Arctic shrub colonization lagged peak postglacial warmth: Molecular evidence in lake sediment from Arctic Canada.

Arctic shrubification is an observable consequence of climate change, already resulting in ecological shifts and global-scale climate feedbacks including changes in land surface albedo and enhanced evapotranspiration. However, the rate at which shrubs can colonize previously glaciated terrain in a warming world is largely unknown. Reconstructions of past vegetation dynamics in conjunction with climate records can provide critical insights into shrubification rates and controls on plant migration, but paleoenvironmental reconstructions based on pollen may be biased by the influx of exotic pollen to tundra settings. Here, we reconstruct past plant communities using sedimentary ancient DNA (sedaDNA), which has a more local source area than pollen. We additionally reconstruct past temperature variability using bacterial cell membrane lipids (branched glycerol dialkyl glycerol tetraethers) and an aquatic productivity indicator (biogenic silica) to evaluate the relative timing of postglacial ecological and climate changes at a lake on southern Baffin Island, Arctic Canada. The sedaDNA record tightly constrains the colonization of dwarf birch (Betula, a thermophilous shrub) to 5.9 ± 0.1 ka, ~3 ka after local deglaciation as determined by cosmogenic 10 Be moraine dating and >2 ka later than Betula pollen is recorded in nearby lake sediment. We then assess the paleovegetation history within the context of summer temperature and find that paleotemperatures were highest prior to 6.3 ka, followed by cooling in the centuries preceding Betula establishment. Together, these molecular proxies reveal that Betula colonization lagged peak summer temperatures, suggesting that inefficient dispersal, rather than climate, may have limited Arctic shrub migration in this region. In addition, these data suggest that pollen-based climate reconstructions from high latitudes, which rely heavily on the presence and abundance of pollen from thermophilous taxa like Betula, can be compromised by both exotic pollen fluxes and vegetation migration lags.

Intact polar lipidome and membrane adaptations of microbial communities inhabiting serpentinite-hosted fluids.

The generation of hydrogen and reduced carbon compounds during serpentinization provides sustained energy for microorganisms on Earth, and possibly on other extraterrestrial bodies (e.g., Mars, icy satellites). However, the geochemical conditions that arise from water-rock reaction also challenge the known limits of microbial physiology, such as hyperalkaline pH, limited electron acceptors and inorganic carbon. Because cell membranes act as a primary barrier between a cell and its environment, lipids are a vital component in microbial acclimation to challenging physicochemical conditions. To probe the diversity of cell membrane lipids produced in serpentinizing settings and identify membrane adaptations to this environment, we conducted the first comprehensive intact polar lipid (IPL) biomarker survey of microbial communities inhabiting the subsurface at a terrestrial site of serpentinization. We used an expansive, custom environmental lipid database that expands the application of targeted and untargeted lipodomics in the study of microbial and biogeochemical processes. IPLs extracted from serpentinite-hosted fluid communities were comprised of >90% isoprenoidal and non-isoprenoidal diether glycolipids likely produced by archaeal methanogens and sulfate-reducing bacteria. Phospholipids only constituted ~1% of the intact polar lipidome. In addition to abundant diether glycolipids, betaine and trimethylated-ornithine aminolipids and glycosphingolipids were also detected, indicating pervasive membrane modifications in response to phosphate limitation. The carbon oxidation state of IPL backbones was positively correlated with the reduction potential of fluids, which may signify an energy conservation strategy for lipid synthesis. Together, these data suggest microorganisms inhabiting serpentinites possess a unique combination of membrane adaptations that allow for their survival in polyextreme environments. The persistence of IPLs in fluids beyond the presence of their source organisms, as indicated by 16S rRNA genes and transcripts, is promising for the detection of extinct life in serpentinizing settings through lipid biomarker signatures. These data contribute new insights into the complexity of lipid structures generated in actively serpentinizing environments and provide valuable context to aid in the reconstruction of past microbial activity from fossil lipid records of terrestrial serpentinites and the search for biosignatures elsewhere in our solar system.

Bacterial and archaeal lipids trace chemo(auto)trophy along the redoxcline in Vancouver Island fjords.

Marine oxygen minimum zones play a crucial role in the global oceanic carbon, nitrogen, and sulfur cycles as they harbor microbial communities that are adapted to the water column chemistry and redox zonation, and in turn control the water column chemistry and greenhouse gas release. These micro-organisms have metabolisms that rely on terminal electron acceptors other than O2 and often benefit from syntrophic relationships (metabolic coupling). Here, we study chemo(auto)trophy along the redoxcline in two stratified fjords on Vancouver Island (Canada) using bacterial bacteriohopanepolyols and archaeal ether lipids. We analyze the distribution of these lipid classes in suspended particulate matter (SPM) to trace ammonia oxidation, anaerobic ammonium oxidation (anammox), sulfate reduction/sulfur oxidation, methanogenesis, and methane oxidation, and investigate ecological niches to evaluate potential links between their respective bacterial and archaeal sources. Our results show an unparalleled BHP and ether lipid structural diversity that allows tracing the major redox-driven metabolic processes at the time of sampling: Both fjords are dominated by archaeal ammonia oxidation and anammox; sulfate-reducing bacteria may be present in Deer Bay, but absent from Effingham Inlet; methanogenic Euryarchaeota and archaeal and bacterial methanotrophs are detectable at low abundance. Correlation analysis reveals distinct biomarker clusters that provide constraints on the biogeochemical niches of some orphan BHP and ether lipids such as in situ-produced adenosyl-BHPs or unsaturated archaeols.