**4. Microbial ecology in glaciers**

Considering the horizontal stratification of glaciers, they can be divided into three parts: the supraglacial ecosystem, the subglacial system, and the englacial ecosystem [2, 5] (**Figure 3**). These three ecosystems differ in terms of their solar radiation, water content, nutrient abundance, and redox potential [2]. These factors greatly determine the biogeochemical cycles, the type of metabolism, and the diversity and abundance of microbial populations inhabiting glaciers.

#### **4.1. The supraglacial ecosystem**

The main habitats in the supraglacial ecosystem are the snowpack, cryoconite holes, supraglacial streams, and moraines. On the glacier surface, the absorption of solar radiation by dark organic matter causes snow and ice melting yielding liquid water that is necessary for microorganisms. Meltwater dissolves nutrients from adjacent debris and even directly from the atmosphere [20].

The sunlit and oxygenated supraglacial surface are populated by autotrophic microorganisms such as microalgae and diatoms; by chemolitotrophic bacteria, which feed on inorganic sand particles; and by heterotrophic bacteria and microeukaryotes.

**Figure 3.** A schematic of different habitats of a glacier colonized by microorganisms. Supraglacial ecosystem is dominated by phototrophic algae and cyanobacteria that take advantage of sunlight and by heterotrophic bacteria and microeukaryotes that feed on organic particles from atmospheric deposition. Microorganisms in englacial ecosystem can be chemoautotrophs, but they can also be heterotrophic bacteria that feed of solubilized products. Subglacial ecosystem is dominated by microorganisms which obtain energy from inorganic compounds and occupy basal ice/till veins and subglacial lakes.

The lithotrophic microorganisms degrade the till and black carbon on the surface of the glaciers. So, the concentration of dissolved ions in water increases and its melting point decreases. This fact develops cryoconite holes, vertical cylindrical melt holes in a glacier surface, which have a thin layer of sediment at the bottom and are filled with water [21]. The materials comprising cryoconite can be divided into two main types: organic and inorganic [22]. Organic matter includes living and dead microorganisms and their products of decomposition, while inorganic matter in cryoconites is dominated by mineral fragments, mainly silicates [22]. Cryoconites are an important microbial habitat in supraglacial ecosystems [1]. Cryoconite holes may converge and origin small streams of liquid water that run downhill [21]. Food webs in cryoconite are dominated by photoautotrophs, mainly cyanobacteria, which provide substrate for heterotrophic communities from a wide range of bacteria. All major groups of heterotrophic bacteria and many fungal groups are represented in cryoconite holes [5]. In addition, microbial eukaryotes such as ciliates are crucial for nutrient recycling through the metabolism of primary producers [23]. Heterotrophic activities in supraglacial habitats are substantial but typically occur at lower rates than the rates of photosynthetic production, which leads to the accumulation of organic matter over time.

Microorganisms inhabiting glacial surface produce a wide diversity of pigmented molecules, which allow their adaptation to cold conditions and solar radiation. They use pigments to obtain energy [24], develop photosynthesis [25], stress resistance [26], and for ultraviolet light protection [27, 28]. For instance, green snow is caused by young, trophic stages of snow algae, whereby more mature and carotenoid-rich resting stages result in all shades of red snow [29]. Dominant species on snow fields belong to the unicellular Chlamydomonaceae. Additionally, some examples of cold-adapted bacteria that produce pigments are the bacterium *Sphingobacterium antarcticus*, which produces zeaxanthin, b-cryptoxanthin, and b-carotene [30]. Other examples include the polar bacteria *Octadecabacter arcticus* and *Octadecabacter antarcticus*, producers of xanthorhodopsin [31], and *Shewanella frigidimarina* which produces the red cytochrome c3 [32, 33]. Colored melanized fungi also live on glaciers, for instance, the oligotrophic genus *Cladosporium* [34]. These pigments absorb solar light and heat, melting snow on glacial surfaces. Microorganisms on glacial surfaces also bear high solar radiation, but in a way, this radiation is beneficial for them. In spring, light radiation melts the glacial surface and leads to the increase in wet areas and the dilution of solutes on snow and ice surfaces, which facilitates the growth of microbial mats [14].

