Viruses that infect other microbes may influence the movement of more than a billion tonnes of carbon in soil, according to the first attempt at quantifying their role in one of the planet’s main carbon stores.
“While there are still gaps, we’re understanding that viruses can have a huge impact on soil carbon,” says at Pacific Northwest National Laboratory in Washington state.
Earth’s soils are packed with an estimated 2.5 trillion tonnes of carbon, contained mostly within decaying plant and microbial mass. Exactly how much of this carbon is released back into the atmosphere as carbon dioxide, with consequences for the climate, depends on microbial activity.
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Such activity is in turn shaped by an uncountable number of viruses – the top metre of soil alone contains an estimated 3.8 million trillion trillion viral particles. In the ocean, viruses are known to influence carbon storage by killing plankton that sink to the bottom, and a similar process could occur in soil, says Hofmockel. But how this works in a less fluid environment remains poorly understood.
Hofmockel and her colleagues gathered existing research on the soil virosphere to make what she calls a “first stab” estimate of how viruses contribute to the terrestrial carbon cycle.
Despite their vast numbers, soil viruses are so small that all of their bodies contain a middling 100 million tonnes or so of carbon, the researchers estimate. “Yet they can have this outsized impact on carbon cycling,” says Hofmockel.
One way is by splitting open the microbial cells they infect. The team estimates this process moves at least 1.2 billion tonnes of carbon from living cells to the soil’s pool of dissolved organic matter, assuming the microbes are killed by viruses at a similar rate as in the ocean. That’s more carbon than is contained within all land animals. “If they are killing bacteria, those dead cells are contributing to persistent carbon,” says Hofmockel.
While the team’s estimates raise interesting questions, so little is known about soil viruses that “empirical constraints on these calculations are seriously lacking”, says at the University of California, Davis, adding clearer constraints will take many years of research.
For example, viruses could be helping store even more carbon if the microbes they kill end up in the deeper and larger pool of dead cells, or “necromass”, in soil – but the rate at which this happens remains unknown. Another big variable left out of the count is the effect viruses have on soil fungi, which make up the majority of the microbial biomass. Also unclear is how environmental changes that boost viral activity, such as rising temperatures, influence all of this.
at the University of Colorado Boulder agrees such yawning knowledge gaps make it difficult to generalise about viruses in soil. “At the same time, the complexities of these relationships and the large number of fundamental knowledge gaps are thrilling,” he says. “We don’t need to travel to outer galaxies to find important scientific problems to solve. We can just look at the ground beneath our feet.”
Global Change Biology
