Trajectories of freshwater microbial genomics and greenhouse gas saturation upon glacial retreat.

In this study, we did fieldwork in the remote high Arctic close to 80 oN at Svalbard as well as in the alpine, glaciated landscapes of Norway. The question we address here was how the aquatic microbial communities change along a glacial retreat gradient combined with analysis of greenhouse gases.
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A nice aspect of natural sciences, is the possibility, under lucky circumstances, to combine fieldwork at scenic sites with serious scientific issues. In this study, we did fieldwork in the remote high Arctic close to 80 oN at Svalbard as well as in the alpine, glaciated landscapes of Norway. Few places in the world have the temperature increase been as striking as in these areas, putting its toll on the local glaciers. In fact, the Arctic is warming four times faster than the global average in recent decades; in some high-Arctic areas even exceeding 2 °C per decade.

Moving glaciers and permafrost melt provides organic matter to arctic lakes and fjords

The question we set out to address here was how the aquatic microbial communities change along a glacial retreat gradient (chronosequence), and this was done by sampling DNA from transects from the glacier to the fjord (Svalbard) or to the forested valley (mainland alpine and subalpine Norway). In parallel, we analysed greenhouse gases (GHGs; CO2, CH4 and N2O) at the same stations to link microbial diversity and their functional genes and metabolic pathways to greenhouse gas production.

Exotic (and potentially risky) field-work

The alpine areas on mainland Norway offer no real risk beyond bad weather and glacier cracks. It does however offer magic scenarios of barren landscapes and glaciers, rivers, waterfalls, and patches of flowering plants, sometimes contrasted with green and lush vegetation further down the valley. The high Arctic offers an even more breathtaking landscape with its peaks, numerous glaciers, blue ice floating on the fjord, and amazing birdlife. There is wildlife too, the specific subspecies of reindeer – and the real risky element; polar bears.

When doing the sampling, on foot and bikes, we had to carry flaring pistols and guns together with dry-shippers (cooled by liquid nitrogen) for snap-freezing of DNA samples, gear for water and gas sampling, and numerous bottles for supporting samples. Heavyweight, but rewarded by strolls in landscapes where you do not encounter people, plus really “new” landscapes and lakes from which the glacier has recently withdrawn. And we had close encounters with polar bears in fact, but no attacks. However, a kind of experience that adds flavour to the already exotic fieldwork. Fieldwork in such remote wilderness also allows for reflections of what we risk losing if the heat continues to rise.

Sampling of gas and eDNA at high Arctic sites, Svalbard

After many years of research in the high Arctic we can witness the major change, not only in glacial retreat but also a progressing permafrost thaw which in places generates CH4 seepage of “fossil” methane from the coal deposits below. Another trend is the striking increase in breeding geese populations that have a strong impact on the fragile ecosystems by their grazing and defecation, fertilizing land and ponds. This increase is also to a large extent driven by climate change, offering an extended breeding season and new breeding grounds for these birds.

 

System age, productivity, and goose impact drive community shifts and GHG production

Leaving those scenic mountains behind we got into the labs and in front of our computers in Oslo. There we used whole genome sequencing and reconstructed microbial genomes to infer relationships between microbial metabolism and greenhouse gas concentrations along 5 distinct lake chronosequences. Our study revealed the genomic succession from chemolithotrophy to photo- and heterotrophy and increases in methane supersaturation in freshwater lakes upon glacial retreat. Arctic lakes at Svalbard also revealed strong microbial signatures from nutrient fertilization by birds. Although methanotrophs were present and increased along lake chronosequences, methane consumption rates were low even in supersaturated systems. The same major patterns were revealed both in the high Arctic and mainland alpine sites. Nitrous oxide oversaturation and genomic information suggest active nitrogen cycling across the entire deglaciated landscape. In the high Arctic, the increasing geese populations served as major modulators at many sites. Our findings show diverse microbial succession patterns, and trajectories in carbon and nitrogen cycle processes representing a positive feedback loop of deglaciation on climate warming.

This study forms part of our attempts at the Centre for Biogeochemistry in the Anthropocene at University of Oslo where we study carbon cycling in northern ecosystems, both northern permafrost and the boreal biome. It is important to reveal feedback mechanisms that affect the climate, and well as to link ecosystems (land to lakes and rivers). More available organic carbon as we see in many of the northern aquatic ecosystems will boost the microbial greenhouse gas production, feeding back on the climate. It is also important to go beyond descriptive studies by understanding how microbial metabolism shifts and responds to these changes.

https://www.nature.com/articles/s41467-023-38806-w

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