Ocean Acidification and Warming Lead to Increased Growth and Altered Chloroplast Morphology in the Thermo-Tolerant Alga Symbiochlorum hainanensis

Ocean acidification and warming affect the growth and predominance of algae. However, the effects of ocean acidification and warming on the growth and gene transcription of thermo-tolerant algae are poorly understood. Here we determined the effects of elevated temperature (H) and acidification (A) on a recently discovered coral-associated thermo-tolerant alga Symbiochlorum hainanensis by culturing it under two temperature settings (26.0 and 32.0°C) crossed with two pH levels (8.16 and 7.81). The results showed that the growth of S. hainanensis was positively affected by H, A, and the combined treatment (AH). However, no superimposition effect of H and A on the growth of S. hainanensis was observed under AH. The analysis of chlorophyll fluorescence, pigment content, and subcellular morphology indicated that the chloroplast morphogenesis (enlargement) along with the increase of chlorophyll fluorescence and pigment content of S. hainanensis might be a universal mechanism for promoting the growth of S. hainanensis. Transcriptomic profiles revealed the effect of elevated temperature on the response of S. hainanensis to acidification involved in the down-regulation of photosynthesis- and carbohydrate metabolism-related genes but not the up-regulation of genes related to antioxidant and ubiquitination processes. Overall, this study firstly reports the growth, morphology, and molecular response of the thermo-tolerant alga S. hainanensis to future climate changes, suggesting the predominance of S. hainanensis in its associated corals and/or coral reefs in the future.