**3. Glaciers as biomes**

that are contrasted with the accumulation of ice in winter, producing an imperceptible but constant dynamism. This is the basis of the life of many of the organisms that inhabit them [2]. Microbial communities in glaciers are different depending on the type of glacier and the area studied. Thanks to DNA sequencing methods, a lot of information about their biodiversity and ecology has been acquired. Firstly, glaciers are of various kinds [3, 4] (**Figure 2**). Some of them have a marine margin and finish in a calving front (**Figure 2(C)**), this establishes important differences with respect to glaciers with a land margin (**Figures 2(A), (B)**). In glaciers ending on land, there is continuous permafrost at ice front (**Figure 2(A)**), while calving glaciers present partly or completely temperate tidewater tongues [4]. Secondly, the growth areas of the glacier (accumulation area) are oligotrophic media for microorganisms. They establish a vertical food chain, from surface photosynthesizers in the upper illuminated layers to protists and bacteria confined in the inner part [5]. These microorganisms are greatly influenced by the melting of surface layers. The diversity of microorganisms in the areas of regression of glaciers (ablation zone and glacial lake) can be lower than in the accumulation area [6], although they are usually more abundant. Predatory species are numerous in these areas, so microbial

**Figure 2.** Types of glaciers. (A) Gébroulaz glacier ending on land. (B) Literola glacier at Pyrenees, ending on a lake and

river. (C) Marine glacier at Livingston Island, South Shetland Antarctica.

**Figure 1.** Ecosystems in cold environments. (A) Polar deserts, (B) Glaciers, (C) Icy seas and (D) Icebergs.

106 Glacier Evolution in a Changing World

Among cold environments, glaciers are considered biomes that should be recognized as such in their own right [1, 2]. A great diversity of microorganisms belonging to the three main domains (Bacteria, Eucarya and Archaea) has been discovered inhabiting these cold environments. Most of the microorganisms isolated from cold environments are psychotolerant (also called psychrotrophs) and psychrophiles. Psychotolerant organisms can grow at temperatures close to 0°C but have their optimum growth temperature at about 20°C. However, psychrophiles have their optimal growth temperature at 15°C or less [15].

Glaciers are inhabited by microorganisms which maintain active biochemical processes.

To grow efficiently at low temperatures, microorganisms have developed complex structural and functional strategies for their adaptation [16]. The study of these adaptation strategies aims to identify the limits of life at these temperatures. Adaptations include the production of psychrophilic enzymes that are functional at low temperatures with a high catalytic efficiency; the incorporation of unsaturated fatty acids in the cell membrane to improve its fluidity; the synthesis of certain proteins that allow synthesizing others at low temperatures [17]; and the production of compounds that allow the cell to protect itself from frostbite (e.g. sugars, extracellular polysaccharides, antifreeze proteins) [18, 19].
