Metabolic Strategies Under Heat Stress in Non-Adapted and High-Temperature-Adapted Lines of a Marine Diatom.

In the last decade, numerous laboratory experiments have demonstrated that when marine phytoplankton are exposed to thermal stress, they can evolve high temperature tolerance in a short time (months). This evolutionary potential may ensure the persistence of marine phytoplankton species under current and future global warming. However, the effect of such adaptation on the phytoplankton's interaction with the environment and other organisms depends on how cellular metabolism shifts during the evolutionary process. In order to elucidate which cellular strategies allow the emergence of thermo-tolerant lines, we analysed the proteomic changes in a marine diatom (Chaetoceros simplex) following both thermal acclimation and evolutionary adaptation. We found that the high temperature-tolerant lines exhibit a conservative cellular strategy when acclimated to high, above-optimal temperature, where nitrogen recycling, reallocation of carbon and nitrogen, and protein repair or protection are favoured at the expense of new proteins and photosynthetic structures biosynthesis and rapid growth. While this strategy gives the lines that evolved high temperature tolerance an advantage under thermal stress, the shift to resource reallocation may explain the absence of high-temperature adaptation when cells are exposed to low nitrate availability.