Dark algae could accelerate melting of Greenland ice sheet

Dark algae that grow on the surface of Arctic ice sheets are likely to expand their range in the future, a trend that will exacerbate melt, sea level rise and climate warming.

“These algae are not a new phenomenon,” says James Bradley at the Mediterranean Institute of Oceanography in Marseille, France. “But if they bloom more intensely, or the bloom is more widespread, then it would be an important thing to consider in future projections of sea-level rise.”

Greenland’s ice sheet, which covers most of the island, is rapidly melting due to rising temperatures, making it the biggest single contributor to sea-level rise worldwide.

The alga Ancylonema blooms on ice patches, called ablation zones, which are exposed as the snowline recedes on the ice sheet each summer. The blooms darken the ice, reducing its reflectivity and absorbing more heat, thereby enhancing melt on affected ice by an estimated 10 to 13 per cent.

To better understand this feedback loop, Bradley and his colleagues collected Ancylonema samples from the south-west tip of the ice sheet and examined their cells with advanced imaging techniques.

The results revealed that the algae are highly adapted to nutrient-poor conditions, suggesting that they could make inroads into ice at higher elevations, where nutrients are scarce.

Climate warming is already causing the snowline to retreat farther over time, exposing more ice, which is less reflective than snow and therefore accelerates melt. Ice algae add yet another layer to these interactions that will need to be accounted for in future climate projections.

“We’ve been studying glacier algal blooms for several years now but one of the big questions remaining has been how they are able to grow to such high numbers in such nutrient-poor ice,” says Christopher Williamson at the University of Bristol, UK, who was not involved in the project. “One big part of understanding this puzzle is how much nutrient is needed by the glacier algal cells and whether they are able to efficiently uptake and store the scarce nutrients available in the system. This study does a great job of demonstrating these things using cutting-edge methodologies.”