**7. Microorganisms as indicators of the conservation status of glaciers**

In the last few decades, recently deglaciated areas present in different glacial zones in the world, are available for colonization and primary succession, especially initiated by pioneer microorganisms [71] followed by plants [72] and animals [73].

In retreat glaciers, the microbial populations of the glaciers are very different from those found in the surrounding soils. Some reports [61] have demonstrated that both bacterial and fungal community structures show significant differences between deglaciated sites and successional sites that had been ice-free over more than 100 years [74]. These changes have a strong influence on the processes of colonization and succession in the areas where glacier ice has melted [72]. Firstly, microbial communities change as psychrophilic microorganisms are replaced by mesophilic microorganisms. Then, plants and animals colonize these newly formed environments. Additionally, characteristic bacterial species can be found in each glacier zone and not found in the others. So, there are "type species" that can subsist thanks to their special metabolism and molecular mechanisms of adaptation. An example is shown in **Figure 6**, in which the population of microorganisms in three habitats: glacier snow [51, 75, 76], glacier front [6], and forefield [6, 62, 71] are compared. Although the compared glaciers are located in very different places around the world, it can be observed that the distribution of the main groups of microorganisms is different for each of the three habitats.

### **7.1. Glacier snow**

is open to the elements, rocks appear on the surface, whose minerals dissolve and change the physical and chemical characteristics of the runoff water. Occasionally, the waters become even toxic because of the presence of heavy metals [68]. Sometimes the formation of alkaline lakes can be observed, due to the mineral salts entrained by the water of runoff in the ground that had been occupied by a glacier, like the Amarga Lagoon in Chile (**Figure 5(A)**). These lagoons can be inhabited by cyanobacteria which grow forming laminar colonies whose calcareous skeletons fossilize generating sedimentary rocks named stromatolites (**Figure 5(B)**). They are formed when cells build up a carbonate skeleton, integrating particles present in the lake water. Otherwise, when certain minerals such as pirite are exposed to air and water, a slow chemical reaction with molecular oxygen occurs. While this abiotic reaction can lead to the development of acidic conditions, the degree to which acid mineral drainage becomes an overwhelming burden on the environment results from the oxidative dissolution, a reaction catalyzed by microorganisms [69]. This process affects differently to the terrestrial and marine glaciers. In land glaciers, the runoff waters become more acidic, which affects the rivers and lakes that receive their waters. This can affect the flora and fauna, the crops and the human populations that live downstream. When this fact affects the marine glaciers, the composition of marine tidewater tongues changes their chemical composition: water salinity decreases, and at the same time, water becomes more acidic. Acidification of sea water impacts ocean species to varying degrees. A more acidic environment has a dramatic effect on some calcifying species,

114 Glacier Evolution in a Changing World

including oysters, clams, sea urchins, corals, and calcareous plankton [70].

**7. Microorganisms as indicators of the conservation status of glaciers**

microorganisms [71] followed by plants [72] and animals [73].

Calcareous skeleton of a stromatolite of Amarga Lagoon at Chile.

In the last few decades, recently deglaciated areas present in different glacial zones in the world, are available for colonization and primary succession, especially initiated by pioneer

**Figure 5.** Alkalinization of runoff waters. (A) Formation of an alkaline lake (pH 9.1) due to the mineral salts entrained by the water of runoff in the ground that had been occupied by a glacier. The inset shows the location of **Figure 5(B)**. (B) Several reports have been published [77] about the little microbial abundance observed on glacier snow. For example, in Alpine snow packs, bacterial abundances range between 10<sup>3</sup> and105 cells/ml [78, 79], and in Svalbard archipelago, snow bacterial abundances are about 2 × 104 cells/ml [75, 80]. In cell counts performed on Antarctic snow, it was observed that the microbial abundance was even lower (<103 cells/ml).

Regarding microbial diversity in glacier snow (**Figure 6(A)**), the effect of snow melt on bacterial community structure and diversity of surface environments of a Svalbard glacier has been

**Figure 6.** Bacterial community structure along a glacier front based on 16S rRNA gene sequences. Pie charts represent relative abundances of bacterial classes for three glacier environments: glacier snow, glacier front and forefield. The data come from Refs. [51, 76, 77] for glacier snow, from Ref. [6] for glacier front and from Refs. [6, 62, 74] for forefield.

examined using analyses of 16S rRNA genes [51]. In these studies, it was observed that the bacterial community structure depends on the type of snow deposition. However, the most interesting fact, from the point of view of monitoring the state of conservation of the glacier is that slush (the product of decomposition of snow when it melts) contains lineages of bacteria completely different from those of freshly fallen snow, which implies a change in the composition of the community structure that is post-depositional.

