T he comminution of bedrock in subglacial systems promotes weathering processes by exposing fresh minerals with a high surface area (1-5). Subglacial water chemical profiles (e.g., see references 5 and 6), field-and laboratory-based microcosm experiments (e.g., see references 7-9), and molecular analyses (e.g., see references 6, 10, and 11) indicate the presence of an active and diverse subglacial microbiome founded on chemical energy that functions to enhance rates of mineral weathering (8). Given that ice covers approximately 10% of the present-day continental landmass, the subglacial environment is a widespread habitat for microbial life and for mineral weathering.Aqueous geochemical data collected from the meltwaters of numerous glaciers suggest that pyrite (FeS 2 ) oxidation and the concomitant production of hydrogen ions are key drivers of subglacial bedrock weathering (5, 6, 9, 12, 13). It has also been inferred that FeS 2 weathering in the subglacial environment may be microbially mediated (3, 6, 9). This inference is supported by DNA-based molecular data that show the presence in subglacial systems of a number of taxa closely related to organisms capable of Fe and S oxidation (6,7,9,11,14,15). Moreover, Mitchell et al. (7) showed that microbial communities colonizing FeS 2 incubated in situ in a subglacial meltwater stream at Robertson Glacier (RG), Canada, were phylogenetically more similar at the level of 16S rRNA genes to communities associated with native subglacial sediments and suspended sediments than communities colonizing other iron-bearing minerals (i.e., magnetite, hematite, and olivine) and carbonate minerals (i.e., calcite) (7). These data suggest a relationship between microbial community structure and bedrock mineralogy and imply that bedrock minerals serve as a source of energy for subglacial microbial communities. Further evidence for the role of FeS 2 in supporting subglacial microbial communities comes from the recovery of 16S rRNA gene transcripts from RG sediments that exhibit a close affiliation with known Fe-and S-oxidizing taxa (11). However, FeS 2 oxidation pathways in the subglacial system and the role of microbes in these geochemical transformations, especially those FeS 2 oxidation processes that occur under hypoxic or anoxic conditions thought to characterize significant sectors of subglacial drainage networks, are poorly understood (16).In acidic (pH Ͻ 4) environments, abiotic FeS 2 oxidation is known to occur through both oxic and anoxic processes (17)(18)(19)(20)
Snow algae can form large-scale blooms across the snowpack surface and near-surface environments. These pigmented blooms can decrease snow albedo, increase local melt rates, and may impact the global heat budget and water cycle. Yet, underlying causes for the geospatial occurrence of these blooms remain unconstrained. One possible factor contributing to snow algae blooms is the presence of mineral dust as a micronutrient source. We investigated the bioavailability of iron (Fe) -bearing minerals, including forsterite (Fo, MgFeSiO), goethite, smectite and pyrite as Fe sources for a - bacteria co-culture through laboratory-based experimentation. Fo was capable of stimulating snow algal growth and increased the algal growth rate in otherwise Fe-depleted co-cultures. Fo-bearing systems also exhibited a decrease in bacteria:algae ratios compared to Fe-depleted conditions, suggesting a shift in microbial community structure. The co-culture also increased the rate of Fo dissolution relative to an abiotic control. Analysis of 16S rRNA genes in the co-culture identified , and , all of which are commonly found in snow and ice environments. Archaea were not detected. and , which are known to enhance mineral weathering rates, comprised two of the top eight (> 1 %) OTUs. These data provide unequivocal evidence that mineral dust can support elevated snow algae growth under otherwise Fe-depleted growth conditions, and that snow algae can enhance mineral dissolution under these conditions. Fe, a key micronutrient for photosynthetic growth, is necessary to support the formation of high-density snow algae blooms. The laboratory experiments described herein allow for a systematic investigation of snow algae-bacteria-mineral interactions and their ability to mobilize and uptake mineral-bound Fe. Results provide unequivocal and comprehensive evidence that mineral-bound Fe in Fe-bearing Fo was bioavailable to snow algae within an algae-bacteria co-culture. This evidence includes: 1) an observed increase snow algae density and growth rate; 2) decreased bacteria:algae ratios in Fo-containing cultures relative to cultures grown under similarly Fe-depleted conditions with no mineral-bound Fe present; and 3) increased Fo dissolution rates in the presence of algae-bacteria co-cultures relative to abiotic mineral controls. These results have important implications for the role of mineral dust in supplying micronutrients to the snow microbiome, which may help support dense snow algae blooms capable of lowering snow albedo, and increase snow melt rates on regional, and possibly global, scales.
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