Supraglacial habitat

The supraglacial habitat is mainly composed of surface snow and ice that melt during the summer. This zone receives solar radiation and contains photosynthetic algae, fungi (Hu et al. Reference Hu, Fair, Liu, Wang, Duan and Lu2023) and coloured bacteria (Garcia-Lopez & Cid Reference Garcia-Lopez, Cid and Singh2016) that give the glacial surface reddish, greenish or brownish tones (Martín-Cerezo et al. Reference Martín-Cerezo, García-López and Cid2015). The most abundant representatives of these microorganisms are unicellular algae of the family Chlamydomonaceae (included in Archaeplastida), mainly the species Chlamydomonas nivalis (Garcia-Lopez & Cid Reference Garcia-Lopez, Cid and Singh2016). The juvenile stages of these algae synthesize green pigments involved in photosynthesis. When mature, the algae produce reddish carotenoids that protect them from excess solar radiation (Lutz et al. 2014, Khan et al. Reference Khan, Dierssen, Scambos, Höfer and Cordero2021).

In the supraglacial habitat, the important role played by cryoconite holes must be emphasized. Cryoconite is defined as the sediment that forms due to the interaction of mineral particles deposited on the glacier surface and the microbial communities that inhabit them. Over time, the surface snow melts, giving rise to cryoconite holes filled with liquid water. These holes are true oases for the development of microbial communities, containing sufficient nutrients for their growth (Hassan et al. Reference Hassan, Mushtaq, Ganiee, Zaman, Yaseen and Shah2024). Cryoconite holes are populated mainly by primary producers such as cyanobacteria (Fig. 3a) and microalgae (Fig. 3b; Hodson et al. Reference Hodson, Anesio, Tranter, Fountain, Osborn and Priscu2008). It has been demonstrated that significant metabolic heat is produced within cryoconite holes, which contributes to ice melting and in turn increases the size and depth of these pools (Gladkov et al. Reference Gladkov, Kimeklis, Tembotov, Ivanov, Andronov and Abakumov2024). Some cryoconite holes generate at least 13% of the observed runoff on the one glacier for which this has been measured (Fountain et al. Reference Fountain, Tranter, Nylen, Lewis and Mueller2004).

Englacial habitat

The englacial habitat, consisting of the area and veins occurring within a glacier, is the least understood part of the glacial environment. It is also the most inhospitable area of these glaciers due to the presence of low temperatures, high pressures and total darkness. Researching this habitat is difficult as such research requires the extraction of ice cores under sterile conditions to avoid contamination. The first studies on the microbiology of these cores were carried out by Abyzov (Reference Abyzov1993). The microorganisms in this stratum could come from the upper layer and the marginal moraines by dragging through the ice. They reside in the air bubbles, veins and water microchannels that form between the ice crystals. It has been shown that the populations inhabiting these environments are not in a quiescent state but are functional (Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019). The microorganisms of this glacial zone are generally chemoautotrophs. They can also be heterotrophs that feed on the organic debris carried by the ice. They usually perform anaerobic respiration due to the lack of oxygen.

Subglacial habitat

Subglacial environments represent a largely unexplored component of Earth’s biosphere. This environment is generated as a result of ice friction on rocks, fragmenting minerals and organic debris that, combined with subglacial water, create optimal niches for microbial life. It has been described that under glaciers there is often a stream of water. This water may originate from the melting of the ice by geothermal heat from the underlying rock or from supraglacial channels that cross these glaciers and form lakes on their bedrock (Anesio & Laybourn-Parry Reference Anesio and Laybourn-Parry2012). Although, as mentioned earlier, this ecosystem was one of the first glacial environments to come to the attention of researchers in the 1980s thanks to the exploration of the microbiology of Ace Lake (Franzmann et al. Reference Franzmann, Liu, Balkwill, Aldrich, Conway de Macario and Boone1997) and Lake Vostok (Shtarkman et al. Reference Shtarkman, Koçer, Edgar, Veerapaneni, D'Elia and Morris2013), few studies have sampled subglacial environments given the difficulties associated with accessing these sites. In these surveys, several species of archaea, bacteria and fungi have been found (Garcia-Lopez et al. Reference Garcia-Lopez, Ruiz-Blas, Sanchez-Casanova, Peña Perez, Martin-Cerezo and Cid2022).

