Viruses aren’t all bad: In the ocean, some help fuel the food web – a new study shows how

Virus. The word evokes images of illness and fears of outbreaks. Yet, in the oceans, not all viruses are bad news.

Some play a helpful, even critical, role in sustaining marine life.

In a new study, we and an international team of scientists examined the behavior of marine viruses in a large band of oxygen-rich water just under the surface of the Atlantic Ocean. What we discovered there – and its role in the food web – shows marine viruses in a new light.

Studying something so tiny

Viruses are incredibly small, typically no more than tens of nanometers in diameter, nearly a hundred times smaller than a bacterium and more than a thousand times smaller than the width of a strand of hair.

In fact, viruses are so small that they cannot be seen using conventional microscopes.

Decades ago, scientists thought that marine viruses were neither abundant nor ecologically relevant, despite the clear relevance of viruses to humans, plants and animals.

Then, advances in the use of transmission electron microscopes in the late 1980s changed everything. Scientists were able to examine sea water at a very high magnification and saw tiny, circular objects containing DNA. These were viruses, and there were tens of millions of them per milliliter of water – tens of thousands of times greater than had been estimated in the past.

A theory for how viruses feed the marine world

Most marine viruses infect the cells of microorganisms – the bacteria and algae that serve as the base of the ocean food web and are responsible for about half the oxygen generated on the planet.

By the late 1990s, scientists realized that virus activity was likely shaping how carbon and nutrients cycled through ocean systems. We hypothesized, in what’s known as the viral shunt model, that the marine viruses break open the cells of microorganisms and release their carbon and nutrients into the water.

This process could increase the amount of nutrients reaching marine phytoplankton. Phytoplankton provide food for krill and fish, which in turn feed larger marine life across the oceans. That would mean viruses are essential to a food web that drives a vast global fisheries and aquaculture industry producing nearly 200 million metric tons of seafood.

Watching viruses in action

In the new study in the journal Nature Communications led by biologists Naomi Gilbert and Daniel Muratore, our international team demonstrated the viral shunt in action.

The team took samples from a meters-thick band of oxygen that spreads for hundreds of miles across the subtropical Atlantic Ocean. In this region, part of the Sargasso Sea, single-celled cyanobacteria known as Prochlorococcus dominate marine photosynthesis with nearly 50,000 to upwards of 100,000 cells in every milliliter of seawater. These Prochlorococcus can be infected by viruses.

By sequencing community RNA – molecules that carry genetic instructions within cells – our team was able to look at what nearly all viruses and their hosts were trying to do at once.

We found that the rate of virus infection in this oxygen-rich band of the ocean is about four times higher than in other parts of the surrounding ocean, where cyanobacteria don’t reproduce as quickly. And we observed viruses causing massive infections in Prochlorococcus.

The viruses were attacking cells and spilling organic matter, which bacteria were taking up and using to fuel new growth. The bacteria respired away the carbon and released nitrogen as ammonium. And this nitrogen appears to have been stimulating photosynthesis and the growth of more Prochlorococcus cells, resulting in greater production that generated the ribbon of oxygen.

The viral infection was having an ecosystem-scale impact.

Understanding the microscopic world matters

Viruses can cause acute, chronic and catastrophic effects on human and animal health. But this new research, made possible by an open-ocean expedition supported by the National Science Foundation, adds to a growing range of studies that demonstrate that viruses are central players in how ecosystems function, including by playing a role in storing carbon in the deep oceans.

We are living on a changing planet. Monitoring and responding to changes in the environment require an understanding of the microbes and mechanisms that drive global processes.

This new study is a reminder of how important it is to explore the microscopic world further – including the life of viruses that shape the fate of microbes and how the Earth system works.

Steven Wilhelm's work on this study was supported by The National Science Foundation, The National Institute of Environmental Health Science, the Simons Foundation and the Allen Family Philanthropies.

Joshua Weitz's work on this study was supported by The National Science Foundation, the Simons Foundation, and the Blaise Pascal Chair of the Île-de-Paris Region.