Warming oceans intensify viruses that kill coral reefs

A groundbreaking three-year study has found that viruses may increase their attacks on the photosynthetic algae that live inside reef-building corals during marine heat waves. The study, published in the journal ISME Communications, analyzed the reef-wide prevalence, persistence, triggers, and health impacts of “dinoflagellate-infecting RNA viruses” (dinoRNAVs), which are single-stranded RNA viruses that infect the symbiotic algae living inside corals.

Lead author Lauren Howe-Kerr, a postdoctoral researcher at Rice University, said that coral and marine disease researchers are paying closer attention to coral viruses following evidence suggesting that viral infections of symbiotic dinoflagellates might be responsible for stony coral tissue loss disease (SCTLD). This is one of the deadliest coral diseases ever recorded and has been decimating reefs in Florida and the Caribbean since it was first identified in 2014.

“While this study is not focused on SCLTD, it builds our understanding of coral viruses, and particularly RNA viruses that infect coral endosymbionts,” said Howe-Kerr. She co-authored the study with more than a dozen colleagues from Rice, Northeastern University, the University of Oregon, the University of the Virgin Islands, Rutgers University, Oregon State University, George Mason University, New Zealand’s National Institute of Water and Atmospheric Research, and the Mote Marine Laboratory’s Coral Reef Research and Restoration Center in Summerland Key, Florida.

“Our work provides the first empirical evidence that exposure to high temperatures on the reef triggers dinoRNAV infections within coral colonies, and we showed those infections are intensified in unhealthy coral colonies,” Howe-Kerr said.

The study was conducted at the Moorea Coral Reef Long-term Ecological Research station on the Pacific Ocean island of Moorea in French Polynesia. Samples from 54 coral colonies around the island were collected twice a year between August 2018 and October 2020. The warmest water temperatures during that span were in March 2019. Reefs across the island suffered heat-related stress during this period, including widespread bleaching.

The study sites were located in a variety of reef zones that were subject to different kinds of environmental stress. For example, ocean-facing forereefs are deeper, with cooler and more consistent water temperatures, while near-shore fringing reefs in lagoons are subjected to the highest temperatures and greatest temperature variability.

During her doctoral studies in the Rice lab of marine biologist Adrienne Correa, Howe-Kerr conducted a study on viral dynamics in coral colonies that was recently published. Howe-Kerr earned her doctorate from Rice in 2022 and completed a yearlong Ocean Policy Fellowship at the National Science Foundation. According to Correa, the sampling and analysis for the study were conducted over a three-year period and required a tremendous team effort to locate and sample the same coral colonies twice a year. The pandemic added to the complexity of the study by preventing sampling in March 2020. However, despite the challenges, the team was able to gather significant data about reef-wide viral dynamics.

Previous studies indicated that corals “harbor lots of diverse viruses,” but little was known about how specific viral types were distributed across a reef. A 2022 study led by Correa’s former student, Carsten Grupstra, revealed that the viral activity of a single viral group, dinoRNAVs, increased in corals under heat stress in tank-based experiments. Correa explained that the new study builds on this finding by demonstrating that heat-induced increases in viral production also occur in the ocean. The research presents one of the first observations of how dinoRNAVs behave in time and space over reefs and reef zones.

Howe-Kerr noted that the study characterized the diversity of dinoRNAVs and their prevalence in colonies across multiple years and reef environments. The team detected dinoRNAV infections in over 90 percent of the sampled colonies at some point during the three years, and the composition and diversity of viruses in those infections differed among reef zones. This indicates that environmental conditions play a role in the dynamics of these outbreaks.

Correa revealed that in the three-year experiment, all 54 colonies survived, but there was a partial mortality rate of 50 percent. The forereefs facing the ocean were the hardest hit, and they were almost three times more likely to experience partial mortality compared to the corals in the fringing reefs. This may be attributed to the fact that the corals in the fringing reefs are more accustomed to dealing with high temperatures in shallower waters close to the shore.

According to Correa, there was a wider variety of RNA viruses found in heat-stressed colonies in 2019, suggesting an increase in viral production. This pattern was most prominent in colonies that suffered partial mortality, indicating that there may be specific host-virus interactions that could drive ecosystem impacts.

Correa pointed out that “viral productivity will likely increase as ocean temperatures continue to rise,” emphasizing the need to understand host-virus interactions. Such interactions could potentially alter the foundational symbiosis that supports coral reef ecosystems.

Climate change is having a significant impact on coral reefs worldwide. The rise in global temperatures and increased levels of carbon dioxide in the atmosphere are leading to the following consequences for coral reefs:

1. Coral Bleaching: The increase in sea surface temperatures is causing coral bleaching, a process in which corals expel the symbiotic algae living in their tissues, leaving them white and vulnerable to disease.
2. Ocean Acidification: Increased carbon dioxide in the atmosphere is being absorbed by the ocean, leading to ocean acidification. This makes it difficult for corals to form their calcium carbonate skeletons and can ultimately lead to their death.
3. Sea Level Rise: As sea levels rise, coral reefs are becoming more exposed to air and are experiencing more frequent and severe damage from storms and waves.
4. Changing Ocean Chemistry: The changing ocean chemistry due to climate change is causing changes in the chemistry of seawater, which is affecting the ability of corals to grow and reproduce.

Overall, the combination of these impacts is leading to significant declines in coral reef health and biodiversity. It is important to take action to mitigate the effects of climate change on coral reefs and protect these vital ecosystems.

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