Cyanobacteria, which explode in sunlight, are mixed with this sand and melted ice. As a cryoconite hole gets deeper, its bottom comes out of direct sunlight, which means there is less energy available for the cyanobacteria that live there. But, says Leidman, “when it rains, or when there is a strong thaw, the sediment from these cryoconites is washed away and washed into these supraglacial currents, where they accumulate in floodplains.”
Now the bacteria are exposed to all the sunlight they could ever dream of, especially because of the declining cloud cover over Greenland. As they proliferate, 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 degradation of dead bacteria and can provide UV protection for surviving bacteria. The latter is a glue-shaped mixture that helps cyanobacteria stabilize their local environments.
The second way, says Leidman, is that “they change the structure of the sediment, group it together so that it can retain water more easily and adhere more easily to surfaces. Therefore, just the fact that it is clustered means that it can absorb more sunlight. ”The accumulated buildup in supraglacial currents is significantly darker than the ice itself.
Flying with drones around the Greenland ice sheet, Leidman and colleagues found that sediment can cover up to 25 percent of the bottom of a stream. In addition, they estimated that without the bacteria acting to collect the sand, only 1.2 percent of the bottom would be covered, because the smaller loose particles would wash off instead of settling.
Researchers still face many unknowns. Given that cyanobacteria work in sunlight, they are likely to proliferate as Greenland warms. But how hot too hot? “We don’t really know if these bacteria will survive with higher temperatures or higher flows, or how rivers will change their shape,” Leidman says. But, he adds, “as the temperature rises, there is likely to be more bacterial growth. Therefore, although it is definitely not the main cause of increases in merger rates, it is likely to be a significant factor.