Microcosms to Study Climate Change

Microcosms to Study Climate Change

As discussed in previous blogs, climate change has the potential to change marine microbial communities. A change in microbial communities could in turn affect oceanic and truly global processes such as carbon and nitrogen cycling. A lot of previous work has focussed on long term microbial surveys and correlations with sea surface temperatures. So far this work had been somewhat convincing as strong correlations and patterns are clear to see, in particular with regard to increased temperature and vibrios. However, correlations are not causations, and as we’ve discussed climate change per se is multifaceted and complex. Krause et al. (2012) were particularly interested in ocean acidification, an element of climate change which is only just beginning to be researched. Ocean acidification describes the process of increased dissolution of CO₂ into the sea; CO₂ then disassociates into bicarbonate, carbonate, carbonic acid and hydrogen ions, overall reducing pH. Since the beginning of the industrial period, the oceans have taken up approximately one-third of anthropogenic CO₂ emissions which has already lead to a reduction in pH of 0.1 units. Given that oceanic pH has remained above 8.1 for the last 23 million years, recent and predicted ocean acidification is causing concern amongst many marine biologists.

Krause et al. (2012) specifically aimed to investigate the direct effects of reduced pH on microbial communities. The study consisted of highly-replicated (n=5 microcosms per treatment) microcosm laboratory acidification experiments with a natural bacterial community taken from the North Sea. Seasonal variability was accounted for by repeating the experiment in spring, summer, autumn and winter. An interesting dilution approach was used to select for different ecological strategies, i.e. oligotrophic vs high nutrients. Three pH levels were investigated:  “in situ” (pH 8.12-8.22) was the current day control, pH 7.82 and pH 7.67 represented two predicted scenarios for the North Sea by 2100 and were the “ocean acidification” treatments. After four weeks in the microcosms the community structure was analysed using automated ribosomal intergenic spacer analysis (ARISA) and 16S ribosomal amplicon pyrosequencing. Overall season and dilution treatment had the biggest effect on community structure, however pH was also responsible for some of the changes. The response to reduced pH was context-dependent, i.e it was different in different seasons and dilution treatments, and pH did not affect the abundance of microorganisms.

The reviewed paper represents the first highly-replicated, statistically convincing evidence that projected ocean acidification will change bacterial community structure. Unlike previous surveys, the high amount of control in this experiment allows true causation to be assigned to ocean acidification. Whilst the controlled microcosms allowed for a tentative causal relationship to be established, it should also be noted that they didn’t truly replicate what is likely to occur in the field. Firstly microcosms were kept in the dark, secondly the seawater was acidified using HCL rather than CO₂ (resulting in differences in alkalinity) and thirdly many microorganisms were filtered leaving only the bacteria. The issue of environmental relevance however does not detract from the main finding that reduced pH changes bacterial community structure. Furthermore the highly controlled experiment reviewed here can now be integrated into the current survey data to increase our overall understanding of the biological affects of ocean acidification.

Krause, E., Wichels, A., Giménez, L., Lunau, M., Schilhabel, M. B., & Gerdts, G. (2012). Small changes in pH have direct effects on marine bacterial community composition: a microcosm approach. PloS one, 7(10), e47035.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0047035