Abiotic factors affecting the bacterial and fungal diversity of permafrost in a rock glacier in the Stelvio Pass (Italian Central Alps)
Introduction
Almost 25% of the Northern Hemisphere and 17% of the Earth's exposed land surface is underlain by permafrost (Gruber, 2012) that is defined as a material (ground or rock) with a temperature remaining at or below 0 °C for at least two consecutive years (Shur et al., 2011). Although Alpine permafrost in Europe is quite widespread especially in barren ground, steep bedrock slopes, debris slopes, screes, or morainic deposits, it is above all concentrated within rock glaciers.
Recent atmospheric warming (over the last century) in the European Alps has been roughly twice the global average (Böhm et al., 2001; Auer et al., 2007) dramatically reducing the extent of permafrost and thickening its active layer (Etzelmüller et al., 2020). Within the European Alps, the Stelvio area exhibits these effects in a particularly evident way, with important consequences on microbial ecosystems (Ponti et al., 2021). Despite their frozen status, many microbial species inhabiting permafrost cores are considered metabolically active, and this activity can obviously increase in the case of permafrost thawing (even temporary) (Bakermans et al., 2003; Rivkina et al., 2004, Rivkina et al., 2018; Steven et al., 2006; Tuorto et al., 2014). Therefore, recent studies have included the monitoring of microbial parameters in order to gain a more complete picture of the interactions between biotic and abiotic factors occurring in these changing ecosystems (Wagner et al., 2007; Hollesen et al., 2011; Graham et al., 2012; Donhauser and Frey, 2018).
Research carried out during the last decade has focused on the detailed characterisations of the microbial communities occurring in permafrost ecosystems in Arctic and Antarctic regions, and the Tibetan plateau; the observed microbial diversity was often associated with a habitat-specific distribution (Yergeau et al., 2010; Zucconi et al., 2012; Goordial and Whyte, 2014; Hu et al., 2015; Chen et al., 2017; La Ferla et al., 2017; Borruso et al., 2018; Donhauser and Frey, 2018; Papale et al., 2019; Xue et al., 2019; Sannino et al., 2020a). On the other hand, there is little knowledge about the microbial communities inhabiting the European Alpine permafrost. Frey et al. (2016) studied both bacterial and fungal diversity, finding many unknown taxa in the active layer (1.60 m depths) of the permafrost (Muot-da-Barba-Peider, Eastern Switzerland), suggesting that these microorganisms may have developed peculiar structural and functional adaptation mechanisms to survive and even proliferate in such ecosystems.
The abiotic parameters affecting the microbial diversity inhabiting permafrost have previously been investigated (Zhang et al., 2013; Frank-Fahle et al., 2014; Gittel et al., 2014a; Borruso et al., 2018; Sannino et al., 2020a). Furthermore, recent studies have found that bacterial and fungal communities occurring in the Alpine active layer and permafrost were differently affected by an increase in temperatures (Donhauser and Frey, 2018; Luláková et al., 2019). However, despite the above studies, the knowledge about which abiotic factors may be able to affect the microbial diversity in Alpine permafrost remains largely incomplete. Considering the probability that Alpine glaciers may disappear in the coming decades, the importance of reducing these gaps in knowledge is becoming evident (Zemp et al., 2006). Hence, the study of microbial diversity inhabiting Alpine permafrost and the role of abiotic factors affecting the taxonomic structure of bacterial and fungal communities has a great strategic value.
In light of the above statements, the aims of the present study are: i) to evaluate the bacterial and fungal (including yeasts) diversity of a permafrost core drilled in a rock glacier of the Mount Scorluzzo, Stelvio Pass, Italy along a depth gradient; ii) to estimate what chemical-physical (abiotic) factors could putatively affect bacterial and fungal diversity inside this ecological niche, and ii) to investigate co-occurrences among bacterial and fungal taxa inhabiting the permafrost core.
Section snippets
Study site and sample collection
The Scorluzzo rock glacier is located close to the Stelvio Pass (46°31′ N, 10°25′ E) within the Stelvio National Park at an elevation of 2740 m above sea level in the Italian Central Alps (Fig. 1). The area around the rock glacier is characterised by bedrock outcrops (mainly paragneiss and mica schists), as well as some Holocene till and talus deposits (Ponti et al., 2021). The vegetation is very scarce and typical of the Alpine and nival belts mainly related to pioneer plant communities
Bacterial and fungal diversity
After bioinformatic analyses, a total of 702,630 reads grouped into 3292 ASVs for bacteria and 821,790 reads grouped into 860 ASVs for fungi were found in the six units (U1-U6, each one analysed in triplicate = a total of 18 samples) under study. Almost the entirety (99.99%) of bacterial reads were classified at the phylum level, 99.88% at the class level, 96.36% at the order level, 90.54% at the family level and 86.10% at the genus level. On the other hand, 97.665% of fungal reads were
Discussion
Permafrost is considered to be an important marker of global climate change (Harris et al., 2003). The observed LOI values ranging between 0.18 and 0.35% can be considered reasonable values even in harsh environments such as the rock glacier under study. Furthermore, the TC values (one order of magnitude higher) could probably due to the circulation of “hard water” linked to the occurrence of lenses of dolostones (as sources of carbonates) embedded in the metamorphic rock surrounding and
Conclusion
This study reports the taxonomic structure of bacterial and fungal communities in a permafrost core collected in an Alpine rock glacier. The abiotic factors affecting microbial diversity have also considered. The depth gradient, water content, pH and LOI affected the taxonomic structure of bacterial communities (in particular, the abundance of bacterial amplicon sequence variants - ASVs - assigned to Afipia sp., Chloroflexi, Gaiella sp., Oryzihumus sp., and Serratia sp.) while fungal
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of competing interest
The authors declare that they have no conflicts of interest.
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