Mixed with this grit and melted ice are the cyanobacteria, which run on sunlight. As a cryoconite hole deepens, the soil moves out of direct sunlight, meaning less energy is available for the cyanobacteria that live in it. But, says Leidman, “when it rains, or there is a severe melting event, the sediment in those cryoconites is washed away and flushed into these supraglacial streams, where they accumulate in flood plains.”
Now the bacteria are exposed to all the sunlight they could ever dream of, especially given the decreased cloud cover over Greenland. As they multiply, the cyanobacteria have two ways of darkening that sediment. First, they produce a dark substance themselves, a combination of humic acids and what scientists call extracellular polymeric substances. The former results from the breakdown of dead bacteria and can provide UV protection for surviving bacteria. The latter is a gluey sludge that helps the cyanobacteria stabilize their local environment.
The second way, says Leidman, is that “they change the structure of the sediment, clump it together so that it can hold water more easily and attach to surfaces more easily. The mere fact that it’s clumped together means it can absorb more sunlight. “The accumulation in the supraglacial currents is considerably darker than the ice itself.
By flying drones around the Greenland ice sheet, Leidman and his colleagues found that the sediment can cover up to 25 percent of the bottom of a stream. (Check out their beautiful images below.) In addition, they estimated that without the bacteria collecting the grit, only 1.2 percent of the soil would be covered, as the smaller loose particles would wash away instead of settling.
However, the researchers are still grappling with many unknowns. Since the cyanobacteria work on sunlight, they are likely to multiply as Greenland warms. But how hot is it to warm? “We don’t really know whether these bacteria will survive at higher temperatures or faster flow rates, or how the rivers change shape,” says Leidman. But, he adds, “if the temperature rises, more bacteria growth is likely to occur. So while it is certainly not the main cause of rising melt rates, it is most likely a non-negligible factor. “