Bacteria

It has been reported that all major phyla of bacteria are represented in glaciers, especially Acidobacteriota, Actinobacteriota, Bacteroidota, Firmicutes, Gemmatimonadota, Planctomycetota, Proteobacteria and Verrucomicrobiota (Anesio & Laybourn-Parry Reference Anesio and Laybourn-Parry2012).

In this work, great bacterial diversity has been observed when compared with other Antarctic glaciers (Grzesiak et al. Reference Grzesiak, Zdanowski, Górniak, Świątecki, Aleksandrzak-Piekarczyk and Szatraj2015). According to our results, it is confirmed that all major phyla of bacteria are represented in glaciers. Most of the phyla of the englacial ecosystem were identified as Firmicutes, Proteobacteria and Bacteroidota (Fig. 4, level 2), and the major genera in this habitat were Polaromonas, Pedobacter, Sulfobacillus, Flavobacterium, Clostridium and Sphingomonas (Fig. 4, level 6).

Regarding the subglacial ecosystem, samples from two glaciers on Deception Island were analysed (Garcia-Lopez et al. Reference Garcia-Lopez, Ruiz-Blas, Sanchez-Casanova, Peña Perez, Martin-Cerezo and Cid2022). These samples were collected in the basal margins of the glaciers, in contact with the lower bedrock. The major phyla in the subglacial ecosystem were Actinobacteriota, Bacteroidota, Cyanobacteria and Proteobacteria (Fig. 4, level 2). Most of the bacterial genera of the subglacial habitat were Hymenobacter, Pontibacter, Frankia, Cyanobacterium and Symploca (Fig. 4, level 6).

It is worth highlighting the presence of sentinel microorganisms of human contamination inside the glacial ice, which have altered the composition of native microbial populations. In a study comparing the gradient of microbial populations across the archipelago, it was observed that the western islands contained greater microbial diversity, possibly due to the existence of volcanic minerals that serve as nutrients for chemolithotrophic bacteria (Garcia-Lopez et al. Reference García-Lopez, Serrano, Calvo, Peña Perez, Sanchez-Casanova and García- Descalzo2021). However, in the eastern islands, where the number of research stations and tourists is higher, the diversity of chemolithotrophs decreased and the existence of microorganisms associated with human presence was observed. These microbial populations include various species of enterobacteria such as Escherichia coli, as well as other microorganisms such as Helicobacter, Bacteroides, Streptococcus and Oscillospira (Power et al. Reference Power, Samuel, Smith, Stark, Gillings and Gordon2016). The presence of animal parasitic microeukaryotes such as Cryptosporidium was also observed (Kleinertz et al. Reference Kleinertz, Christmann, Silva, Hirzmann, Hermosilla and Taubert2014). The problems associated of human frequentation through tourism and the expansion of scientific operations have been reported previously, and these activities can have serious repercussions such as increased human-mediated antimicrobial resistance (Clark et al. Reference Clark, Gregson, Greco, Evans and Hwengwere2025).

A comparison of the microbial populations of the glaciers at the two poles shows that the differences between the two geographical regions are significant, and these disparities condition their observed environmental microbiology. The Arctic is an ocean surrounded by continents. Arctic glaciers are heavily influenced by marine microorganisms that have colonized the land from the sea, and vice versa. Sentinel microorganisms of this transfer such as the genus Hymenobacter have been described (Garcia-Lopez et al. Reference Garcia-Lopez, Rodriguez-Lorente, Alcazar and Cid2019). This process is less observed in Antarctica, where the continent is surrounded by the ocean. It is also significant that the microbial diversities at both poles are very different, and there are taxonomic groups that only exist in either one or the other of them. Also noteworthy is the presence of microorganisms of the same genus (e.g. Psychrobacter) but different species (e.g. Psychrobacter frigidicola and Psychrobacter arcticus) in the glaciers of both poles, which may be due to evolutionary convergence or past migrations (Bowman et al. Reference Bowman, Cavanagh, Austin and Sanderson1996, Bakermans et al. Reference Bakermans, Ayala-del-Río, Ponder, Vishnivetskaya, Gilichinsky, Thomashow and Tiedje2006).

