Microbial community composition and the transformation of dissolved organic matter in supraglacial environments
Relating microbial community composition to ecosystem function is a fundamental goal in ecological analyses, with physico-chemical parameters largely controlling this relationship. This investigation aimed to elucidate the impact of physicochemical factors on biodiversity in glacial habitats, with an emphasis on dissolved organic matter (DOM). DOM is a complex mixture of organic compounds and the primary substrate for microbial activity. Considering the variety of DOM sources in aquatic systems, little is still known about the biological release and bio-transformation of microbially-derived, autochthonous DOM. Continental Antarctica, typically lacking terrestrial carbon inputs, is largely governed by autochthonous DOM, making it an ideal site to investigate microbial biodiversity and the microbial formation of DOM. Different glacial ecosystems were selected, with a strong focus on the supraglacial Cotton Glacier stream, to investigate: i) the microbial diversity and underlying environmental factors governing biogeographical trends, ii) the contribution of exuded carbon to the DOM pool and subsequent heterotrophic uptake/transformation, and iii) how biofilm influences nutrient cycling in supraglacial environments. Findings from this study highlight distinct microbial assemblages in meltwater streams/sediments, ice, snow, and cryoconite across local and regional geographic scales. Specifically, nutrient availability and DOM quality influenced trends in microbial diversity. In situ DOM exudation was sufficiently high to support bacterial carbon demands, while the spatial organization of microorganisms in biofilms was advantageous in transferring nutrients between community members. Furthermore, compared to other more recalcitrant and chemically heterogeneous DOM sources, the highly labile supraglacial DOM was unable to sustain the same magnitude of microbial metabolism. The present study revealed dynamic carbon cycling in supraglacial environments, mediated by the tight coupling between in situ carbon fixation, DOM exudation, and rapid consumption. Statistical analyses failed to show the impact of any physical parameters on community composition. However, data from the Greenland Ice Sheet imply that interactions between community composition and meltwater dynamics are susceptible to environmental changes, shifting ecosystem function and microbial communities, with unforeseen consequences to downstream environments. A multi-scale approach contributed to a better understanding of microbial biogeography, carbon cycling, and cellular spatial organization in glacial surface environments.