Desempeño Académico

2.4.1 Bare glacial ice

The filamentous ice algae that dominate the bare ice surfaces have only recently been described more in detail (Remias et al, 2012a; Remias et al, 2009; Yallop et al, 2012).

Ice algae belong to the Zygnematophyceae and the most described species are Ancylonema nordenskiöldii Berggren 1871 and Mesotaenium berggrenii Lagerheim. Ice algal mass blooms cause a grey colouration of the ice and they show cosmopolitan occurrence in many permanently frozen alpine and polar settings (Kol, 1968; Remias et al, 2012a; Remias et al, 2009; Takeuchi et al, 2006; Uetake et al, 2010; Yallop et al, 2012). The metabolic season is limited to a short period in summer when liquid water is available. Due to the lack of any flagellated stage, the glacial ice algae are restricted to the ice surface and cannot actively move upwards into the snow layers. In contrast to snow algae, they are able to persist harsh conditions with relatively thin and less rigid cell walls and without formation of cysts (Remias et al, 2009). Mesotaenium berggrenii cells exposed to -25ºC could be revived and show that they are well adapted to overwintering in a frozen state (Ling & Seppelt, 1990).

Raphidonema sempervirens can be abundant on glacial ice surfaces as well. However, it is a typical permafrost algae and not a true snow algal species.

Laboratory experiments (Leya et al, 2009) also demonstrated that Raphidonema sempervirens does not share one of the main cryophilic properties of true snow algae, i.e., the production of secondary carotenoids (e.g., astaxanthin). Yet, in culture and under optimal conditions Raphidonema sempervirens is able to produce significant amounts of primary carotenoids, i.e. xantophylls (Leya et al, 2009). Furthermore, Stibal and Elster (2005) have suggested that Raphidonema sempervirens is likely introduced onto glacial surfaces by wind rather than through in-situ propagation. Thus, it remains unclear whether this species is a critical player in the ecology of glaciers.

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Figure 2.6: Filamentous ice algae Ancylonema nordenskiöldii (Source: Yallop et al., 2012). Scale bar = 10 µm

2.4.2 Ecophysiological adaptation strategies

Ancylonema nordenskilöldii and Mesotaenium berggrenii accumulate a hydrophilic brownish vascuolar pigment with a tannin nature, which was identified by Remias et al. (2012b) as purpurogallin carboxylic acid-6-O-b-D-glucopyranoside, whereas secondary carotenoids seem to be absent (Remias et al, 2009). The brownish pigment has a broad absorption range in the visible as well as the UV-A and UV-B range and therefore plays an important role in shielding the chloroplast and avoid photoinhibition. The same pigmentation can be found in algal cells thriving in less UV irradiated settings, suggesting that this compound may additionally act as an antimicrobial agent against grazers apart from the photoprotective role (Remias et al, 2012b).

Due to their dark pigmentation and their wide occurrence glacial ice algae play an important role in reducing the albedo of bare ice fields during summer and thus in increasing glacial melting (Yallop et al., 2012).

2.4.3 Cryoconites

First described and named by the Swedish explorer A. E. Nordenskjöld during his Greenland expedition 1870, cryoconites are derived from ‘cryo’ meaning ice and ‘conite’ meaning dust (Nordenskjöld, 1875). Among glacial surface habitats, cryoconite holes (cyanobacteria dominated water-filled holes formed by the preferential melt of organic and inorganic dark particles) have been by far the more extensively studied habitats (Cameron et al, 2012; Edwards et al, 2014).

Cryoconites form from solar-heated organic and inorganic debris that melts into the ice by absorbing more radiation than the surrounding ice due to the lower albedo. Small individual holes can coalesce into larger holes or can become connected by meltwater channels. There are three types of cryoconites: cryoconite holes, stream cryoconites and dispersed cryoconite granules. They can be found in the ablation zones of glaciers (Porazinska et al, 2004).

They are unique environments filled with liquid water and seeded by material from the local environment. Nutrient and microbe input can be either wind-blown, via melt or rain water or direct lateral input through englacial piping (Kaštovská et al, 2005)

They can cover up to 10% of a glacier’s surface and contain highly diverse microbial communities including viruses, bacteria (mainly cyanobacteria and proteobacteria), algae (mainly Chlorophyceae), fungi, and even ciliates, copepods, insects, tardigrades, nematodes, and rotifers (Anesio et al, 2009; Anesio et al, 2007; Christner et al, 2003; Edwards et al, 2010; Hodson et al, 2007; Müller et al, 2001a).

Anesio et al. (2007) found that in cryoconite holes photosynthesis exceeds respiration and they have the potential to fix up to 64 Gg of carbon per year, which is important in the context of a globally changing climate. Despite the low temperatures, microbial activity in cryoconite sediments can be exceptionally high (Säwström et al, 2002) and approximate more temperate and nutrient rich regions (Anesio et al, 2007). This makes them hot spots for biogeochemical cycling

(Hodson et al, 2007; Telling et al, 2011). Cyanobacteria and heterotrophic bacteria are the main nutrient cyclers (Kaštovská et al, 2005; Margesin et al, 2002). This is even enhanced by their viral-induced lysis and subsequent release of nutrients (Anesio et al, 2007).

In winter, the top of cryoconite holes is usually frozen, but they still absorb more solar energy than the surrounding due to the underlying dark sediment, which may allow the existence of liquid water. In summer, they are open to the atmosphere, allowing an exchange of gases, nutrient and microbes (Hodson et al, 2007). The chemistry and microbiology of individual cryoconites are often very distinct from other cryoconites due to hydraulic isolation from glacial melt water (Tranter et al, 2004).