative limits of this relationship across microbial diversity have not been systematically evaluated. Here, we constructed a dataset of 14,023 microbial growth-temperature records to examine the relationships among Tmin, Topt, and Tmax. While Topt showed strong overall correlations with both Tmax (r = 0.910, R2 = 0.827, Padj < 0.05) and Tmin (r = 0.793, R2 = 0.628, Padj < 0.05), the strength of these relationships varied markedly across the temperature spectrum. Within the lower-temperature range, the association between Topt and Tmin became weak and statistically insignificant (r = 0.106, Padj = 0.200). Furthermore, within 10–30 °C Topt range, microorganisms sharing similar Topt values frequently exhibited substantially different Tmin values, indicating considerable variation in their capacity for growth at low temperatures despite comparable Topt. These findings demonstrate that Topt and Tmin provide complementary rather than interchangeable information about microbial thermal characteristics, and Topt alone is insufficient to predict lower thermal growth limits. Our study provides the most comprehensive quantitative assessment to date of the relationship between microbial cardinal growth temperatures, defining the limitations of using Topt to predict lower thermal growth limits and highlighting the importance of explicitly reporting Tmin in ecological, evolutionary and taxonomic studies.