As the Earth warms, carbon stored in thawing Arctic permafrost soil is gradually converted to carbon dioxide which leads to more warming. Soil microbes take part in the conversion process, but when carbon reaches open water, they are not the only part of the equation.
Arctic permafrost contains about half of all the organic carbon trapped in soil on Earth.
As permafrost thaws, bacteria turn the carbon into greenhouse gases, either carbon dioxide or methane. While much of this conversion process takes place in the soil, a large amount of carbon is washed out of the soils and into rivers and lakes where it's exposed to sunlight.
Sunshine's Down Side
Scientists wondered what was happening to all that carbon, so a research team turned their focus to the runoff waters and found a striking result.
In one study, they analyzed water released from permafrost landslides at 27 sites across the Arctic. They found that the carbon dioxide released by bacteria was 40 percent higher when the water was exposed to ultraviolet light than when it was dark. That's because ultraviolet light alters the soil carbon and speeds up the metabolism of microbes, increasing the amount of carbon dioxide they can produce.
In another study, they measured the carbon dioxide output from many types of rivers and lakes in the Alaskan Arctic. They found that, along with bacteria, sunlight is creating carbon dioxide through a process called photochemical oxidation.
In virtually all of the freshwater systems they measured, sunlight was always faster than bacteria at converting organic carbon into carbon dioxide. They estimate that this chemical reaction accounts for 30 percent of the carbon dioxide produced in lakes and streams of the tundra.
"Surface Exposure To Sunlight Stimulates CO2 Release From Permafrost Soil Carbon In The Arctic" (PNAS)
"Sunlight Controls Water Column Processing Of Carbon In Arctic Fresh Waters" (Science)