Roughly a tenth of the land surface of the Earth is covered by glacial ice. However, glaciers are retreating ever further and ever faster as a consequence of global warming. As they do so, they expose new landscapes which for millennia have been covered in ice, with extremely limited contact with air, light, and nutrients: conditions that are very challenging for life to survive. After glacial ice melts and retreats, various microbial lifeforms colonize the now accessible bedrock, accumulating nutrients and forming new soils and ecosystems. As soil can be a significant carbon store under the right circumstances, how exactly new soils form after the melting of glaciers is a question of great scientific and societal relevance.

The very first pioneers of the inhospitable terrain are microorganisms such as bacteria and fungi. "Microbes determine how much carbon and nitrogen can be stored in the young soils," explains Professor William Orsi from the Department of Earth and Environmental Sciences at LMU. "But little is known about the exact processes behind this nutrient stabilization through microbial activity." To better understand them, Orsi and his team studied soils in the Arctic that have recently been exposed. Their investigations were part of the dissertation of Orsi's doctoral student Juan Carlos Trejos-Espeleta and were carried out in close cooperation with Arctic biogeochemist and CNRS researcher Dr. James Bradley from the Mediterranean Institute of Oceanography in France. The study was funded by the German National Science Foundation (DFG) the Natural Environment Research Council (NERC), and the National Science Foundation (NSF). The results of the study, in which other researchers from the United States, the United Kingdom, and Switzerland were involved, have now been published in the journal Proceedings of the National Academy of Sciences (PNAS).

Timeline of colonization

The object of their analyses was the glacier foreland of Midtre Lovénbreen, a retreating valley glacier in the northwest of Spitsbergen. "In the high Arctic, the melting of glaciers is particularly dramatic," says Orsi. "Ice-free terrestrial environments are expanding there at an extraordinarily fast rate." James Bradley, who first worked at the site in 2013, said: "A decade ago I was drilling ice cores into the glacier. When we returned in 2021, the glacier had shrunk and instead of ice there were barren, seemingly lifeless soils." But upon laboratory-based analyses of these soils, the researchers found that they contain incredibly diverse communities of microbes.

The newly exposed areas are ideal for researching incremental changes in the soil. The closer soil is to the glacier margin, the younger it is; whereas the further away soil is, the more time life has had to colonize the terrain. Immediately beyond the ice, there is a zone of glacial rocky debris where no visible plant life exists, followed by moraines with isolated mosses and lichens, and after this only then do flowering plants and soil begin to form in an advanced stage of development. As such, receding glacier edges are ideal natural laboratories for observing the various stages of soil development. The ecosystems are some of the most pristine, delicate, and vulnerable habitats on the planet, and they are rapidly colonised by specialised microbes, even though they are subject to extremes in temperature, light, water and nutrient availability.

Orsi's team investigated the microbial composition of the various areas by means of DNA analysis while also measuring the cycling and flow of carbon and nitrogen. Through experiments involving isotope labeled amino acids, they were able to precisely follow the microbial assimilation and metabolism of organic carbon. "We were especially interested in what proportion of carbon microorganisms lock in the soil as biomass and how much they release back into the atmosphere as carbon dioxide," says Juan Carlos Trejos-Espeleta.

Pioneer fungi sequester carbon in the soil

Their main focus was on fungi -- a class of organism that is much better than bacteria at storing a lot of carbon in the soil and keeping it there. The ratio of fungi to bacteria is an important indicator of carbon storage: More fungi mean more carbon in the soil, while more bacteria generally lead to the soil emitting more CO₂. "In high Arctic ecosystems, the variety of fungi is particularly high compared to that of plants, which increases the likelihood that fungal communities could play a key role there as ecosystem engineers," reckons Orsi. Discovering more about the carbon assimilation processes of fungal and bacterial populations and carbon flow processes in the ecosystem is crucial for making accurate predictions about how terrestrial ecosystems in the Arctic will respond to future warming.

And indeed, the researchers were able to show that fungi -- or more precisely, specific basidiomycete yeasts -- play a decisive role in the early stabilization of the assimilated carbon. According to the study, they are the fungal pioneers in the young postglacial soils and make a decisive contribution to the enrichment of organic carbon. The research team found that these specialized fungi are not only able to colonise the harsh Arctic landscapes before any other more complex life, but that they also provide a foothold for soil to develop by building up a base of organic carbon which other life can use. In soils in medium and late stages, bacteria increasingly dominate amino acid assimilation, leading to a significant reduction in the formation of biomass and an increase in CO₂ from respiration. "Our results demonstrate that fungi will play a critical role in future carbon storage in Arctic soils as glaciers shrink further and more of Earth's surface area is covered by soil" summarizes Orsi.