Evidence for a role of viruses in the thermal sensitivity of coral photosymbionts

Symbiodinium, the dinoflagellate photosymbiont of corals, is posited to become more susceptible to viral infections when heat-stressed. To investigate this hypothesis, we mined transcriptome data of a thermosensitive and a thermotolerant type C1 Symbiodinium population at ambient (27 °C) and elevated (32°C) temperatures. We uncovered hundreds of transcripts from nucleocytoplasmic large double-stranded DNA viruses (NCLDVs) and the genome of a novel positive-sense single-stranded RNA virus (+ssRNAV). In the transcriptome of the thermosensitive population only, +ssRNAV transcripts had remarkable expression levels in the top 0.03% of all transcripts at 27 °C, but at 32 °C, expression levels of +ssRNAV transcripts decreased, while expression levels of anti-viral transcripts increased. In both transcriptomes, expression of NCLDV transcripts increased at 32 °C, but thermal induction of NCLDV transcripts involved in DNA manipulation was restricted to the thermosensitive population. Our findings reveal that viruses infecting Symbiodinium are affected by heat stress and may contribute to Symbiodinium thermal sensitivity.

Levin RA, Voolstra CR, Weynberg KD, van Oppen MJH (2016) Evidence for a role of viruses in the thermal sensitivity of coral photosymbionts. The ISME Journal. Available: http://dx.doi.org/10.1038/ismej.2016.154.

The Australian Institute of Marine Science supplied the Symbiodinium strains (aims-aten-C1-WSY, aims-aten-C1-MI) used in this study. Yi Jin Liew provided assistance with the bit score analysis. Rhys T Graham provided technical support for MATLAB. The Centre for Marine Bio-Innovation at the University of New South Wales, King Abddullah University of Science and Technology (KAUST), The Joyce W Vickery Scientific Research Fund Grant awarded to Rachel A Levin from the Linnean Society of New South Wales, and Future Fellowship No FT100100088 awarded to Madeleine JH van Oppen from the Australian Research Council contributed financial support. Raw, processed and annotated sequencing data are available through NCBI GEO (accession: GSE77911). The novel +ssRNAV genome (TR74740|c13_g1_i1) and highly related, partial +ssRNAV genome (TR74740|c13_g1_i2) discovered in this study have been deposited in GenBank (accession nos: KX538960 and KX787934, poly(A) tails were trimmed from RNA-Seq reads prior to de novo assembly of transcripts).

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