Abstract
The Arctic environment is particularly affected by global warming, and a clear trend of the ice retreat is observed worldwide. In proglacial systems, the newly exposed terrain represents different environmental and nutrient conditions compared to later soil stages. Therefore, proglacial systems show several environmental gradients along the soil succession where microorganisms are active protagonists of the soil and carbon pool formation through nitrogen fixation and rock weathering. We studied the microbial succession of three Arctic proglacial systems located in Svalbard (Midtre Lovénbreen), Sweden (Storglaciären), and Greenland (foreland close to Kangerlussuaq). We analyzed 65 whole shotgun metagenomic soil samples for a total of more than 400 Gb of sequencing data. Microbial succession showed common trends typical of proglacial systems with increasing diversity observed along the forefield chronosequence. Microbial trends were explained by the distance from the ice edge in the Midtre Lovénbreen and Storglaciären forefields and by total nitrogen (TN) and total organic carbon (TOC) in the Greenland proglacial system. Furthermore, we focused specifically on genes associated with nitrogen fixation and biotic rock weathering processes, such as nitrogenase genes, obcA genes, and genes involved in cyanide and siderophore synthesis and transport. Whereas we confirmed the presence of these genes in known nitrogen-fixing and/or rock weathering organisms (e.g., Nostoc, Burkholderia), in this study, we also detected organisms that, even if often found in soil and proglacial systems, have never been related to nitrogen-fixing or rock weathering processes before (e.g., Fimbriiglobus, Streptomyces). The different genera showed different gene trends within and among the studied systems, indicating a community constituted by a plurality of organisms involved in nitrogen fixation and biotic rock weathering, and where the latter were driven by different organisms at different soil succession stages.
Highlights
Due to global warming, a clear trend of glacial ice retreat has been observed in recent decades worldwide (Moon et al, 2018; Maurer et al, 2019)
We focused on two of the processes that shape forefield dynamics and nutrient bioavailability the most: nitrogen fixation, exploring nitrogenase genes, and rock weathering processes, looking at the obcA genes that are involved in the first step of oxalate biosynthesis (Nakata, 2011), genes involved in cyanide synthesis, and genes involved in siderophore synthesis and transport
Glacial forefields constitute ideal systems to study microbial successions, as they are characterized by different environmental gradients, such as an increase in total organic carbon (TOC), total nitrogen (TN), and vegetation coverage and a decrease in pH; these vary in different soil genesis stages from the bedrocks close to the ice edge to the more developed soil
Summary
A clear trend of glacial ice retreat has been observed in recent decades worldwide (Moon et al, 2018; Maurer et al, 2019) This rapid loss of the cryosphere is leading to an expansion of proglacial systems, exposing bedrocks that have been covered by ice for thousands of years (Fountain et al, 2012; Heckmann et al, 2016). Other key environmental and ecological processes, such as nitrogen fixation, take place in proglacial systems These environments are habitats for diverse diazotrophic communities that, using the enzyme nitrogenase, progressively enrich the soil with ammonia and bioavailable nitrogen sources to non-diazotrophic organisms (Bradley et al, 2014; Nash et al, 2018)
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