Cyanobacteria, which work in sunlight, mix with this sand and melted ice. As a cryoconite hole deepens, its bottom moves out of direct sunlight, which means there is less energy available to the cyanobacteria that live there. But, says Leidman, “when it rains or there is a strong melt, the sediments of these cryoconites are washed away and washed in these supraglacial streams, where they accumulate in the floodplains.”
Now the bacteria are exposed to all the sunlight they could possibly want, especially given Greenland’s dwindling cloud cover. By proliferating, cyanobacteria have two ways of darkening this sediment. On the one hand, they themselves produce a dark substance, a combination of humic acids and what scientists call extracellular polymeric substances. The first comes from the breakdown of dead bacteria and can offer the surviving bacteria UV protection. The latter is a glue-like ooze that helps cyanobacteria stabilize their local environment.
The second way, says Leidman, is that “they change the structure of the sediment, aggregate it so that it can more easily retain water and more easily adhere to surfaces. So the simple fact that it’s sintered means that it can absorb more sunlight. The accumulation accumulated in supraglacial streams is distinctly darker than the ice itself.
By flying drones around the Greenland ice cap, Leidman and his colleagues discovered that sediment can cover up to 25% of a stream’s bottom. (Check out their beautiful pictures below.) Additionally, they estimated that without the bacteria acting to collect the sand, only 1.2% of the bottom would be covered, as the smallest loose particles would wash off instead of settling.
Researchers are still grappling with many unknowns. Since cyanobacteria work in sunlight, they will likely proliferate as Greenland warms. But how hot it is too hot? “We don’t really know if these bacteria will survive higher temperatures or higher flows, or how rivers will change shape,” says Leidman. But, he adds, “as the temperature rises, there will likely be more bacterial growth. So while this is certainly not the main cause of increased fusion rates, it is probably a significant factor. “