Spatially consistent microbial biomass and future cellular carbon release from melting Northern Hemisphere glacier surfaces

Earth's glaciers and ice sheets are melting at an alarming rate, releasing meltwater, microbes, nutrients, and sediment into various environments. The accelerating loss of ice, particularly outside Greenland and Antarctica, is projected to peak within the next 50 years. This meltwater discharge will significantly impact the microbial productivity, biogeochemical cycles, and biodiversity of glacier-fed ecosystems. A crucial aspect of this impact is the organic carbon (OC) delivered by glacial meltwaters, which supplements downstream environments with bioavailable OC. However, fluctuations in OC delivery can lead to complex ecological responses. One significant component of glacially-derived OC is the active microbial ecosystem found on melting glacier surfaces, which contributes dissolved and particulate organic carbon (DOC and POC) and microbial assemblages to meltwater. While estimates exist for Greenland, similar assessments for other glaciers are lacking. The 'weathering crust' on the glacier surface provides diverse microbial habitats, including saturated and unsaturated subsurface ice and cryoconite holes. This near-surface photic zone, formed during the melt season, is rich in particles and meltwater, supporting a community of ice algae, cyanobacteria, bacteria, and other protists. Despite estimates of vast microbial cell numbers in Earth's weathering crust, assessments of their activity and role in OC cycling remain scarce. The weathering crust connects glacier surface habitats with downstream environments via hydrological networks, but this connection and its role in regulating biomass and OC transport are not well understood. The current study aims to evaluate microbial abundance in glacial weathering crust meltwaters and examine its association with supraglacial hydraulic properties across various latitudinal and climatological settings from eight Northern Hemisphere glaciers and two Greenland Ice Sheet sites to improve understanding of these processes in this 'peak melt' century.

Existing research highlights the significant impact of glacier melt on downstream ecosystems, focusing on the increased delivery of meltwater, nutrients, and organic carbon. Studies have shown the importance of organic carbon as a bioavailable resource, but also the complex ecological responses to fluctuations in its delivery. The active microbial ecosystems thriving on glacier surfaces are known to contribute both dissolved and particulate organic carbon, along with microbial assemblages, to meltwater. However, comprehensive characterization of these communities and their contribution to global carbon cycling has been limited, especially outside of Greenland. Previous studies have touched upon the microbial communities within various parts of the glacier system, such as cryoconite holes and subglacial environments. However, a comprehensive, spatially consistent assessment of microbial abundance across diverse glacier systems and the subsequent projection of future carbon export was largely missing until this study.

Fieldwork was conducted over two years (July 2014-July 2016) at ten sites across the Northern Hemisphere. Samples were collected from both weathering crust and stream environments during the summer melt season. Microbial enumeration employed an optimized flow cytometry (FCM) protocol. Meltwater samples were fixed using paraformaldehyde or glutaraldehyde, stored, and then analyzed using a Sony SH-800EC cell sorter. A multi-stage gating protocol was used to identify and count microbes, with size categories ranging from <1 to >15 µm. Hydrological data including weathering crust hydraulic conductivity (K), water table depth, electrical conductivity (EC), water temperature, and meltwater discharge (Q) were collected alongside microbial abundance measurements. Suspended sediment concentration was also measured. Statistical analyses were conducted to examine correlations between microbial abundance and hydrological variables. Cellular carbon export from glacier surfaces was estimated by upscaling the averaged cellular carbon in supraglacial meltwaters using projected future glacial discharge under different Representative Concentration Pathways (RCPs). Cellular carbon content per unit meltwater volume was calculated using abundance-size derived cell biovolume and allometric conversion ratios. Glacier runoff projections were obtained from Global Glacier Evolution Model (GloGEM) and regional climate model MAR, for glaciers and the Greenland Ice Sheet respectively. Uncertainty ranges for microbial carbon export were calculated considering uncertainties in both cell concentration and modelled discharge.

The study revealed a mean microbial abundance of 2.2 × 10⁴ ± 5.5 × 10⁴ cells mL⁻¹ in weathering crust meltwaters across the ten sites. No significant difference was found between weathering crust and in-stream abundances. While there were differences in cell concentrations between glaciers, the mean abundances remained within the same order of magnitude (10⁴ cells mL⁻¹) across all locations. The modal size class of microbes was 1–2 µm. Suspended sediment concentration showed the strongest correlation with microbial abundance, possibly reflecting co-mobilization under similar hydrological conditions or the presence of microbes within sediment aggregates. Hydraulic conductivity, electrical conductivity, and water temperature showed weak or insignificant relationships with microbial abundance. The study projected that under a medium emission scenario (RCP 4.5), annual average cellular carbon export from the study regions and the Greenland Ice Sheet would be 0.65 Mt yr⁻¹ over the 21st century. This represents a significant portion of POC flux from Arctic rivers. 'Peak carbon' export is projected to occur between 2040 and 2059 under the medium emission scenario. The Greenland Ice Sheet will become an increasingly large contributor to total carbon export. The study also suggests that supraglacially-derived cells are a primary source of POC exported from glaciers, especially outside major ice sheets.

The findings highlight the significant contribution of supraglacial microbial communities to glacial meltwater OC fluxes. The consistent microbial abundance across diverse glacier settings indicates a broad stability in this ecosystem despite varying hydrological conditions. The strong correlation with suspended sediment concentration suggests a potential link between sediment transport and microbial mobilization. The lack of strong correlations with other hydrological parameters points to the influence of other ecological factors, such as nutrient availability or biotic interactions, which warrant further investigation. The projections of future cellular carbon export underscore the importance of considering supraglacial ecosystems when assessing the downstream biogeochemical impacts of glacier retreat. The higher proportion of surface-derived POC in proglacial settings compared to the Greenland Ice Sheet likely reflects shorter subglacial drainage pathways in smaller glaciers.

This study provides a critical baseline assessment of microbial abundance and cellular carbon export from Northern Hemisphere glacier surfaces. The consistent abundance of ~10⁴ cells mL⁻¹ across diverse settings and the substantial projected carbon export emphasize the importance of these communities in downstream ecosystems. The study highlights the need for further research to explore the ecological controls on microbial abundance, the fate of exported carbon, and the ecological implications of changing carbon fluxes in deglacierizing regions. Further investigations into seasonal variations, specific nutrient limitations, and biotic interactions within the weathering crust are crucial for refining our understanding of this important ecosystem and its role in global carbon cycling.

The study's carbon export projections rely on several assumptions, including consistent microbial community composition and abundance over the next 80 years, efficient advection of supraglacial carbon downstream, and the equivalence of supraglacial cellular carbon to total supraglacial POC. The extrapolation to unsampled regions is based on broad similarities in glaciological and climatological settings, which may not fully capture local variations. The lack of seasonal data limits the assessment of seasonal variations in microbial concentrations, which could affect the accuracy of carbon export estimates. Finally, the study focuses on the Northern Hemisphere, excluding High Mountain Asia and the Southern Hemisphere, limiting its global applicability.