Archaea

The greatest number and diversity of archaea on the planet exist in cold environments (Caviccioli Reference Caviccioli2006). Archaea have been reported both within glacier ice and beneath glaciers (Skidmore et al. Reference Skidmore, Foght and Sharp2000). Methanogenic archaea represent one of the most abundant resident groups in glaciers, especially in the subglacial environment (Tung et al. Reference Tung, Bramall and Price2005). For example, Methanogenium frigidum and Methanococcoides burtonii are two of the most studied such species, and they were isolated from samples from the aforementioned Ace Lake, Antarctica (Franzmann et al. Reference Franzmann, Liu, Balkwill, Aldrich, Conway de Macario and Boone1997, Saunders et al. Reference Saunders, Thomas, Curmi, Mattick, Kuczek and Slade2003). Other archaea that are not methanogenic have also been described, such as Halorubrum lacusprofundi, which was isolated from Deep Lake, Antarctica. This psychrophilic microorganism is also capable of living in a subglacial lake with a salinity that is 10 times higher than that of seawater (Caviccioli Reference Caviccioli2006). In our sequence analyses, although they were not specifically designed to search for archaea, some archaea of the Halobacterota and Euryarchaeota groups were identified (Fig. 4).

Microeukaryotes

Eukaryotic microorganisms mostly populate the surface of glaciers, especially cryoconite holes (Christner et al. Reference Christner, Kvitko and Reeve2003). However, eukaryotic microorganisms have also been found in the englacial ecosystem. The majority of the groups of the englacial ecosystem belonged to Opisthokonta (41.57%, of which 65.27% were fungi), SAR (38.09%) and Archaeplastida (12.58%), although 7.68% of the sequences were assigned as unspecified eukaryotes (Fig. 5, level 2).

Figure 5.

Phylogenetic tree and taxonomy of microeukaryotes identified in South Shetland archipelago glaciers. The phylogenetic tree was generated from 18S amplicon sequences previously obtained for Martinez-Alonso et al. (Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019) and Garcia-Lopez et al. (Reference García-Lopez, Serrano, Calvo, Peña Perez, Sanchez-Casanova and García- Descalzo2021). Sequence analysis and tree construction were performed as previously described (Ruiz-Blas et al. Reference Ruiz-Blas, Muñoz-Hisado, Garcia-Lopez, Moreno, Bartolomé and Leunda2023). Barplots were generated from taxonomic annotations using the SILVA rRNA database. The first barplot ring (closer to the tree) represents the glacial environments, the second barplot ring represents taxonomic level 1, the third barplot ring represents taxonomic level 2 and the most external barplot ring represents taxonomic level 6 (genus). The legend corresponding to the genera (level 6) has not been represented due to its large size, but the major genera are detailed in the text.

The most abundant genera of eukaryotes in the englacial environment were diatoms such as Thalassiosira and Cocconeis, green algae such as Hydrurus and Urospora, cercozoa such as Glissomonadida and Bodomorpha and fungi such as Phytophthora, Sporobolomyces and Hyphozyma (Fig. 5, level 6). It has been previously reported that the lack of light and the anoxic conditions of the englacial and subglacial environments favour the growth of numerous fungal species (Anesio et al. Reference Anesio and Laybourn-Parry2012).

Some microorganisms that denote environmental contamination, such as the microeukaryote Cryptosporidium, a characteristic gastrointestinal parasite of emperor penguins, were identified in the glaciers Traub (Greenwich Island) and Ecology and Machu Picchu (King George Island; Garcia-Lopez et al. Reference García-Lopez, Serrano, Calvo, Peña Perez, Sanchez-Casanova and García- Descalzo2021). The genus Giardia, an anaerobic flagellated parasite of vertebrates, was also detected in the englacial habitat of Mount Pond glacier (Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019).

Viruses

Most of the studies conducted on Antarctic viruses have been done using freshwater samples from lakes (Lopez-Bueno et al. Reference Lopez-Bueno, Tamames, Velázquez, Moya, Quesada and Alcamí2009) or seawater samples taken under the Antarctic ice shelves (Lopez-Simon et al. Reference Lopez-Simon, Vila-Nistal, Rosenova, De Corte, Baltar and Martinez-Garcia2023). However, very few studies exist on the diversity of viruses in Antarctic glacier ice. Yet, by analysing the genomes of bacteria and microeukaryotes, the presence of viruses that infect microorganisms (phages) can be detected (Boetius et al. Reference Boetius, Anesio, Deming, Mikucki and Rapp2015). It is known that viruses in cold environments play an important role in controlling the mortality and diversity of microbial communities, but further research is needed on ice-hosted viral populations, which remain poorly understood (Anesio & Laybourn-Parry Reference Anesio and Laybourn-Parry2012). As a consequence of climate change, glaciers and permafrost are melting. This means that pathogenic viruses could awaken and infect the surrounding fauna and flora (Varghese et al. Reference Varghese, Patel, Kumar and Sharma2023).

