The Maximum Growth Temperature for Eukaryotes Is Thermodynamically Driven but Ecologically Contingent.

Temperature is a state variable that affects all life. While it is known that archaea can grow at 120 °C and many bacteria can grow at over 100 °C, no eukaryote is known to complete a life cycle at above 65 °C. This paper explores why the difference in the maximum temperature of eukaryotes and other kingdoms of life might occur. It finds that chemical and genome structural differences between the domains of life are unlikely to explain the difference in maximum growth temperature, with the exception of the Saccharomycotina, which are different from other fungi, possibly because of their unique ecology. The distribution of inherently disordered proteins (IDPs), however, is significantly correlated with maximum and minimum growth temperature in fungi, and with the range of temperatures over which fungi can grow. I also demonstrate that the range of temperatures over which a species can grow is correlated with its maximum temperature. I postulate that the range is correlated with maximum temperature because all real-world ecologies fluctuate between elevated and average surface temperatures, and the thermodynamics of IDP-based structures in eukaryotes inherently limits the range over which they can operate. Thus, the 65 °C maximum temperature limit for eukaryotes is a result of a combination of thermodynamic properties of their organization and the temperature regime on the modern Earth; I suggest an experimental approach to testing this.