Lazy microbes make possible the soils on which we all depend, a new study suggests. Finding a way to make more of these lazy, or "cheater," microbes might help fight global warming.
Earth's soils hold an estimated 2,500 gigatons of carbon, compared to less than 800 gigatons in the atmosphere. A 10 percent increase would eliminate global warming, but this is a daunting challenge. These soils have built up over thousands of years; moreover, our understanding of the process is surprisingly poor.
In Nature Communications, researchers at Austria's International Institute of Applied System Analysis identified microbes that sponge off those around them as unexpected keys to soil production.
When plants die, soil microbes break down their bodies, releasing the carbon dioxide the plants absorbed from the atmosphere during their lifespan. However, some of the carbon gets left behind, adding to the soil in which the microbes live. The same thing happens with leftover nitrogen. Gradually, this builds up into soil rich in the nutrients needed by future generations of plants.
As essential as this process is for the planet, it is not obvious why it happens. Individual microbes don't know that they need to leave something in the soil for the future. It was once assumed that the soil was made of material the microbes couldn't digest. However, recent studies have found that much of the material incorporated into soils can indeed be broken down, which begs the question of why it isn't.
Many of these soil microbes produce enzymes to split larger portions of dead plants into pieces small enough for them to absorb. Others, particularly at the periphery of decomposition patches, skip enzyme production and rely on enzymes released by their neighbors to start the decomposition process, digesting only material that has already been partly broken down.
"It's a strategy we see all over nature," senior author Dr. Ulf Dieckmann said in a statement. "Cheaters are everywhere – across many contexts, it's an evolutionarily successful strategy to save resources and thus become more competitive." Normally, such free riders impose costs on the system as a whole, but it seems soil is an exception.
First author Dr. Christina Kaiser added: “The presence of microbial cheaters ultimately reduces the total amount of enzymes produced by the microbial community, while the total amount of microbial biomass stays about the same."
Without enough enzymes to go around, some material gets incorporated into the soil, rather than processed back into carbon dioxide or ammonia. The nitrogen-rich products remaining are essential for plant growth; all terrestrial life depends on these cheating microbes being too lazy to produce their own enzymes.
The authors add that microbial communities are flexible, with the ratio of cheaters to working microbes adjusting to suit circumstances.
“Soil, however, is a complex system characterized by nonlinear interactions among functionally different microorganisms,” the paper notes, making its production challenging to control. However, if we can find ways to bolster cheaters, we may increase soil production, storing more carbon so less escapes into the air.
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