Image of the Baltoro Glacier, courtesy of wikicommons.
Bacteria are amazing, and as a recent article by Ambrosini and colleagues reminds us, they are quite literally, just about everywhere. Before reading this article, I have to admit, I was a little rusty on my definition of cryoconite holes, so I did a little reading. Essentially they are small ponds that form on glacier surfaces. For the pond to form, first cryoconite accumulates on glaciers, which is a dust composed of small rock particles, bacteria, and soot.
The buildup of this fine dust on glaciers has been termed “biological darkening”, and as you might guess, changes how the ice surface absorbs heat. This black dust can induce melting of the ice surface, forming cryoconite holes, which are basically pools of water that can be tiny (few centimeters across) or quite large (a meter across), and have been found to host microbial communities.
“These environments are an underexploited reservoir of biological functions with high biotechnological potentials, and the study of biological processes acting in them gained interest in the recent years owing to increasing evidence that they can strongly affect the engergy flows of glaciers and, consequently the climate.”
Figure 1. Location of sampling sites. Ambrosini et al., (2016)
It is important to understand the dynamics involved in changes that affect glaciers and ice sheets as they are extremely sensitive to climate change and have a huge influence on the rest of the planet. Since cryoconite holes potentially play a part in altering glacier melting rates, understanding these relatively biologically active sites and how they influence the glacier is important.
While previous research has shown that the microbial communities on different glaciers and in diverse regions (low-latitude mountain vs polar glaciers) can vary, there is still very little known about the characteristics of these communities and the mechanisms driving their formation and furthermore, how they affect the glacier on a large scale. The authors set out to study 30 different cryoconite holes on the Baltoro Glacier, which is 63 km long and runs through the Karakoram mountain range in Pakistan.
Figure 2. Mean relative abundance of OTUs sampled. Ambrosini et al., (2016)
Ambrosini and colleagues extracted the DNA from the water samples from the cryoconite holes, which were located in one of four main areas defined by differential elevations, and then amplified the V5-V6 hypervariable regions of the 16S rRNA gene. Finally, they sequenced everything using Illumina MiSeq and clustered the sequences into OTUs using a 97% cut-off value. They found that the communities in the cryoconite holes were dominated by Betaproteobacteria. Specifically, the genera Polaromonas and Limnohabitans of the order Burkholderiales were particularly abundant.
Figure 4. Association network among different OTUs. Ambrosini et al., (2016)
Overall, the five most abundant bacterial orders plus the Cyanobacteria made up the vast majority of bacteria at each site (more than 70%). One of the main environmental drivers that seemed to change the community composition was pH, which significantly affected bacterial community structure in one of the main 4 areas sampled.
As the authors note, obtaining additional environmental measurements such as nutrient concentration and salinity might have allowed them to verify the influence of these other important parameters. The sampling sites were clearly difficult to access, and transportation of the samples, as well as sample preservation, are understandably challenging. Regardless, the authors summarized the pitfalls in the study as well as the diversity of bacterial communities sampled.
In conclusion, our results suggest that microbial communities of cryoconite holes on Boltoro Glacier were dominated by Betaproteobacteria probably due to their highly versatile metabolism.
I thought the article was interesting in part since, even as a microbial molecular ecologist, I don’t always think about the role that bacteria have in diverse environments that aren’t a part of my day to day work. Piecing together how microbial communities in extreme environments influence the planet is important, it’s impressive to think that microscopic cells literally affect some of the largest and most dramatic and beautiful landscapes on Earth.