The role of subsurface ice in sustaining bacteria in continental and maritime glaciers

In supraglacial environments, surface and subsurface ices are two distinct and connected microhabitats in terms of physicochemical and biological aspects. At the frontline of climate change, glaciers lose tremendous ice masses to downstream ecosystems, serving as crucial sources of both biotic and abiotic materials. In this study, we studied the disparities and relationships of microbial communities between surface and subsurface ices collected from a maritime and a continental glacier during summer. The results showed that surface ices had significantly higher nutrients and were more physiochemically different than subsurface ices. Despite lower nutrients, subsurface ices had higher alpha-diversity with more unique and enriched operational taxonomic units (OTUs) than surface ices, indicating the potential role of subsurface as a bacterial refuge. Sorensen dissimilarity between bacterial communities in surface ices and subsurface ices was mainly contributed by the turnover component, suggesting strong species replacement from surface to subsurface ices due to large environmental gradients. For different glaciers, the maritime glacier had significantly higher alpha-diversity than the continental glacier. The difference between surface and subsurface communities was more pronounced in the maritime glacier than in the continental glacier. The network analysis revealed that surface-enriched and subsurface-enriched OTUs formed independent modules, with surface-enriched OTUs having closer interconnections and greater importance in the network of the maritime glacier. This study highlights the important role of subsurface ice as a bacterial refuge and enriches our knowledge of microbial properties in glaciers.