**2. Microbial community structure through reconstruction of microbial genomes in polar glaciers**

Glaciers have recently been considered authentic biomes [18]. It has been observed that microbial community composition depends on the area of the glacier studied [19]. In most of them, three well-defined and interconnected ecosystems can be defined: supraglacial, englacial, and subglacial ecosystems. These ecosystems are different in their solar radiation, water content, nutrient abundance, and redox potential [20]. These factors influence in the abundance and diversity of microbial populations inhabiting glaciers (**Figure 2**). They also affect the type of functionality and the biogeochemical cycles in these ecosystems.

*Microbial Community Structure and Metabolic Networks in Polar Glaciers DOI: http://dx.doi.org/10.5772/intechopen.84945*

**Figure 2.**

*Bacterial community structure in polar glacier ecosystems based on 16S and 18S rRNA gene sequences. Pie charts represent relative abundances of bacteria, archaea, and eukarya for three glacier ecosystems: supraglacial, englacial, and subglacial. The data are from [20, 11, 10, 4, 13, 8] for bacteria, from [11, 15, 5, 6, 14] for archaea, and from [19, 7, 12, 14] for eukarya.*

## **2.1 The supraglacial ecosystem**

The supraglacial ecosystem is the one which has best been studied. It has been reported that the main habitats in the supraglacial ecosystem are the snowpack, cryoconite holes (vertical cylindrical melt holes in a glacier surface), supraglacial streams, and moraines [20].

## *2.1.1 The glacial snow*

The sunlit and oxygenated supraglacial surface is populated by autotrophic microorganisms such as cyanobacteria, microalgae, and diatoms [21], by chemolithotrophic bacteria, which feed on inorganic sand particles, and by heterotrophic bacteria and microeukaryotes [22] (**Figure 2**). The main bacterial classes that have been described in this ecosystem are *Betaproteobacteria*, *Actinobacteria*, and *Bacteroidetes* [11, 23]. The genus *Polaromonas* is one of the most abundant in the supraglacial area of Arctic [9] and Antarctic glaciers [24].

Among microeukaryotes, snow is mainly populated by pigmented algae, which have been observed in Arctic and Antarctic glaciers [25]. They belong to several taxa, mainly *Chlamydomonas*, *Chloromonas*, *Raphidonema*, and *Chrysophyceae* [11].

Fungi, especially basidiomycetous yeasts and *Chytridiomycota*, have been reported in glacial snow and ice [26]. It is believed that they act as saprophytes and parasites, yet their diversity and function in this ecosystem are poorly known [25].

Archaea have also been identified in glacial snow and ice, although they have not been found in all the studies that have been carried out. They belong to Nitrososphaerales, which are known as important ammonia oxidizers [11].

## *2.1.2 Ice surfaces*

Under the recently fallen snow, there is a layer of hard ice. This layer arises to the surface in the episodes of melting that occur during the polar summer. The ice surface constitutes a distinct type of supraglacial microhabitat that is different from cryoconite holes. It is mainly populated by microalgae (Zygnematophyceae) and by cyanobacteria [17].

## *2.1.3 Cryoconite holes*

Cryoconite holes are predominantly inhabited by cyanobacteria [27]. Filamentous cyanobacteria such as *Phormidesmis*, *Oscillatoria*, *Leptolyngbya*, *Phormidium*, and *Nostoc* play an important role in cryoconites [28]. They produce organic material and extracellular polymeric substances (EPS), which act as cryo- and osmo-protectants [27]. Additionally, bacteria of the class *Actinobacteria* (*Microbacteriaceae* and *Intrasporangiaceae*) are also important members of cryoconite holes, followed by *Proteobacteria*, *Bacteroidetes*, and *Cyanobacteria*. Archaea and eukarya are the least abundant and the least representative members of this environment [10].

#### **2.2 The englacial ecosystem**

In englacial ecosystems, live motile bacteria can reach more than 3000 m of depth. These bacteria reside in clay particles and ice channels. According to their metabolism, they can be both chemoautotrophs (i.e., *Streptomyces*, *Nocardia*, *Bacillus*) and heterotrophs (i.e., *Proteobacteria*, *Actinobacteria*) (**Figure 2**). The later bacteria feed on solubilized organic products from pollen grains and from other dead microorganisms. At great depth, anaerobic respiration takes place [29] and methanogens (for instance, *Firmicutes* and *Euryarchaeota*) are also active [20].

#### **2.3 The subglacial ecosystem**

The subglacial ecosystem is dominated by aerobic and anaerobic bacteria in basal bedrock and subglacial lakes. It does also contain diverse and metabolically active archaeal, bacterial, and fungal species [25] (**Figure 2**).

Among bacteria, species with chemolithotrophic activity have been identified; an example is *Sideroxydans lithotrophicus*, which is an iron sulfide oxidizer. Other bacterial taxa found in this ecosystem are *Thiobacillus* and *Thiomicrospira*, both associated with the sulfur and iron cycles [15].

Archaea in these anoxic environments are mainly represented by methanogenic and methanotrophic species [25]. Methanogenesis, the production of methane in an anaerobic process mediated exclusively by methanogenic archaea, is a very plausible process in the subglacial ecosystem. In glacier samples from this environment, methanogenic archaea of the euryarchaeal orders Methanosarcinales [5] and Methanomicrobiales have been detected [6].

Eukaryotes have only been found in some of the studied subglacial environments [19]. Among them, mainly fungi have been described [26]. *Basidiomycetes* predominate, among which *Cryptococcus* and *Rhodotorula* are the dominant genera.
