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Ocean Acidification Enhances the Resilience of Marine Diatoms

Paper Reviewed
Valenzuela, J.J., de Lomana, A.L.G., Lee, A., Armbrust, E.V., Orellana, M.V. and Baliga, N.S. 2018. Ocean acidification conditions increase resilience of marine diatoms. Nature Communications 9: 2328, DOI: 10.1038/s41467-018-04742-3.

Writing as background for their study, Valenzuela et al. (2018) say that the concept of ecological resilience is defined as "the amount of disturbance that can be tolerated by a system without changing state and still persist," citing the works of Holling (1973), Gunderson (2000), Botton et al. (2008) and Griffiths and Philippot (2013). By this definition, therefore, a more resilient system is one that requires a larger disturbance to force the system into an alternate state.

Acting on the above principle, Valenzuela et al. set out to investigate the ecological resilience of the marine diatom Thalassiosira pseudonana. To do so, they designed a stress test to "quantify resilience of [the] diatoms under simulated mid-twentieth century and future oceanic conditions, by assaying their ability to tolerate and recover from progressively larger amounts of stress."

Mid-twentieth century and future conditions were defined as seawater with pH levels that corresponded with atmospheric CO2 levels of 300 and 1000 ppm, respectively. Cultures of the diatoms were grown for successive generations under these conditions while being subjected to incrementally increasing amounts of low-dose ultraviolet A and B radiation and nitrogen-limiting conditions. The ultimate aim of the stress test was "to investigate whether growth under elevated CO2 would alter diatom resilience."

The results of their experiment indicated that all cultures eventually collapsed under the stress test conditions. However, diatoms growing at 300 ppm CO2 succumbed sooner. In contrast, diatoms growing at 1000 ppm CO2 survived and sustained themselves for a significantly longer time period, demonstrating, in the words of the authors, an innate ability "to recover from ultraviolet radiation and adopt a physiologic state that was matched to the external environment" of the stress test.

Commenting on this incredible feat, Valenzuela et al. say that the enhanced resilience at 1000 ppm CO2 likely resulted (at least in part) from a reduction in energy requirement of the species' carbon concentrating mechanism, which likely conserved some 3-6 percent of the energy expended during carbon fixation than that expended under 300 ppm CO2. Alternatively, the resilience could have come from differential gene expression induced at higher levels of CO2. In either case, the researchers conclude that, "under elevated CO2 conditions diatoms sustain relational resilience over a longer timeframe, demonstrating increased resilience to future acidified ocean conditions." And that is great news for the fate of the world's food web, where diatoms account for approximately 40 percent of marine primary production.

References
Botton, S., van Heusden, M., Parsons, J.R., Smidt, H. and van Straalen, N. 2008. Resilience of microbial systems towards disturbances. Critical Reviews in Microbiology 32: 101-112.

Griffiths, B.S. and Philippot, L. 2013. Insights into the resistance and resilience of the soil microbial community. FEMS Microbiology Reviews 37: 112-129.

Gunderson, L.H. 2000. Ecological resilience--in theory and application. Annual Review of Ecology and Systematics 31: 425-439.

Holling, C.S. 1973. Resilience and stability of ecological systems. Annual Review of Ecology and Systematics 4: 1-23.

Posted 26 October 2018