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Elevated CO2 Alleviates Irradiance Stress in a Marine Phytoplankton

Paper Reviewed
García-Gómez, C., Gordillo, F.J.L., Palma, A., Lorenzo, M.R. and Segovia, M. 2014. Elevated CO2 alleviates high PAR and UV stress in the unicellular chlorophyte Dunaliella tertiolecta. Photochemical & Photobiological Sciences 13: 1347-1358.

Phytoplankton play a key role in the transport of carbon from the atmosphere to the ocean. However, in the future this transport may be interrupted. Some have postulated, for example, that "in a warmer acidified ocean, phytoplankton may be exposed to a shallower mixing depth and hence a higher average light dose" (García-Gómez et al., 2014), which increase in irradiance is further postulated to negatively impact marine photosynthesis and slow the transport of carbon into the ocean. Under such circumstances, it would be expected that more CO2 will remain in the atmosphere, acting as a positive feedback to amplify CO2-induced global warming.

The recent work of García-Gómez et al. attempts to evaluate a critical portion of the above hypothesis by studying "the effects of increased CO2 and irradiance on the physiological performance of the chlorophyte Dunaliella tertiolecta." More specifically, the authors grew samples of the phytoplankton under two CO2 concentrations (390 and 1,000 ppm) and four irradiance values (200, 400, 800 and 1,100 µmol PAR quanta m-2 s-1) for a period of five days, after which they performed a series of tests directed toward answering the following questions: (1) whether or not increased CO2 resulted in higher rates of primary production, even under conditions of high irradiance, and (2) whether or not elevated CO2 played a role in the regulation of essential repair mechanisms in the phytoplankton when under irradiance stress. Their results were quite illuminating.

For starters, the Spanish researchers report that, as expected, growth rates of D. tertiolecta declined as irradiance increased. However, for any given irradiance, growth rates were always higher under elevated CO2 conditions. Furthermore, they note that the differences between growth rates at high and low CO2 were greater with increasing irradiance, meaning elevated CO2 ameliorated the irradiance stress to a greater degree as the stress increased.

With respect to the role of CO2 in regulating cellular health and repair, García-Gómez et al. report that cell stress "was alleviated in high CO2 with respect to low CO2 as evidenced by a decrease in reactive oxygen species accumulation." As for DNA damage, elevated CO2 proved provident here too, as the presence of CPDs (cyclobutane-pyrimidine dimers) -- a "direct indicator of DNA damage" -- at high CO2 was approximately one-quarter of the value observed at low CO2.

Based on all their findings, García-Gómez et al. conclude, in the final sentence of their abstract, "our results demonstrate that future scenarios of global change result in alleviation of irradiance stress by CO2-induced photoprotection in D. tertiolecta." And by all indications, that fact represents good news that global warming alarmists would do well to consider.

Posted 29 October 2014