The changing face of the invisible Arctic
Have you watched the BBC One documentary series Frozen Planet? It’s an amazing show that really highlights the changing nature of the frozen wilderness. It’s hard not to feel sorry for the Arctic polar bears as the environment they live in continues to disappear. Understandably, the programme focuses on large, impressive beasts, but I wondered what was happening at the other end of the food chain and wanted to see how microbes are coping with the changing temperature (yes, I’m an absolute scream at dinner parties).
I found a paper in PLoS One that looks at this very subject. Canadian scientists looked at how the communities of bacteria, archaea and tiny eukaryotes changed between 2003 and 2010 in the Beaufort Strait. This timeframe is significant because September 2007 saw Arctic ice shrink to a record low. This was due to the melting of ‘multiyear sea ice’ – ice that stays frozen through the summer months and contains less brine (and, therefore, is less salty) than other ice in the Arctic. In particular, the scientists looked at a layer of the sea known as the subsurface chlorophyll maxima, or SCM, which contains a high number of photosynthetic plankton.
As a result of the multiyear sea ice melting, the area the researchers tested is becoming more layered, with a clear divide between the fresh and salty water. This increasing division might reduce the cycling of nutrients between the layers.
The results showed that levels of nitrate found in the waters had decreased significantly between 2002 and 2010 as a result of the increased ice melt, which is important because nitrates are the limiting factor for photosynthesis in the seas. Large differences were also seen in the community of sea microorganisms before and after the 2007 record ice low. These differences were seen in the Bacteria, Archaea and Eukarya. Bacterial diversity fell, with a large decrease seen in the phylum Bacteroidetes. These bacteria prefer to grow on complex organic matter, and the authors suggest that their depletion is a marker for the changing levels of organic matter in Arctic waters.
Although the research only looked at microscopic eukaryotes between 0.2 and 3.0 microns in diameter, the work showed that ciliates became more common after 2007 and marine stramenophiles (which feed on bacteria) became harder to find, possibly because of the lower level of Bacteroidetes bacteria for them to eat.
It was among members of the Archaea that some of the most striking changes were seen. Marine Group I Phylum Thaumarchaeota (Thaum-MG-I), which had previously accounted for approximately 60 per cent of all Archaeal DNA sequences identified before 2007, had declined to less than 10 per cent by 2010. These microbes use ammonia as an energy source, which is one of the early steps in its conversion into nitrate. The authors of the paper speculate that this is contributing to the drop in nitrate levels. This, in turn, could promote the growth of organisms better at competing for the available nitrates, further upsetting the ecological niche.
This work shows that our seas, and the microbes that live within them, are changing in response to the warming planet. The increasing divisions between fresh and salty water and changes within microbial communities could have a substantial impact, not only on the complex and delicate Arctic food chain but also in the cycling and sequestering of carbon dioxide. Whether these initial results represent the tip of the iceberg remains to be seen.
Next time you see a polar bear struggling at the top of the Arctic food pyramid, spare a thought for the microorganisms that make up its base. If that crumbles, the whole thing could collapse.
Comeau AM, Li WK, Tremblay JÉ, Carmack EC, & Lovejoy C (2011). Arctic Ocean microbial community structure before and after the 2007 record sea ice minimum. PloS one, 6 (11) PMID: 22096583