Large variability in abundance of viruses that infect ocean microorganisms

This may affect the flux of carbon and nutrients on a global scale.

26 January 2016

Viruses infect more than humans or plants. For microorganisms in the oceans – including those that capture half of the carbon taken out of the atmosphere every day – viruses are a major threat. A recently published paper in the journal Nature Microbiology shows that the relative size of these viral populations varies over 100 times more than scientists believed until now. The study highlights another source of uncertainty governing climate models and other biogeochemical cycles. For the Netherlands, UvA professor by special appointment Corina Brussaard contributed to this research.

Influenza and measles immediately come to mind when most people think of viruses, but actually the bulk of world’s viruses infect microorganisms. Estimates suggest that a single liter of seawater typically contain more than ten billion viruses. To better understand this population, researchers conducted a meta-analysis of the microbial and virus abundance data that had been collected over multiple decades, by scientists based in the United States, Canada and Europe. 

Viruses and bacteria

Image of viruses (large number of small dots) and bacteria (small number of large dots). c:NIOZ

Large variability

Because viruses are parasites, it was thus far assumed that there was a constant relationship between the number of the number of viruses and the number of microbial cells they attack. For years, scientists had utilized a baseline ratio of 10 to 1 (ten times more viruses than microbes). However, collecting and re-examining more than 5,600 estimates of ocean microbial cell and virus populations recorded over the past 25 years, researchers have found that viral populations in seas and oceans vary dramatically from location to location, and at differing depths . The ratio of viruses to microbes varied from approximately 1 : 1 to 150 : 1. A marine environment with 100-fold more viruses than microbes may have very different rates of microbial recycling than an environment with far fewer viruses. This study challenges the notion of a uniform ecosystem role for viruses.

Consequences for food webs

Marine microorganisms play a critical role in capturing atmospheric carbon. A key target for viruses are tiny photoautotrophs – marine microorganisms that obtain their energy through photosynthesis in a process that takes carbon out of the atmosphere. What happens to the carbon these tiny organisms remove may be determined by whether they are eaten by larger grazing creatures such as animal plankton – or die from viral infections. When these photoautotrophs die from infections, their carbon is likely to remain in the top of the water column, where it can nourish other microorganisms. If they are eaten by larger creatures, their carbon is likely to sink into the deeper ocean as the grazers excrete the carbon in their feces, or when they when they die.

Thus, marine viruses have an important role in shunting some of the carbon away from the deep ocean and keeping it in the surface ocean. Quantifying the strength of the viral shunt of the marine food web remains a vital issue which directly affects which fraction of the total carbon mass in microbial cells is transferred to higher trophic levels, such as fish and shellfish, and which fraction is recycled in the microbial loop. Future research should focus on examining the relationship between ocean microorganisms and viruses at the scale of relevant interactions. Especially more ocean surveys are needed to fill in the many blanks for this critical part of the carbon cycle. Indeed, virus infections of microbes could change the flux of carbon and nutrients on a global scale.

Publication details

Charles H. Wigington, et al.(2016),'Re-examination of the relationship between marine virus and microbial cell abundances', Nature Microbiology, January. doi:10.1038/nmicrobiol.2015.24

NIOZ Royal Netherlands institute for Sea Research

Published by  Faculty of Science