#### **4.2. The englacial ecosystem**

**4. Microbial ecology in glaciers**

108 Glacier Evolution in a Changing World

**4.1. The supraglacial ecosystem**

the atmosphere [20].

subglacial lakes.

Considering the horizontal stratification of glaciers, they can be divided into three parts: the supraglacial ecosystem, the subglacial system, and the englacial ecosystem [2, 5] (**Figure 3**). These three ecosystems differ in terms of their solar radiation, water content, nutrient abundance, and redox potential [2]. These factors greatly determine the biogeochemical cycles, the type of metabolism, and the diversity and abundance of microbial populations inhabiting glaciers.

The main habitats in the supraglacial ecosystem are the snowpack, cryoconite holes, supraglacial streams, and moraines. On the glacier surface, the absorption of solar radiation by dark organic matter causes snow and ice melting yielding liquid water that is necessary for microorganisms. Meltwater dissolves nutrients from adjacent debris and even directly from

The sunlit and oxygenated supraglacial surface are populated by autotrophic microorganisms such as microalgae and diatoms; by chemolitotrophic bacteria, which feed on inorganic

**Figure 3.** A schematic of different habitats of a glacier colonized by microorganisms. Supraglacial ecosystem is dominated by phototrophic algae and cyanobacteria that take advantage of sunlight and by heterotrophic bacteria and microeukaryotes that feed on organic particles from atmospheric deposition. Microorganisms in englacial ecosystem can be chemoautotrophs, but they can also be heterotrophic bacteria that feed of solubilized products. Subglacial ecosystem is dominated by microorganisms which obtain energy from inorganic compounds and occupy basal ice/till veins and

sand particles; and by heterotrophic bacteria and microeukaryotes.

The englacial ecosystem presents a minor impact upon nutrient dynamics [2]. Surface meltwaters flood the englacial sediments by means of drainage channels. In englacial ecosystems, live motile bacteria that can reach more than 3000 m of depth. These bacteria live at grain boundaries and other interstices. Mineral substrates such as clay particles [35] provide nutrients and a supply of water for microorganisms. Microorganisms can also live in narrow veins between ice crystals. When the water freezes, dissolved and particulate impurities (including microorganisms) are excluded from the ice matrix into interstitial aqueous channels at the ice-grain boundaries [11]. In turn, these microorganisms and impurities diminish the growth of ice crystals and even break them, facilitating the existence of liquid water. The liquid vein habitat provides water, energy, and nutrients. In contrast with this, the metabolism of microbes encased in solid ice must overcome the diffusion of nutrients in a solid media [36].

Microorganisms in englacial ecosystems can be chemoautotrophs, but they can also be heterotrophic bacteria that feed on solubilized products from pollen grains, invertebrates, and other microorganisms. At great depth, anaerobic respiration can take place [35, 37], and methanogens could be active [2].

## **4.3. The subglacial ecosystem**

At glacial sediments and bedrock, debris contains minerals and sedimentary organic carbon that, combined with subglacial water, create microniches where microorganisms can live [5]. A strong coupling is likely to exist between the hydraulic conditions at the glacier bed and the bacterial processes that take place [20]. The subglacial system is dominated by aerobic/ anaerobic bacteria and probably viruses in basal bedrock and subglacial lakes. It also contains diverse, metabolically active archaeal, bacterial and fungal species [38]; although eukaryotes have not been detected in all subglacial environments examined [5].

As there is no sunlight, chemoautotrophic or chemolithotrophic bacteria obtain energy from inorganic compounds. The inorganic processes associated with chemoautotrophs and chemolithotrophs may make these bacteria one of the most important sources of weathering and erosion of rocks on Earth [39].
