Light-dependent microbial metabolisms drive carbon fluxes on glacier surfaces

Abstract

Biological processes on glacier surfaces affect glacier reflectance, influence surface energy budget and glacier response to climate warming, and determine glacier carbon exchange with the atmosphere. Currently, carbon balance of supraglacial environment is assessed as the balance between the activity of oxygenic phototrophs and the respiration rate of heterotrophic organisms. Here we present a metagenomic analysis of tiny wind-blown supraglacial sediment (cryoconite) from Baltoro (Pakistani Karakoram) and Forni (Italian Alps) glaciers, providing evidence for the occurrence in these environments of different and previously neglected metabolic pathways. Indeed, we observed high abundance of heterotrophic anoxygenic phototrophs, suggesting that light might directly supplement the energy demand of some bacterial strains allowing them to use as carbon source organic molecules, which otherwise would be respired. Furthermore, data suggest that CO 2 could be produced also by microbiologically mediated oxidation of CO, which may be produced by photodegradation of organic matter. The ISME Journal Climate change is determining a global cryosphere shrinkage and mountain glacier environments are declining (IPPC, 2014). The consequent loss of biodiversity is yet to be fully assessed, particularly the loss of functional biodiversity in extreme environments (Stibal et al., 2012;Boetius et al., 2015). Cryoconite holes, that is, small depressions on glacier surfaces whose formation is because of windborne debris (cryoconite), are the most biologically active environments on glaciers (Boetius et al., 2015).We used whole-metagenomic sequencing to investigate the main functions of six cryoconite holes from Forni (Italian Alps) and six from Baltoro (Pakistani Karakoram) glaciers. We focused on carbon and energy metabolisms by comparing the total coverage of marker genes for photosynthesis, use of inorganic and organic compounds as energy source and autotrophy/heterotrophy. We also used metagenomic sequences for the taxonomic attribution of microorganisms carrying specific metabolic genes Table S4 reports the marker genes whose coverage (mean number per base of reads mapping the genes) was used to infer the abundance of each metabolism. Supplementary Table S5 and Figure 1 report their normalized coverages. Finally, we measured chemical/physical parameters of cryoconite holes and oxygen consumption rates on the days of sampling, both under light and dark conditions (Supplementary Table S3 and Supplementary Figure S2). The main hypothesis tested was whether oxygenic phototrophy and organotrophic respiration represent the only significant metabolisms affecting carbon balance on glacier surface, as currently conceived, or other microbial processes could contribute to it.On the basis of 16S rRNA gene sequencing, Cyanobacteria represented 22 and 3% of the microbial community on Forni and Baltoro, respectively (Supplementary Figure S3). High abundance of cyanobacteria has been already observed in polar and alpine cryoconite (Segawa et al....

Post-Depositional Biodegradation Processes of Pollutants on Glacier Surfaces

“…A whole shotgun metagenomics analysis of the bacteria occurring in the cryoconite of the same glacier allowed the genome reconstruction of a bacterial population that could be responsible of the pesticide biodegradation [31]. These putative CPF-degrading bacteria resulted to be photoheterotrophic Burkholderiales, thus strengthening previous findings on the metabolic versatility and the importance of Betaproteobacteria in the heterotrophic metabolisms of cold environments [37,61,62].…”

“…A whole shotgun metagenomics analysis of the bacteria occurring in the cryoconite of the same glacier allowed the genome reconstruction of a bacterial population that could be responsible of the pesticide biodegradation [31]. These putative CPF-degrading bacteria resulted to be photoheterotrophic Burkholderiales, thus strengthening previous findings on the metabolic versatility and the importance of Betaproteobacteria in the heterotrophic metabolisms of cold environments [37,61,62]. Ferrario and colleagues [16,31] detected high concentrations (2-3 mg kg −1 ) of the pesticide chlorpyrifos (CPF) in the cryoconite and meltwater of an Alpine glacier, while in agriculture and termiticidal initial soils (i.e., in the application area) its concentrations vary from 10 to 1000 mg per kg of soil respectively [59].…”

“…Cryoconite in holes has higher concentration of organic matter and inorganic nutrients (nitrogen, phosphorus...) than the surrounding ice and supraglacial sediments [35,36]. This characteristic leads, on the one hand, to the incorporation of hydrophobic compounds such as POPs into the cryoconite organic matter [6] and, on the other hand, it promotes the presence of biodiverse and active microbial communities [37], which can biodegrade the contaminants.…”

Impact of impurities and cryoconite on the optical properties of the Morteratsch Glacier (Swiss Alps)

“…Nevertheless, a lot of uncertainty still exists regarding its formation and evolution. Moreover, most of the literature focused on Arctic, Antarctic and Asian glaciers, while little information is available for the European Alps Franzetti et al, 2016), despite their high sensitivity to environmental and climatic changes (Beniston, 2005). The Alps rise on the top of the Po river plain, one of the most industrialized and densely populated areas of Europe.…”

The power of glacial microbes

“…However, evidence is emerging of unexpected light-driven metabolisms in glacial systems, such as carbon monoxide oxidation 9 . This may introduce older sources of inorganic and organic carbon into the pool of actively cycled organic material on the surface of glaciers.…”