Sex, Scavengers, and Chaperones: Transcriptome Secrets of Divergent Symbiodinium Thermal Tolerances

Using transcriptomics, we show that Symbiodinium acclimation to elevated temperature involves up-regulated expression of meiosis genes followed by up-regulated expression of numerous reactive oxygen species scavenging genes and molecular chaperone genes. Our study connects Symbiodinium transcriptional regulation with physiological heat stress responses as well as known bleaching responses of corals harboring these same Symbiodinium. By uncovering these critical links, we greatly advance understanding of the bleaching susceptibility of corals, which is a key process responsible for global coral reef health. Overall design: We analyzed gene regulation in response to heat stress by two Symbiodinium C1 populations with contrasting thermal tolerances and bleaching thresholds. The thermo-sensitive South Molle (SM) Symbiodinium population and thermo-tolerant Magnetic Island (MI) Symbiodinium population were cultured in filtered seawater supplemented with Daigo IMK (Wako Pure Chemical Industries, Ltd.). Light was provided at an intensity of 30 µmol quanta m^-2 s^-1 (Crompton 36W cool white fluorescent tubes, 4000 K) with a 12:12 h light:dark cycle. For this study, 50 ml (~1 x 10^6 cells/ml) of each population were added to eight replicate culture flasks per population (n=4 for each temperature treatment). Two flasks per population were randomly assigned to each of four experimental incubators and acclimated at 27°C. After 10 days of acclimation, fresh media was supplied to the cultures. After an additional four days of acclimation (two weeks of acclimation total), two incubators were ramped on day 0 at 0.5°C/h to 32°C for the heat stress temperature treatment, while two incubators remained at 27°C. Temperature and light intensity in the incubators were monitored with HOBO data loggers. Cultures remained in exponential growth phase, determined by the average of three replicate haemocytometer counts for each sample measured throughout the experiment. Transcriptomes were sampled on day -1 (pre-heating), 9, and 13. SM_min250_nr.fasta: South Molle population de novo transcriptome, non-redundant transcripts (min transcript length 250 bp) SM_transcriptome.fasta: preliminary South Molle population de novo transcriptome (min transcript length 150 bp) MI_min250_nr.fasta: Magnetic Island population de novo transcriptome, non-redundant transcripts (min transcript length 250 bp) MI_transcriptome.fasta: preliminary Magnetic Island population de novo transcriptome (min transcript length 150 bp) Trinity_SM_min250nr_annotation_report.complete.5.xls: SM transcriptome annotation spreadsheet Trinity_MI_min250nr_annotation_report.complete.5.xls: MI transcriptome annotation spreadsheet day*nr.counts.txt: gene count matrix across samples