Transcriptomic Analysis of Thermally Stressed Symbiodinium Reveals Differential Expression of Stress and Metabolism Genes.

Sarah L Gierz,William Leggat,Sylvain Forêt

doi:10.3389/fpls.2017.00271

Abstract

Endosymbioses between dinoflagellate algae (Symbiodinium sp.) and scleractinian coral species form the foundation of coral reef ecosystems. The coral symbiosis is highly susceptible to elevated temperatures, resulting in coral bleaching, where the algal symbiont is released from host cells. This experiment aimed to determine the transcriptional changes in cultured Symbiodinium, to better understand the response of cellular mechanisms under future temperature conditions. Cultures were exposed to elevated temperatures (average 31°C) or control conditions (24.5°C) for a period of 28 days. Whole transcriptome sequencing of Symbiodinium cells on days 4, 19, and 28 were used to identify differentially expressed genes under thermal stress. A large number of genes representing 37.01% of the transcriptome (∼23,654 unique genes, FDR < 0.05) with differential expression were detected at no less than one of the time points. Consistent with previous studies of Symbiodinium gene expression, fold changes across the transcriptome were low, with 92.49% differentially expressed genes at ≤2-fold change. The transcriptional response included differential expression of genes encoding stress response components such as the antioxidant network and molecular chaperones, cellular components such as core photosynthesis machinery, integral light-harvesting protein complexes and enzymes such as fatty acid desaturases. Differential expression of genes encoding glyoxylate cycle enzymes were also found, representing the first report of this in Symbiodinium. As photosynthate transfer from Symbiodinium to coral hosts provides up to 90% of a coral’s daily energy requirements, the implications of altered metabolic processes from exposure to thermal stress found in this study on coral-Symbiodinium associations are unknown and should be considered when assessing the stability of the symbiotic relationship under future climate conditions.

Highlights

Unicellular dinoflagellates form symbiotic relationships with reef-building corals and other marine invertebrates
Symbiodinium cell densities were significantly decreased in thermally stressed cultures from day 4 onward of the experiment (Figure 1B)
Through examining the physiological response of Symbiodinium cultures to elevated temperatures, and linking this to molecular processes that are altered under thermal stress we may begin to understand how reefs may be affected in the future

Summary

Introduction

Unicellular dinoflagellates (genus Symbiodinium) form symbiotic relationships with reef-building corals and other marine invertebrates. Modeling of bleaching patterns have shown that short periods of high temperature are highly stressful to corals (Berkelmans et al, 2004) with studies emulating these acute conditions in an attempt to understand mechanisms of coral bleaching (Iglesias-Prieto et al, 1992; Ralph et al, 2001, 2005; Takahashi et al, 2008). Mechanisms of thermal acclimation within Symbiodinium are unknown, with only two recent studies investigating the effect of moderate thermal stress on photobleaching (Takahashi et al, 2013; Fujise et al, 2014)

Methods

Results

Conclusion