The cyanobacteria, which run on sunlight, are mixed in by this granular and molten ice. As a cryoconite hole deepens, the bottom moves out of direct sunlight, which means that less energy is available to the cyanobacteria that live in it. But, says Leidman, “when it rains, or when there is a heavy melting event, the sediment is washed out into the cryoconites and washed into these supraglacial streams, where it accumulates in floodplains.”
Now the bacteria are exposed to all the sunlight they could ever dream of, especially given the reduced cloud cover over Greenland. As they multiply, the cyanobacteria have two ways of darkening the sediment. First, they themselves produce a dark substance, a combination of humic acids and what scientists call extracellular polymeric substances. The former comes from the deterioration of dead bacteria, and can provide surviving bacteria with UV protection. The latter is a gummy outlet that helps the cyanobacteria to stabilize their local environments.
The second way, says Leidman, is that ‘they change the structure of the sediment, clinging together so that it can hold water more easily and stick to surfaces more easily. The fact that it is combined therefore means that it can absorb more sunlight. The accumulated build-up in the supraglacial currents is considerably darker than the ice itself.
By flying drones around the ice of Greenland, Leidman and his colleagues found that the sediment could cover up to 25 percent of the bottom of a stream. (Check out their beautiful footage below.) In addition, they estimated that without the bacteria that the grain collects, only 1.2 percent of the bottom would be covered, because the smaller loose particles wash away instead of sagging.
However, the researchers are still struggling with many unknowns. As the cyanobacteria run on sunlight, they are likely to multiply as Greenland warms up. But how hot is it not also hot? “We do not really know whether these bacteria will survive with higher temperatures or higher flow rates, or how the rivers will change shape,” says Leidman. But, he adds, “as the temperature rises, there will probably be more bacterial growth. While this is certainly not the biggest cause of the increase in melting speed, it is most likely an insignificant factor.”