Adaptation to temperature changes

Antarctic glacier ice shows a wide temperature range. Average summer temperatures are close to 0°C, although temperatures of up to 18.3°C have been recorded on the surface of glaciers, where solar radiation amplified by the albedo heats the ice (Bergstrom et al. Reference Bergstrom, Gooseff, Myers, Doran and Cross2020). During the winter (March–September), there is little to no incoming solar radiation, and temperatures average -30°C (Fountain et al. Reference Fountain, Lyons, Burkins, Dana, Doran and Lewis1999). These enormous temperature variations must be added to the generalized temperature increase that the entire planet is undergoing due to climate change linked to anthropogenic greenhouse gases. Although this change is less pronounced in Antarctica than in other regions of Earth, the temperature increase in the South Shetland archipelago has been ~3°C since the middle of the 20th century (Gorodetskaya et al. Reference Gorodetskaya, Durán-Alarcón, González-Herrero, Clem, Zou and Rowe2023).

Microorganisms adapt to temperature changes by adopting various modifications in their cellular and molecular machinery. These adjustments include the synthesis of heat shock proteins (Hsps) and cold shock proteins (Csps) that are involved in several cellular processes such as transcription, translation, protein folding and the regulation of membrane fluidity (Garcia-Descalzo et al. Reference Garcia-Descalzo, Alcazar, Baquero and Cid2011, Yusof et al. Reference Yusof, Masnoddin, Charles, Thien, Nasib and Wong2022). These proteins have been extensively investigated in the context of the adaptation of Antarctic yeasts and bacteria to temperature increases (Deegenaars et al. Reference Deegenaars and Watson1997, Che et al. Reference Che, Song and Lin2013, Garcia-Descalzo et al. Reference Garcia-Descalzo, García-López and Cid2022). For example, when the bacterium Shewanella frigidimarina is exposed to high temperatures, it accumulates Hsps, as well as other proteins related to stress, redox homeostasis or protein synthesis and degradation and the depletion of enzymes and cell envelope components. In addition, the entire cellular machinery is remodelled, and chaperone proteins, such as Hsps, reorganize their interaction networks with other proteins (Garcia-Descalzo et al. Reference Garcia-Descalzo, García-López, Alcazar, Baquero and Cid2014).

Another adaptation of microorganisms to cold is the modification of the composition of their cell membranes. Cold reduces the fluidity of membranes, and to compensate for this effect, the fatty acid composition of the membranes of these microorganisms is different from that of membranes of other prokaryotes (Wang et al. Reference Wang, Chen, Nian, Ji, Lin, Wei and Zhang2017). Among the modifications to membrane fatty acids, the synthesis of lipids with cis-unsaturated double bonds has been described previously, as well as the incorporation of branched lipids and the incorporation of short fatty-acyl chains that reduce the contacts between adjacent chains (Feller Reference Feller2007, Chen et al. Reference Chen, Zhang, Li, Cui, Yang and Zhang2022).

Recently, high-throughput technologies have been developed to determine the function of a large number of enzymes from psychrophilic microorganisms, combining proteomics techniques and the reconstruction of protein networks (Garcia-Descalzo et al. Reference Garcia-Descalzo, García-López and Cid2022, Ramasamy et al. Reference Ramasamy, Mahawar, Rajasabapathy, Rajeshwari, Miceli and Pucciarelli2023). It has been shown that living at low temperatures is benefitted by the ability to synthesize enzymes that continue to function at near-freezing temperatures. Psychrophilic enzymes constitute an important class of proteins in which structure-function relationships can be investigated by comparing their properties with those of homologous mesophilic and thermophilic counterparts (Ruggiero et al. Reference Ruggiero, Raimo, Palma, Acari and Masullo2007). In this comparison, it has been observed that psychrophilic enzymes are more flexible (the space available for amino acid side chains is larger than that for mesophilic enzymes), and this fact allows them to continue to function at low temperatures. The flexibility of these proteins sometimes affects the whole molecule, but it is sufficient if only the catalytic region has such flexibility (Hochachka & Somero Reference Hochachka and Somero2002, D’Amico et al. Reference D'Amico, Collins, Marx, Feller and Gerday2006, Giordano et al. Reference Giordano, Russo, Di Prisco and Verde2012).

