The Greenland Ice Sheet is the second largest land ice mass on Earth. In the last 25 years, dramatic melting has been observed here. One driving force for this, which has received little attention so far, is the effect of glacier ice algae. They darken the surface and thus reduce the reflection of sunlight. The ice melts faster. Researchers from the University of Leeds (UK) led by postdoctoral fellow Jenine McCutcheon (now University of Waterloo, Canada) and Liane G. Benning, German Research Centre for Geosciences Potsdam GFZ, have identified an important nutrient source for the glacier ice algae: phosphorus from locally generated mineral dust. This finding helps to better predict future developments of algal blooms and ice melt and to optimise climate models. The study was published today in Nature Communications.
Greenland's dark ice
The Greenland Ice Sheet covers about 80 per cent of the land mass of this huge island. But its ice is not as white as one might think. On the west coast, a 30-kilometre-wide dark strip, the so-called "Dark Zone", can be observed. It contains not only soot and mineral dust, but also glacier snow and ice algae. The latter bloom in the summer season and turn dark purple - with fatal consequences for the ice: they reduce the albedo effect, i.e. the reflection of sunlight, and thus accelerate surface melting. Because the Arctic summers are getting warmer and longer, the algal blooms are also increasing in length and extent. An international and interdisciplinary team led by Liane G. Benning from the GFZ and formerly at the University of Leeds has been observing this for several years on a transect across the southwestern edge of the Greenland ice sheet. The team’s measurement campaigns that have now been evaluated took place in the summers of 2016 and 2017 as part of the now completed "Black & Bloom" project. These results also helped lay the foundations for the new ERC Synergy project "DEEP PURPLE", which has started in 2020 and now addresses the complex mechanisms that trigger these blooms.
The role of the glacier ice algae
The researchers wanted to understand what promotes the growth of the glacier ice algae and how these in turn impact on the ice albedo. In their study they showed that although biomass only makes up about five percent of the "dark mass" on the ice, the other 95 percent being soot and mineral dust, it is the dark pigments of glacier ice algae that are essentially responsible for lowering the albedo.
Tracking down the nutrients
The current study focused on the algae's nutrient supply: What do they feed on and how they get their "food"? The challenge: In the mixture of snow, ice and water with bacteria, fungi and algae on the one hand and soot and mineral dust on the other, it is necessary to identify and quantify the individual organic and mineralogical components in order to decipher their complex interactions. Therefore, many different yet complementary measurements and experiments were carried out.
On site, the researchers measured the photosynthesis activity of the glacier ice algae - quasi as a vital function - depending on the administration of various nutrients, including phosphorus and nitrogen species. They only found a significant increase in photosynthesis when they supplied the algae with phosphorus. "This shows us that phosphorus is the limiting nutrient here. The more of it is additionally available, the stronger the algae grow. Additional nitrogen does not cause further proliferation," says geomicrobiologist Jenine McCutcheon.
To verify this influence and find a natural source of phosphorus accessible to the algae, she and her colleagues collected snow and ice samples, melted them, filtered them, and dried and analysed the filtrate. Important indicators are the elements carbon C, nitrogen N and organic phosphorus P(org). Measured in balanced nourished microorganisms, they are in a very specific ratio to each other. Here, too, the team’s data showed that the more mineral phosphorus was contained in the samples, i.e. was available to the algae as food in principle, the closer the quantity ratio C:N:P(org) was to the ideal value.
While the nutrient addition experiments were done on the ice, most of the other analyses took place in special laboratories of the participating institutions. Using mass spectrometry and flow injection analysis carbon, nitrogen and phosphorus were analysed in the biomass, while X-ray diffraction was used to trace the composition of the mineral phases. This was complemented by sequencing data that helped to identify the different types of microbes in the samples.
Source of the phosphorus
Where the mineral phosphorus came from could be derived from the exact chemical composition of the mineral dust. Local rocks were also examined for comparison. The mineral hydroxyapatite was identified as the source of the phosphorus (P), with the chemical composition Ca5(PO4)3(OH). It comes from local rocks and not from Asia or Africa. "Mineral dust can be carried over thousands of kilometres by the wind. But this one is from the local area. As dry areas in northern latitudes become even drier due to climate change, we can expect more dust to be transported and deposited on the Greenland ice sheet, further fuelling the algal bloom," says atmospheric scientist Jim McQuaid of the University of Leeds a co-author of the paper.
Self-amplifying effects - important for climate models
Overall, there is a self-reinforcing effect: the stronger the algal bloom, the stronger the melt. This in turn releases more nutrients that were previously frozen in the ice, which in turn leads to increased algal blooms. "Currently, these important effects are not considered in either ice mass loss modelling or climate modelling. Our quantitative results can change that. They will help predict where algal blooms can be expected in the future and to what extent this will affect melt," says Liane G. Benning.
The glacier ice algae can cover up to 78 percent of the bare ice in the Dark Zone. However, the researchers have observed large fluctuations in the intensity and spread of the bloom over the course of a season and from year to year – which still makes an exact prediction difficult. Therefore, it is all the more important to know other influencing factors more precisely, Benning emphasises.
Further research: DEEP PURPLE
This is on the agenda of the DEEP PURPLE project, which has been running since 2020, and which she obtained as part of a Synergy Grant from the European Research Council ERC. Her outlook: "We need a better understanding of algal blooms throughout the year. Phosphorous is just one factor among others. In particular, it is important to find out what restarts algal growth after 'hibernation'. And how the algae also influence the ice and its structure, because that also plays an important role in the albedo effect."
Original Study: Jenine McCutcheon, Stefanie Lutz, Christopher Williamson, Joseph M. Cook, Andrew J. Tedstone, Aubry Vanderstraeten, Siobhan A. Wilson, Anthony Stockdale, Steeve Bonneville, Alexandre M. Anesio, Marian L. Yallop, James B. McQuaid, Martyn Tranter & Liane G. Benning. 2021. Mineral phosphorus drives glacier algal blooms on the Greenland Ice Sheet. Nature Communications. DOI: 10.1038/s41467-020-20627-w
Interview with Liane G. Benning on DEEP PURPLE in GFZeitung (Dec. 2020, in German)
Prof. Liane G. Benning
Section head Interface Geochemistry
Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Phone: +49 331 288-28970