Going viral: Virus abundances linked to coral bleaching

Written By Danielle Moloney

Introduction

In the midst of a global pandemic caused by the SARS-CoV-2 virus (also known as the novel coronavirus, or Covid-19), coral reefs are probably one of the last things on your mind when you hear the word “virus.” However, a new paper from scientists at Oregon State University suggests otherwise; their research shows a link between the abundance of viruses and severity of bleaching in corals.

Reefs and Viruses 

One of the many characteristics that make corals unique is the symbiotic relationship they have with the algae living inside them. Viruses are already known to be very abundant in corals, in which they are able to infect both the host and the symbiont living inside, but the key here is abundance. While healthy reefs had viruses, bleached ones showed far higher levels. In order to draw this conclusion, Messyasz et al. collected pairs of bleached and unbleached corals living in similar environments,and then determined how similar the viral composition was between the two. The researchers went a step further to test how the genetic makeup of the corals changed under UV stress. Here, they saw that virus-like genes were in fact upregulated under UV stress, which supports the hypothesis that viral infections may be at play during coral bleaching.

Researchers used a scale, shown above, of colors to determine the extent of bleaching in each coral sample. Here, #1 indicates a bleached coral, and #6 indicated a healthy coral, with #2-#5 depicting a mid-range of reef health. Photo courtesy of Messyasz et al. 2020.

They found that bleached corals exhibited a higher abundance of eukaryotic viruses, whereas unbleached corals showed a higher abundance of bacteriophage viruses. The difference is in the host: eukaryotic viruses infect non-bacterial organisms, and bacteriophage viruses infect and replicate within bacteria. The bleached corals in this study were also found to have higher levels of “giant viruses” (known as NCLDV: nucleocytoplasmic large DNA viruses). These are present in healthy corals as well, but in far less abundance. NCLDV are named as such because they are complex viruses with double stranded DNA. These have the capability to infect a range of hosts, from single celled organisms all the way up to humans. Messyasz suggests that this means the virus may play a role, more specifically, in the onset of bleaching or it’s severity. According to Messyasz, this study is the first to “…generate [the] draft genome of a giant virus that might be a factor in coral bleaching.”

NCLDV like particles are seen here in coral samples. The virus like particle (VLP) is indicated with the black arrow, and surrounded by a vacuole like structure. Panel B shows another VLP exhibiting characteristics similar to those found in NCLDVs. Photo courtesy of Messyasz et al. 2020

Why does it matter?

As some of the most biologically significant structures on the planet, reefs are very well-studied systems, and yet the mechanisms that control coral bleaching have long eluded scientists. Coral bleaching is one of the main drivers of reef mortality, so research that aims to clarify the source of bleaching is imperative.

Science has long been stumped by the initiator of bleaching: could it be the thermal tolerance of the coral itself maxing out under heat stress, the thermal tolerance of the symbiont algae, or could the differential resilience from individual to individual? Through their important work on the genome of viruses found in reefs, Messyasz et al. are tipping the scale closer to a world in which scientists can pinpoint the causes of coral bleaching.

The authors hypothesize that corals that become infected with viruses during times of moderate temperature stress experience earlier and more severe loss of both symbiont and host cells. This would explain why infected corals appear bleached earlier in a thermal stress event compared to their non-bleached “neighbors.” Hopefully the group can continue to pursue research in this arena to further investigate the origin of coral bleaching during times of heat stress.

Please contact the author with any questions: dmoloney@fandm.edu.

*An earlier version of this post stated that experiments on how coral genetic makeup changes under UV stress were conducted by Messyasz et al 2020. This research was conducted by Lawrence et al 2017.

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