At first glance, the cryosphere – including all frozen water on Earth – appears to comprise vast, cold expanses devoid of biology. However, even the most remote, hostile and unlikely icy locations in both hemispheres have been found to harbour diverse and active microbial life. It is hard to imagine ice offering many viable places for microbes to exploit; but liquid water and energy sources exist beneath, within, and especially on the surfaces of glaciers and ice sheets. Research has shown us that not only are ice-dwelling microbial communities crucial stores of biodiversity, they might be important amplifiers of global climate change.
The area around the base of a glacier is known as the sub-glacial zone. Here, extremely high pressures are exerted by the overlying body of ice – which can be up to 4 km thick in some areas of Antarctica – and a lack of sunlight prohibits photosynthesis, making this an extreme environment suitable only for micro-organisms with specific adaptations. Of particular importance is the ability to ‘breathe’ or respire in the absence of oxygen, which is sometimes achieved through methanogenesis, a process which adds methane – an extremely potent greenhouse gas – to the atmosphere. Within the main body of ice exists a further microbial habitat: liquid water at the boundaries between adjacent ice crystals. These habitats within the ice might represent analogs for habitats elsewhere in the solar system, for example on Mars, Titan or Europa.
By far the most active part of a glacier is its surface – the ‘supra-glacial zone’. Here, liquid water is plentiful, there is direct contact with the atmosphere, nutrients are replenished by melt running off the surface and organisms are illuminated by the sun. So much so, in fact, that supraglacial microorganisms can be at risk of photo-inhibition – ‘sunburn’ which overwhelms their ability to photosynthesise. Fortuitously, most microbes on the ice surface live in cryoconite holes – cylindrical depressions in ice, tens of centimetres in diameter. Within each depression lies a thin layer of granules formed of mineral fragments encased in organic material. It is on the surfaces, and to some extent in the interiors, of these holes where the majority of the biological activity resides. Stable conditions, created from water that acts as protection from the local weather, allow microbes to photosynthesise and respire in peace.
Holes in the ice
Cryoconite holes form because the dark granules efficiently absorb solar radiation, and the ice beneath them melts more quickly than bare ice. Wind and water from melted ice deliver microbes to these holes, of which some are tolerant to cold conditions and survive to establish complex communities. Rates of activity in these habitats vary greatly depending upon their location. On unstable, fast moving alpine glaciers and at the edges of ice sheets respiration is often the dominant process, fueled primarily by carbon blown in from other regions. This results in net release of carbon dioxide into the atmosphere. In the stable interiors of large ice sheets and glaciers however, cryoconite holes are stable, open to atmospheric exchange and nutrient fluxes and, crucially, illuminated by sunlight so able to happily photosynthesise. This results in a drawdown of carbon dioxide from the atmosphere and biomass production, making big, stable ice masses potentially important carbon sinks.
Observations suggest that cryoconite dust usually exists in layers only one grain deep. When further sediment is added, thermodynamic mechanisms increase the hole diameter and allow the sediment to spread out, maintaining the single grain layer arrangement. This has two important implications: firstly, cryoconite holes cover the maximum possible surface area – important for the glacier surface ‘albedo’ (reflectivity of solar radiation) and therefore the amount of melt; secondly, overlapping of grains is minimised, surface area of each grain exposed to sunlight is maximised, and therefore photosynthetic rates are optimised. Both of these processes represent feedbacks which are currently not included in climate models.
Holes are not the only icy habitat. Scientists from the Universities of Bristol and Sheffield identified an algal community inhabiting ice surfaces on the Greenland ice sheet during a field campaign in 2010. These algae cling directly to weathered surface ice and photosynthesise. Despite sometimes being invisible to the naked eye, algal communities darken ice surfaces and enhance glacier melt – in some regions to a far greater extent than cryoconite holes. Carbon fixation associated with these algae has been estimated to outweigh that of cryoconite communities due to their very high spatial coverage. Combined, surface algae and cryoconite hole communities on stable ice might represent a significant sink of carbon which could be built in to future climate models.
Ice surface microbes – implications for climate
Clearly, glacier surfaces represent an active microbial biome. It has been suggested that this biome could be a climate amplifier, meaning the action of supraglacial microbes might exacerbate climate changes. The primary mechanism for this is thought to be related to ice surface albedo (reflectivity). Albedo is a phenomenon with which we are all familiar: when we wear black clothing we feel hotter than when we wear white because dark surfaces more efficiently absorb solar radiation. Where sediment exists on a glacier surface, the albedo is much lower than where ice is clean and bare, so melting under dark material is enhanced. This means that cryoconite dust that covers a larger area of the ice surface, also further reduces its albedo and ability to reflect radiation. The presence of microbes in and around cryoconite grains makes them darker still. Warmer temperatures might encourage these microbes to grow and proliferate, enhancing their effect to lower albedo and accelerating glacier melting.
Faster melting reduces the amount of reflective ice covering Earth’s surface and promotes absorption of solar radiation, exacerbating temperature rise. Therefore, microbial processes on glacier surfaces might be an important amplifier of temperature changes. Furthermore, there could be complex feedbacks associated with the release of atmospheric carbon by microbial respiration, and the drawdown and fixation of atmospheric carbon by photosynthesis by glacier surface microbes, which could be climatically significant at least at the regional scale.
We know that the climate is changing for the warmer, and the response of ice’s microbial communities is currently uncertain. For example, if faster melt brings more nutrients will microbes fix more carbon? Will increased biomass enhance further reduce the albedo and amplify the warming? How do these processes feed back into the climate system? Answering these questions, amongst others, will play a significant role in understanding the response of glacier and ice sheet surfaces to future climate change.
Glacier microbiologists are adopting increasingly sophisticated techniques (flow cytometry, microscopy, infra-red gas analysis and fluorescence) to examine the response of microbes to climate change. However, very recently advanced molecular methods have been employed to great effect. For example, Arwyn Edwards and his team recently used a metagenomic approach to take a ‘snapshot’ of all the genes present within an Alpine cryoconite hole. We might see greater incorporation of sophisticated molecular techniques in the near future as we begin to tackle the remaining big questions regarding icy life and climate.
In whatever way we progress from here, microbiology on the surface of ice will remain not only a fascinating science, but also one with unique and specific challenges. Glacier microbial communities represent potentially important climate amplifiers, primarily through their impact upon ice albedo. Work done in the field of ice microbiology over the past few decades has provided illuminating, surprising and important knowledge about the robust and diverse nature of life on planet Earth.