Transcriptomic responses to thermal stress and varied phosphorus conditions in Symbiodinium kawagutii

Abstract

20Symbiodinium species are essential symbionts of tropical reef-building corals and 21 the disruption of their symbiosis with corals as a consequence of seawater warming 22 and other stress conditions leads to the globally widespread coral bleaching. As coral 23 reefs live in the oligotrophic environment, Symbiodinium photosynthesis can also face 24 nutrient stress. How metabolic pathways in Symbiodinium respond to thermal stress 25 and phosphate depletion is poorly understood and underexplored for many species.26 Here we conducted RNA-seq analysis to investigate transcriptomic responses to 27 thermal stress, phosphate deprivation and glycerol-3-phosphate (Gro3P) replacement 28 in S. kawagutii. RNA-seq and bioinformatic analysis were conducted for the above-29 mentioned three treatments and a control. We identified 221 (2.04%) genes showing 30 no significant differential expression among all conditions, and defined them as 31 "core" genes of S. kawagutii, which mostly were in the Gene Ontology terms of 32 catalytic activity and binding. Using algorithms edgeR and NOIseq in combination, 33 we identified a set of differentially expressed genes (DEGs) for each treatment 34 relative to the control. Under heat stress 357 (4.42%) DEGs were found, with 35 predicted roles in active molecular (protein-protein/RNA/DNA) interaction, cell wall 36 modulation and transport (including nutrients, iron, and oxygen). About as many 37 DEGs (396, 4.73%) were identified under P deprivation while nearly double of that 38 (671, 8.05%) were detected under Gro3P utilization; in both cases most of the DEGs 39 were up-regulated and predicted to function in photosystem and defensome. Further 3 40 KEGG pathway comparison revealed different molecular responses between 41 phosphate deprivation and Gro3P utilization. Catalytic activity and binding seem to be 42 two important core functions in S. kawagutii. The most significant transcriptional 43 response in S. kawagutii to heat stress was regulation of molecular interaction, cell 44 wall modulation, and transport of iron, oxygen, and major nutrients, suggesting that 45 this species uses a unique mechanism to cope with heat stress, possibly conferring 46 thermal tolerance. The greatest transcriptomic impact of phosphate deprivation and 47 Gro3P replacement were the up-regulation of photosystem and defense. This study 48 provides new clues about molecular mechanisms underpinning responses 49 in Symbiodinium to temperature and nutrient stresses, which will generate new 50 hypotheses and set a new framework for future investigations. 51 52