When the going gets hot the dinoflagellates (sometimes) get going, how viruses might affect coral symbionts

Corals represent more than meets the eye, they host intricate and interesting communities composed of dinoflagellates (also referred to as zooxanthellae), and a suite of microbes that include bacteria, archaea, fungi, protists, and viruses. One such dinoflagellate that often shares a symbiotic relationship with coral is Symbiodinium.

Image courtesy of wikicommons

Image courtesy of wikimedia commons

These algal symbionts are essential to overall reef function and are present in incredible numbers, a healthy coral reef could harbor more than 1010 algal symbionts per meter squared (!!!). These organisms are essential and understanding their role and unique relationship to coral is important, as it can be screwed up by heat stress, ultimately leading to bleaching (eg. when Symbiodinum cells get the heck out of Dodge and the coral then looks completely white).
Simbiodinium fitti. Image: Todd C. LaJeunesse, Penn State University

Simbiodinium fitti. Image: Todd C. LaJeunesse, Penn State University

However, the mechanism triggered by heat stress that leads to bleaching isn’t completely understood. While it was previously suggested that viruses might somehow play a part in such events, a recent short communication published last week in the ISME Journal presented interesting evidence from Levin and colleagues that it is likely that viral infections lead to thermal sensitivity in Symbiodinium and, ultimately, bleaching.
In a previous study, two Symbiodinium populations were cultured from Acropora tenuis at two sites on the Great Barrier Reef. The population from South Molle (SM) Island was thermosensitive and didn’t grow well at 32ºC, while the “thermotolerant” population from Magnetic Island (MI) was cool as a sea cucumber at this elevated temperature. Replicates of both populations (SM and MI) were then maintained at 27ºC or 32ºC and the transcriptomes (sequences of all of the mRNA extracted from the population) were obtained.

Our study exemplifies how RNA-Seq can be used to gain valuable insight into resident viruses.

Levin and colleagues found that only in the thermosensitive population (SM) were incredibly high expression levels of a new RNA virus observed at 27ºC, while at 32ºC anti-viral transcripts increased. At the same time, there was basically no change in the low level of virus RNA expressed in the thermotolerant MI population.

Thus, we conclude TR74740|c13_g1_i1 to be the RNA genome of a novel +ssRNAV, making this the first discovered genome of any virus infecting Symbiodinium

This study is cool because it presents the FIRST genome of a Symbiodinium virus, and provides a possible explanation for thermal sensitivity in coral symbionts that can lead to bleaching events. Understanding the intricate relationships that underlie coral reef function is important as we deal with climate change and attempt to protect such incredibly essential, not to mention beautiful, ecosystems.
Baker, A.C., 2003. Flexibility and specificity in coral-algal symbiosis: diversity, ecology, and biogeography of Symbiodinium. Annual Review of Ecology, Evolution, and Systematics, pp.661-689.
Levin, R.A., Voolstra, C.R., Weynberg, K.D. and van Oppen, M.J.H., 2016. Evidence for a role of viruses in the thermal sensitivity of coral photosymbionts. The ISME Journal.

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