**1. Introduction**

Earth's cold environments have been considered uninhabited for a long time. Icy deserts seemed too hostile to harbor life (**Figure 1**). However, glaciers and ice sheets are unique biomes dominated by microbial communities which maintain active biochemical routes [1].

Glaciers are dominated by very specific environmental characteristics such as the low temperatures associated with precipitation in the form of snow, the exposure to the intense wind, and the extreme solar radiation. In summer, there are processes of melting and sublimation

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

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.

diversity decreases. At last, taking into account the horizontal stratification of glaciers, they can be divided into three ecosystems: supraglacial, englacial, and subglacial. Additionally, there has been an increasing interest in characterizing retreating ice fronts of deglaciated forefields with the aim of getting to know how both the richness and the abundance of microorganisms vary in a glacier due to climate change [6–8]. In forefields, mixed communities are observed, whose composition changes very quickly.