Other studies carried out in Greenland demonstrated that the phylogenetic composition of the microbial communities was different within the snow layers [75]. Proteobacteria, Bacteroidetes, and Cyanobacteria dominated in the middle and top snow layers, although Actinobacteria and Firmicutes were also abundant. In the deepest snow layer, large percentages of Firmicutes and Fusobacteria were found [75]. Large numbers of eukaryotic chloroplasts belonging to Streptophyta and Chlorophyta were also observed, demonstrating that microeukaryotes were also present in snow. Cyanobacteria and algae were almost exclusively found in the top and middle layers of the snow pack which are probably feeding the heterotrophic members of the microbial communities.

Some reports have demonstrated that the composition of snow microbial communities depends on the proximity to the sea [76]. In glacier snow, typical species of marine environments such as the Alphaproteobacteria have been found in samples from Antarctica, although Bacteroidetes and Cyanobacteria are also present [76].

## **7.2. Glacier front**

Microbial communities in glacier fronts have been especially studied in the Antarctic Peninsula which is among the regions with the fastest warming rates, and where regional climate change has been linked to an increase in the mean rate of glacier retreat [6].

Archaeal and bacterial 16S rRNA gene sequences obtained from soil samples collected in the Wanda Glacier forefield showed that the diversity and richness were surprisingly high, and that communities were dominated by Proteobacteria, Bacteroidetes, and Euryarchaeota, with many archaeal and bacterial phylotypes yet unclassified (**Figure 6(B)**). Some of the phylotypes found were also related to marine microorganisms, indicating the importance of the marine environment as a source of colonizers for these recently deglaciated environments [6].

Concerning microbial abundance, some examples have been published. In Greenland glacier fronts, between 6 and 30 × 107 cells/ml, it has been reported [77].

## **7.3. Fore field**

It has been published that microbial abundance in an Antarctic glacier (Ecology Glacier) forefield is increased along several sampling points from the glacier front to the farther outskirts of the glacier [71]. The same effect has been observed in the Peruvian Andes glaciers, where abundances of Cyanobacteria and Diatoms increased over the time of succession [62].

Regarding diversity, new soils from recently deglaciated soils are colonized by a diverse community of microorganisms during the first years following glacial retreat. Taxonomically microorganisms from Ecology Glacier forefield [71] belonged to the alpha, beta, and gamma subdivisions of the Proteobacteria and to the Cytophaga-Flavobacterium-Bacteroides (CFB) group (**Figure 6(C)**). Filamentous fungi were relatively abundant and represented mainly by oligotrophs.

In the recently deglaciated areas of the Peruvian Andes [62], it has been observed that a significant increase in cyanobacterial diversity corresponded with increases in soil stability, heterotrophic microbial biomass, soil enzyme activity, and the presence of photosynthetic and photoprotective pigments.

In glaciers, increasing temperature leads to a rapid retreat of ice, which increases water production [45, 72]. In glacier forefields, the runoff water of the glaciers can origin rivers and lakes [81]. For example, in the High-Arctic, it has been reported that Bacteroidetes, Actinobacteria, and Verrucomicrobia were the most abundant phyla in freshwater, while relatively few Proteobacteria and Cyanobacteria were present. Possibly, light intensity controlled the distribution of the Cyanobacteria and algae which in turn fed the heterotrophic bacteria [75].

Photosynthetic and nitrogen-fixing microorganisms play an important role in acquiring nutrients and facilitating ecological succession in soils during the first years of succession, many years before the establishment of mosses, lichens, or vascular plants [62]. Afterward, species of green soil algae are important pioneers in the colonization process of the areas recently denuded of ice [72].

At last, soil macrofauna and mesofauna colonize the fore fields. The successional chronosequence of an Alpine glacier was studied at several stages from 4 to 150 years of age since deglaciation [73]. Within the first 50 years, macrofauna biomass and mesofauna abundance increased rapidly, and successional age was the major determinant of community composition [73]. Some studies about soil mesofauna in high alpine ecosystems of the Central Alps demonstrated the shifts in species richness and density of arthropod such as oribatid mites [82]. In newly formed soils, some arthropods populate new soils, which in turn, promote the growth of fungi and bacteria and contribute to the formation of the new soil microstructure [82]. Nevertheless, these new fungi and bacteria are different from those that used to live in glaciers, as the novel species of plants and animals, contain associated microorganisms; for example, new microorganisms contained in animal droppings or symbiotic rhizosphere microbial communities associated to plants [65].

Microbial ecology can be a tool for monitoring the biological change that happens in retreat glaciers. Ecological researches conducted along deglaciated chronosequences in some glaciers have been carried out in order to understand the development of ecosystems. In these studies, distance from a receding glacier is used as a proxy for soil age, with older soils being further from the glacier front [62].
