Multiomics studies reveal how ambient temperature changes govern cellular responses of Chlamydomonas.

Photosynthetic protists, known as microalgae, face increasing temperatures due to climate change. The green biflagellate alga Chlamydomonas reinhardtii (Chlamydomonas) serves as a model for thermoregulation. While responses to thermal stress are well characterized, much less is known about the impact of ambient temperature shifts. Understanding microalgal responses to environmental temperature changes is critical, as these primary producers drive ecosystem productivity and food web dynamics. Here, Chlamydomonas grew mixotrophically at ambient temperatures from 18 °C to 33 °C. Transcriptomic profiling revealed extensive reorganization, with over 5,000 transcripts significantly affected, including those involved in algal-bacterial interactions, photoreception, lipid metabolism, photosynthesis, cilia formation, and the secretome. CO2 transfer rates and acetate levels measured at 18 °C and 28 °C suggest decreased photoautotrophic algal growth at 28 °C at first. Antagonistic bacterial activity was sustained longer at lower temperatures. Proteomic analyses of isolated cilia and secreted proteins corroborate major abundance changes within these sub-proteomes, particularly in ciliary intraflagellar transport complexes and mating-related proteins in the secretome. Together, these molecular alterations resulted in pronounced changes in growth, the lengths of cells and cilia swimming behavior, mating ability, and bacterial antagonism. These data reveal major cellular responses caused by ambient, even short-term temperature shifts.