GTPases of immunity associated proteins in the dinoflagellate Breviolum minutum are involved in the response to thermal stress and low light levels

Guanosine Triphosphate (GTP)ases of Immunity Associated Proteins (GIMAP) are small G proteins that in vertebrates, regulate immunity, apoptosis, and autophagy. While previously thought to be limited to vertebrates and plants, this family of proteins was recently characterized in a diversity of dinoflagellates. However, information regarding the function of GIMAPs in dinoflagellates is lacking. Therefore, the goal of this study (conducted from March-November of 2022) was to assess the expression of GIMAPs in the cnidarian symbiont Breviolum minutum (formerly Symbiodinium minutum, referred to as Bm_GIMAPs) in response to low light (25 vs 8 μmol m⁻² s⁻¹ for 9 d), thermal stress (22 vs 28 °C for 48 h), and symbiotic state (cultured vs in hospite). These factors were selected due to their importance in the lifestyle of a photosynthetic symbiont and relevance to the ecologically important issue of coral bleaching. The results indicate Bm_GIMAP expression increased under both low light and elevated temperature, suggesting a role in the response to bleaching related stressors. Cultured B. minutum had lower baseline Bm_GIMAP expression than those in association with a cnidarian host, but a stronger response to thermal stress, suggesting their function is influenced by symbiosis. Lastly, further support for the responsiveness of GIMAPs to thermal stress was provided by a comparison with previously published RNA-seq data from cultured Symbiodiniaceae. Collectively, the results presented here represent the first functional investigation of GIMAPs in an ecologically important group of protists and provide a greater understanding of the molecular response of dinoflagellates to stressors related to bleaching.