**1. Introduction**

Polar glaciers have aroused great interest over the last year, and their study has increased since they are sentinels of climate change. Although both poles are extreme environments (in terms of low temperatures, high UV radiation, lack of light in winter and permanent solar radiation in summer, scarce nutrients, etc.), Arctic and Antarctic glaciers are very different. The North Pole is an ocean surrounded by land, while the South Pole is a continent surrounded by water. This distinction confers them very unique geographical and environmental characteristics.

**Figure 1.**

*Polar maps and localization of the referenced glaciers. (1) Cascade Volcano Arc [4]; (2) Robertson Glacier, Alberta [5]; (3) Western margin of the Greenland ice sheet [6]; (4) southwest part of the Greenland ice sheet [7]; (5) North-Eastern Greenland [8]; (6) Islandic glaciers [11, 12]; (7) Hamiltonbukta, Svalbard [9]; (8) Ny Ålesund, Svalbard [10]; (9) West Antarctic ice sheet [15]; (10) high Antarctic Plateau [13]; (11) Lake Vostok [14] (map source: Google Earth Pro).*

Metabolically active microbial communities have been identified in both the Arctic [1] and Antarctic glaciers (**Figure 1**) [2]. These microbial communities include bacteria, archaea, microeukaryotes, and viruses [3].

Arctic glaciers do not reach such distant latitudes or low temperatures as Antarctic glaciers do. These are some of the reasons why they are being extremely affected by global warming. There are glaciers around the entire Arctic Ocean, but polar glaciers in North America [4, 5], Greenland [6–8], Svalbard [9, 10], and Iceland [11, 12] (**Figure 1**) have been the most widely studied from a microbiological point of view. Antarctic glaciers present exceptional environmental conditions. Being in higher latitudes allows the existence of very low temperatures and high rates of solar radiation in summer. Some published reports on glacial and subglacial microbiology refer to very extreme latitudes that reach −75°S in the high Antarctic Plateau [13], −77°S in Lake Vostok [14], and −84°S in the West Antarctic ice sheet [15] (**Figure 1**).

In the study of glacier microbiology, a variety of techniques have been traditionally used, such as microscopy techniques [16], cell cultures, and isolation of microorganisms [17]. However, the most significant advance has been achieved with the application of metagenomics. This discipline has allowed both the knowledge of the microbial communities' structure and the comprehension of their metabolic potential.
