Ecological paleo-reconstruction of polar lakes in Greenland based on molecular and isotopic analysis of lipid biomarkers

Abstract

<p>While glacial systems are well established as susceptible environments to climatic alterations, they also offer a set of extreme conditions that are optimal for habitability studies and to interrogate the limits of life. In these settings, indigenous microorganisms must endure a prolonged exposure to cold temperatures and to background radiation for geological timescales. Therefore, glaciers and the surrounding cryo-environments (permafrost, glacial lakes or melting streams) arise as relevant scenarios to examine the development of functional microbial cryo-ecosystems and may have implications in the search for past or extant life in icy worlds beyond Earth.</p> <p>Among the multiple perspectives from which thermal susceptibility can be measured, lacustrine systems effectively archive paleoenvironmental information due to their high sedimentation rates and organic matter content. Thus, polar lakes exposed to glacial or periglacial conditions are expected to contain a thriving biology that responds to not only a continuously oscillating level of nutrients, but also to a set of extreme conditions (subzero temperatures and high UV flux) that may geochemically and environmentally resemble potentially habitable locations of the Solar System such as the Martian poles or the subglacial water bodies enclosed in the satellites Europa and Enceladus.</p> <p>The permanent ice sheet in Greenland represents a possible analog of the icy worlds, constituting an important long-term repository of psychrophilic microorganisms. Around it, different geological formations such as glacial lakes, permafrost or further peat soils represent a diverse degree of geobiological succession upon the influence (and retreat) of the ice sheet due to its thermal destabilization. Investigating molecular and isotopic lipid biomarkers of microorganisms inhabiting different cryo-ecosystems at and around the Greenland glacial structures may help obtain insights as of the type of life that could potentially arise on analogous extraterrestrial cold environments (ice sheet). Moreover, it could give clues on cryo-ecosystem evolutionary processes (biological succession) that are triggered when the ice cover retreats and gets exposed to the atmosphere (giving rise to glacier-melting streams, bedrock-erosion sediments, lake sediments and glacial soils). In particular, the focus of our studies is set on exploring the organic geochemistry of sediments in polar lakes representing nearby glacier-influenced ecosystems (glacial lakes) versus meteoric lakes representing longer time-exposed and further developed lacustrine and soil ecosystems (non-glacial lake).</p> <p>Here, we present preliminary results of a biogeochemical study in a meltwater and meteoric lake system within the Kangerlussuaq region by the West coast of Greenland,  representing an advanced state of succession in the thermal exposition gradient of the territory. We employed molecular (lipid biomarkers) and compound-specific isotopic analyses to define ecological (microbial community structure) and metabolic (carbon assimilation-related) fingerprints in this (relatively) long exposed glacial system  compared to more recently exposed lakes in Greenland. Ultimately, we aim to learn about the microbial strategies to adapt to the ever-changing conditions caused by glacier retreat at the aforementioned molecular and isotopic level. The ubiquity of lipids as basic cellular membrane compounds and their recalcitrant hydrocarbon skeletons render these molecules a solid biomarker candidacy for the study of organic matter and habitability in the context of ecology and astrobiology, respectively.</p> <p>Overall, the combination of lipid biomarker analysis and carbon isotopic studies on sediment samples of polar lakes supports the establishment of reliable lipid biomarkers in a cold, extreme environment, as well as their potential psychrophilic biosource. At the same time, determining the lipid content and their metabolic biosynthetic pathways builds a comprehensive perspective on the extent of the influence of environmental changes on the organic matter content in climatically susceptible environments.</p>