![]() Microbial communities have since been found ubiquitously in glacial systems 5, ranging from sub-micron diameter liquid ice veins between ice crystals 21 to large subglacial water bodies, such as Subglacial Lake Whillans situated 800 m beneath the West Antarctic Ice Sheet 22. In 1999, the first communities of microorganisms were discovered beneath a valley glacier in the Swiss Alps 20 driving a major shift in our world-view of glaciers and ice sheets as abiotic systems to extensive icy biomes 5. ![]() Ocean fertilisation by glaciers may be accompanied by significant CO 2 drawdown by phytoplankton, intensifying the biological pump 15, 16. Examples of indirect impacts include the fertilisation of downstream ecosystems, promoted by either the release of nutrient-rich glacial meltwaters 7, 8, 9 or by subglacial meltwater-induced upwelling of nutrient replete marine water at tidewater glacier margins 10, 11, 12, 13, 14. Direct impacts include the release of greenhouse gases (carbon dioxide, CO 2 and methane, CH 4) during the microbial respiration of organic matter (OM) stored within ice sheets. These processes create the potential for ice sheets to directly or indirectly impact the global carbon cycle (Fig. Only in the last 15 years have glacial systems started to be considered as active cyclers of carbon, arising from the discovery that they include a range of aquatic environments 4 which host abundant and diverse populations of microorganisms 5 and are hot spots for biogeochemical weathering 6. However, both these mechanisms represent passive processes of carbon storage and release by glaciers. More recent work has highlighted the capacity of glacier surfaces to act as sinks for carbon-containing aerosols from anthropogenic or natural sources 2, 3. The notion that ice sheets contain significant carbon stores has its roots in the early 2000s–the great ice sheets were hypothesised to advance over soil and vegetation carbon during glacial periods, with this fossil carbon being released back to the atmosphere once exposed by retreating ice during deglaciation 1. For a component of the Earth’s system to impact the global carbon cycle, and potentially influence atmospheric concentrations of carbon dioxide or methane, it must either directly sequester and/or release carbon, or indirectly influence carbon uptake/release in other parts of the Earth system.
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