**3. Metabolic potentials and biogeochemical cycles in polar glaciers through reconstruction of microbial metagenomes**

Living in such extreme environments implies coping with low temperatures, desiccation, low nutrients availability, and ultraviolet irradiation [30]. Over the last

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

years, metagenomics have allowed a great understanding of metabolic potentials and biogeochemical cycles in polar glaciers through reconstruction of microbial genomes (**Figure 3**).

Regarding the supraglacial ecosystem, metagenomic studies have demonstrated the wide diversity of functions in cryoconite holes, with a range of metabolic pathways which depend on their competence to acquire and degrade available nutrients [10]. Functional analyses highlighted the importance of stress responses and efficient carbon and nutrient recycling.

Metagenomic techniques have also been used to identify algal communities in the supraglacial ecosystem and their relationship with geochemical factors [12].

The potential of archaea as important ammonia oxidizers has been another finding achieved by metagenomics [11].

Little is known about the metabolic potential and the biogeochemical cycles of microbial communities inhabiting the englacial ecosystem. It has been reported that microorganisms enclosed in the englacial ice present very low metabolic rates, using energy only to repair damaged biomolecules and not to grow and reproduce [31].

In the subglacial ecosystem, some metagenomics data implied that the most abundant and active component were bacteria within the order *Methylococcales* [6]. Transcripts of the particulate methane monooxygenase from these taxa were detected, demonstrating that methanotrophic bacteria were functional members of this subglacial ecosystem.

At least three modes of carbon fixation were inferred [14]. The most common mode of carbon fixation was the reductive pentose phosphate cycle. The second in frequency was the reductive tricarboxylic acid pathway. This cycle also produces

#### **Figure 3.**

*Overview of the metabolic potentials between dominant microorganisms in the three polar glacial ecosystems. 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.*

precursors for nucleic acid and aromatic amino acid syntheses. The third type of carbon fixation, the reductive acetyl-CoA pathway, is the one used by archaea [14].

These investigations did also identify genes that carry out various parts of the nitrogen cycle, including nitrogen fixation (*Actinobacteria*, *Cyanobacteria*, *Betaproteobacteria*, and *Gammaproteobacteria*), nitrification (*Alphaproteobacteria* and *Betaproteobacteria*), denitrification (*Gammaproteobacteria*), nitrate reduction (*Betaproteobacteria* and *Gammaproteobacteria*), anammox (*Planctomycetes*), assimilation (most microorganisms from these investigations), and decomposition (fungi and other heterotrophs) [14].

Characterization of the Antarctic Blood Falls microbial assemblage revealed taxa that could participate in active sulfur cycling, including autotrophs and heterotrophs such as *Desulfocapsa*, *Geopsychrobacter*, *Thiomicrospira*, and *Thermacetogenium* [32]. Although these microorganisms usually inhabit the subglacial ecosystem, in Blood Falls, they have been identified in brines collected from outflowing fluids (**Figure 3**).
