Abstract
Meltwater streams connect the glacial cryosphere with downstream ecosystems. Dissolved and particulate matter exported from glacial ecosystems originates from contrasting supraglacial and subglacial environments, and exported microbial cells have the potential to serve as ecological and hydrological indicators for glacial ecosystem processes. Here, we compare exported microbial assemblages from the meltwater of 24 glaciers from six (sub)Arctic regions – the southwestern Greenland Ice Sheet, Qeqertarsuaq (Disko Island) in west Greenland, Iceland, Svalbard, western Norway, and southeast Alaska – differing in their lithology, catchment size, and climatic characteristics, to investigate spatial and environmental factors structuring exported meltwater assemblages. We found that 16S rRNA gene sequences of all samples were dominated by the phyla Proteobacteria, Bacteroidetes, and Actinobacteria, with Verrucomicrobia also common in Greenland localities. Clustered OTUs were largely composed of aerobic and anaerobic heterotrophs capable of degrading a wide variety of carbon substrates. A small number of OTUs dominated all assemblages, with the most abundant being from the genera Polaromonas, Methylophilus, and Nitrotoga. However, 16–32% of a region’s OTUs were unique to that region, and rare taxa revealed unique metabolic potentials and reflected differences between regions, such as the elevated relative abundances of sulfur oxidizers Sulfuricurvum sp. and Thiobacillus sp. at Svalbard sites. Meltwater alpha diversity showed a pronounced decrease with increasing latitude, and multivariate analyses of assemblages revealed significant regional clusters. Distance-based redundancy and correlation analyses further resolved associations between whole assemblages and individual OTUs with variables primarily corresponding with the sampled regions. Interestingly, some OTUs indicating specific metabolic processes were not strongly associated with corresponding meltwater characteristics (e.g., nitrification and inorganic nitrogen concentrations). Thus, while exported assemblage structure appears regionally specific, and probably reflects differences in dominant hydrological flowpaths, OTUs can also serve as indicators for more localized microbially mediated processes not captured by the traditional characterization of bulk meltwater hydrochemistry. These results collectively promote a better understanding of microbial distributions across the Arctic, as well as linkages between the terrestrial cryosphere habitats and downstream ecosystems.
Highlights
Glacier meltwater streams connect discrete cryosphere habitats with downstream freshwater and marine ecosystems across the Northern Hemisphere (e.g., Hood et al, 2009; O’Neel et al, 2015; Milner et al, 2017)
Glaciers from Norway and Qeqertarsuaq were sampled at the highest elevations, and samples from the Greenland Ice Sheet (GrIS) had the largest catchment areas and total suspended solids (TSS) concentrations
Iceland and Qeqertarsuaq, both being basaltic localities, clustered together in the principle components analysis (PCA), while other regions did not show substantial overlap (Figure 2). These streams had among the greatest soluble reactive phosphorus (SRP) and dissolved silica (DSi) concentrations, and lowest DiMo ratios
Summary
Glacier meltwater streams connect discrete cryosphere habitats with downstream freshwater and marine ecosystems across the Northern Hemisphere (e.g., Hood et al, 2009; O’Neel et al, 2015; Milner et al, 2017). Subglacial microbes are found at the intersection of the glacier and the underlying bedrock, and are functionally diverse, having been shown to utilize a myriad of metabolic pathways operating over a spectrum of redox conditions (Boyd et al, 2010, 2011, 2014; Stibal et al, 2012a,c; Hamilton et al, 2013; Dieser et al, 2014), which may enable them to influence a host of weathering reactions and biogeochemical transformations (Sharp et al, 1999; Mitchell et al, 2013; Montross et al, 2013; Lamarche-Gagnon et al, 2019) Due to their physical inaccessibility, these habitats are notoriously difficult to investigate, and much of our knowledge of these habitats at present comes from discrete samples taken from marginal areas (e.g., Boyd et al, 2011; Žárský et al, 2018). Supraglacial (surface ice) microbial communities, which are comparatively straightforward to access, can include all three domains of life (Anesio et al, 2017), and include oxygenic, phototrophic and carbon-fixing taxa, with Cyanobacteria playing an integral part in forming the matrix of cryoconite found in depressions on the glacier surface (Langford et al, 2010; Cook et al, 2016; Gokul et al, 2019)
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