Adaptation to high levels of radiation

Microorganisms that are highly resistant to radiation are called ‘radiophiles’. Among them, representatives of the genus Deinococcus such as Deinococcus radiodurans and Deinococcus radiophilus have been described, which are mostly found on the surface of glacial ice, where solar radiation and albedo are very intense (Zhang et al. Reference Zhang, Zhu, Li, Rao, Li and Guo2020, Cheng et al. Reference Cheng, Mu, Zhang, Jiang, Liu and Li2024). These microorganisms are also highly resistant to oxidative damage and dehydration. It has been observed that these bacteria utilize several repair mechanisms (Liu et al. Reference Liu, Li and Zhang2023). The DNA of cells subjected to high doses of radiation is damaged and becomes non-functional, but these bacteria can repair such damaged DNA (Singh & Gabani Reference Singh and Gabani2011).

Another important resistance mechanism of microorganisms to protect themselves from solar radiation is the synthesis of pigments. Bacteria, as well as archaea and algae, are pigment producers (Garcia-Lopez & Cid Reference Garcia-Lopez, Cid and Singh2016). For example, the bacterium Sphingobacterium antarcticus produces zeaxanthin, β-cryptoxanthin and β-carotene (Jagannadham et al. Reference Jagannadham, Chattopadhyay, Subbalakshmi, Vairamani, Narayanan and Rao2000). Another pigment capable of protecting organisms from radiation is xanthomonadin, which is exclusively produced by Xanthomonas (Rajagopal et al. Reference Rajagopal, Sundari, Balasubramanian and Sonti1997). Other pigments produced by halophilic archaea are bacterioruberin and bacteriorhodopsin (Rodrigo-Baños et al. Reference Rodrigo-Baños, Garbayo, Vílchez, Bonete and Martínez-Espinosa2015). Among the microalgae, Chlorella zofingiensis and Chlamydomonas nivalis produce xanthophylls (Smaoui et al. Reference Smaoui, Barkallah, Ben Hlima, Fendri, Mousavi Khaneghah and Michaud2021).

Adaptation to the lack of oxygen

The supraglacial habitat is populated by aerobic microorganisms that use oxygen as an electron acceptor (Maccario et al. Reference Maccario, Sanguino, Vogel and Larose2015). Oxygen is the acceptor of choice for cells as the reduction of oxygen produces a large amount of energy compared to that produced by the reduction of other electron acceptors (Madigan et al. Reference Madigan, Martinko, Bender, Buckley, Stahl and Nakagawa2015). However, in the englacial and subglacial habitats, the oxygen concentration is very limited, and so the respiration of microorganisms in these habitats is anaerobic. These microorganisms use other electron acceptors, such as nitrate reduced to nitrite or nitrogen by Pseudomonas species (Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019). Other bacteria use other inorganic compounds, such as Geobacter using ferric iron and Desulfovibrio using sulphate (Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019). Some methanogenic microorganisms reduce carbonate to methane, whereas acetogens reduce carbonate to acetate. Even organic molecules such as fumarate can be used as electron acceptors in anaerobic respiration (Madigan et al. Reference Madigan, Martinko, Bender, Buckley, Stahl and Nakagawa2015). Similarly to aerobic respiration, anaerobic respiration requires electron transport and uses the enzyme ATPase to synthesize adenosine triphosphate (ATP). Most of the enzymes and proteins used in these cellular processes have been discovered only recently through proteomics analysis (Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019).

Adaptation to the lack of nutrients

Glacial microorganisms play an important ecological role as they participate in global geochemical fluxes. They are essential components of microbial food webs and are often dominant primary producers, playing important roles in chemical weathering and carbon and nitrogen cycling processes (Trejos-Espeleta et al. Reference Trejos-Espeleta, Marin-Jaramillo, Schmidt, Sommers, Bradley and Orsi2024).

