Cryoconite has been studied intensively, but we have only touched upon the redeposition of incumbent microbes to other glacial zones – something we expect to happen more as the climate continues to warm. Whether microbes that fix and respire carbon on glacier surfaces continue to do so when they are washed elsewhere has been pondered but not properly studied, nor have the potential effects on downstream ecosystems. This is an increasingly important question because as glaciers retreat there will be less available ice surface for microbes to inhabit, greater washaway of microbes by melt runoff and increased delivery to other environments (Irvine-Fynn et al, 2013).
Wilhelm et al (2013) engaged with this problem using ecological concepts. They focussed on proglacial streams. Much of the water feeding these streams comes from melting glaciers, and along with it come microbes (predominantly from cryoconite holes). In addition, however, water and microbes are delivered from groundwater, snowmelt and atmospheric deposition. Wilhelm et al (2013) showed that as glaciers retreat, the relative input from each source can change, and the result is markedly different downstream microbial assemblages.
Biodiversity in biofilms in the streams was linked to elevation, glacier coverage and hydrological inputs from icemelt, snowmelt and groundwater, as well as local geomorphology and physicochemistry. These are variables likely to change as glaciers retreat. Furthermore, biodiversity in biofilms was found to differ from both stream water and glacier ice, largely due to environmental ‘harshness’ and the size of each zone’s ‘metacommunity’. The metacommunity refers to all of the microorganisms which interact ecologically in spite of geographical separation. For example, microbes in cryoconite holes are part of the metacommunity of the proglacial streams despite being geographically distinct because some end up transcending the geographical divide and adding to the stream’s biodiversity.
Glacier retreat will reduce the available ice surface area available for habitation. Therefore, ice surface microbes will form a decreasingly important part of the metacommunity for downstream organisms as glacier recede. Soils and groundwater will become more hydrologically important, they will also become increasingly important components of the stream metacommunity.
Which particular microbes from a metacommunity form an assemblage in a specific habitat is controlled predominantly by environmental pressures. However, in stream water these pressures have less impact because cells generally have very short residence times. Biofilms, however, represent selections of organisms from stream water – which microbes form a biofilm is very much dependent upon both upstream metacommunity size and environmental factors. The implication of this is that as glaciers retreat, accompanied by both shifts in metacommunity and environmental pressures, downstream community structures will change markedly.
It is thought that glacier retreat could increase ‘alpha’ biodiversity (biodiversity within a specific habitat) whilst decreasing ‘Beta’ biodiversity (biodiversity between habitats within a metacommunity). This suggests that each habitat will support more diverse microbial assemblages, but that these will be more homogenous across the various locations. Species which are geographically isolated or only sporadic in their occurrence face greater risk of extinction.
In summary, then, Wilhelm et al’s (2013) study has further enlightened us to the potential for multidisciplinary approaches to glaciology, the sensitivity of glacial ecosystems and the potential impacts of glacier retreat on polar biodiversity.
Irvine-Fynn, T.D.L., Edwards, A., Newton, S., Langford, H., Rassner, S., Telling, J., Anesio, A., Hodson, A.J. 2013. Microbial cell budgets of an Arctic glacier surface quantified using flow cytometry. Environmental Microbiology, 14 (11): 2998 – 3012
Wilhelm, L., Singer, G.A., Fasching, C., Battin, T.J., Besemer, K. 2013. Microbial biodiversity in glacier-fed streams. ISME Journal, 7: 1651 – 1660