In the supraglacial ecosystem, there is a large majority of photosynthesizing and heterotrophic microorganisms that feed on organic debris carried by the ice, and so the lack of nutrients is not a limiting factor to their survival (Maccario et al. Reference Maccario, Sanguino, Vogel and Larose2015). However, in the lower layers of glaciers, microorganisms have adapted to nutrient scarcity by developing a chemoautotrophic metabolism in which they feed on rocks and soil (Harrold et al. Reference Harrold, Skidmore, Hamilton, Desch, Amada and van Gelder2016). Most of the microorganisms in these ecosystems are heterotrophic and chemoautotrophic (Hodson et al. Reference Hodson, Anesio, Tranter, Fountain, Osborn and Priscu2008, Martinez-Alonso et al. Reference Martinez-Alonso, Pena-Perez, Serrano, Garcia-Lopez, Alcazar and Cid2019). The englacial and subglacial environments also favour the growth of numerous fungal species. Such fungal growth facilitates the weathering of rocks and the opening of internal channels. These fungi have a heterotrophic metabolism and degrade organic matter remains. In turn, the carbon obtained from this degradation and its transport through the hyphae into the rocks would facilitate the growth of other microorganisms such as heterotrophic bacteria (Butinar et al. Reference Butinar, Spencer-Martins and Gunde-Cimerman2007).

The anaerobic metabolism of microorganisms also include methanogenesis. As stated earlier, the Archaea contain methane-producing microorganisms. There are two groups of methanogenic archaea: methylotrophic and hydrogenotrophic. Hydrogenotrophic methanogens produce methane from H₂ plus CO₂ (or formate). Methylotrophic methanogens produce methane from simple monocarbon compounds such as methanol, methylamines and methylthiols (Madigan et al. Reference Madigan, Martinko, Bender, Buckley, Stahl and Nakagawa2015). It has been documented previously that when glaciers retreat, the methane that had been retained beneath the glacier in pockets of gas, which had been produced by methanogenic microorganisms, escapes into the atmosphere (Kleber et al. Reference Kleber, Hodson, Magerl, Mannerfelt, Bradbury and Zhu2023). Once these glaciers disappear, pedogenesis begins, and the organic matter in glacial forefields becomes subject to remineralization to CO₂ by microbial respiration (Tao et al. Reference Tao, Huang, Hungate, Manzoni, Frey and Schmidt2023). In this succession, fungi play a main role in the initial stabilization of the assimilated carbon (Trejos-Espeleta et al. Reference Trejos-Espeleta, Marin-Jaramillo, Schmidt, Sommers, Bradley and Orsi2024)

Adaptation to dryness

The Antarctic continent is extremely dry, averaging 166 mm of precipitation per year (Vaughan et al. Reference Vaughan, Bamber, Giovinetto, Russell and Cooper1999). The air in Antarctica is also dry, as the low temperatures result in very low absolute humidity levels. As a mechanism of adaptation to the lack of water, some species of bacteria, such as those of the genus Bacillus, produce structures called ‘endospores’. These survival mechanisms enable these organisms to withstand unfavourable growth conditions, such as extreme temperatures, dryness or a lack of nutrients. In addition, these endospores are easily dispersed by wind and water (Ryan-Payseur & Freitag Reference Ryan-Payseur and Freitag2018).

Among eukaryotic microorganisms, fungi, such as the genus Rhizopus identified in the Mount Pond glacier, are also capable of resisting dryness through the formation of a zygosporangium, which can remain dormant and resist dryness and other unfavourable conditions ( Gryganskyi et al. Reference Gryganskyi, Golan, Dolatabadi, Mondo, Robb and Idnurm2019).

Another mechanism of adaptation is the formation of biofilms. This mechanism has been investigated in bacteria of the genus Bacillus, which that can choose between two lifestyles: biofilm formation and flagellum-mediated swimming motility. This choice is made at the individual cell level, with bacterial cells in a population expressing either the genes required for biofilm formation or the genes required for flagellum-mediated swimming motility (Ryan-Payseur & Freitag Reference Ryan-Payseur and Freitag2018). Furthermore, to form a biofilm, the cells need to engage in intercellular communication (quorum sensing) that leads to a choice being made between living freely or forming a biofilm. It has been demonstrated previously that in the genus Pseudomonas quorum sensing induces the expression of a group of genes necessary for the synthesis of the exopolysaccharides that form a biofilm. Intracellular signalling is also involved in biofilm formation in Pseudomonas aeruginosa through the cyclic di-GMP messenger protein, which is unique to prokaryotes (Lichtenberg et al. Reference Lichtenberg, Kragh, Fritz, Kirkegaard, Tolker-Nielsen and Bjarnsholt2022). In glaciers, biofilms can be observed on the surface of cryoconite holes in which there is a liquid water medium and in which cells can move and communicate with each other.