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Positive Effects of Ocean Acidification on a Giant Kelp Species

Paper Reviewed
Zhang, X., Xu, D., Guan, Z., Wang, S., Zhang, Y., Wang, W., Zhang, X., Fan, X., Li, F. and Ye, N. 2020. Elevated CO2 concentrations promote growth and photosynthesis of the brown alga Saccharina japonica. Journal of Applied Phycology

Saccharina japonica, in the words of Zhang et al. (2020), is "one of the most ecologically and economically important brown macroalgae and is used for food, medicinal, agricultural, and industrial products." Yet much remains to be learned in regard to how this key kelp species might respond to projected future changes in seawater pCO2 and temperature. i.e., the dreaded twin evils of so-called ocean acidification and global warming. And so this ten-member Chinese research team set out in search of answers by conducting two separate experiments. In the first, they investigated the effects of temperature (a gradient from 5°C to 20°C) and ocean acidification (a gradient from 400 to 2000 µatm) on non-photochemical quenching (NPQ), which they describe as "one of the most important and most rapid regulatory strategies [designed to] prevent photodamage to the reaction center in PSII" and which helps resist oxidative stress under unfavorable growing conditions. In the second experiment, Zhang et al. studied the effects of temperature and ocean acidification on three S. japonica strains cultured under two levels of pCO2 (400 and 1,000 µatm) for 60 days in order to evaluate the effect of ocean acidification on the growth and development of this marine species over a longer time period.

In reporting their findings of the first experiment, the scientists report that "CO2-induced ocean acidification and high temperature exert antagonistic effects on NPQ." More specifically, elevated CO2 decreased NPQ activity, which is indicative of lower levels of oxidative stress, whereas rising temperature enhanced it (indicative of higher levels of oxidative stress). Consequently, they say the results of this aspect of their study provide "the first evidence that CO2-induced seawater acidification reduces the NPQ response of S. japonica to high light and alleviates the effect of high temperature on NPQ."

In their second experiment, Zhang et al. found that elevated CO2 "dramatically promoted growth in all three [macroalgae] strains," which "positive effect of ocean acidification," they add, is most likely attributable to "the increased concentration of inorganic carbon resources" in the seawater medium. At higher seawater pCO2 an energy savings accrues as the kelp's carbon concentrating mechanism is down regulated, allowing those energy savings to be applied to processes involved with photosynthesis and growth.

Commenting on these several findings, Zhang et al. conclude that "predicted ocean acidification may alleviate the effect of high temperatures on giant kelp, conserving energy for growth and photosynthesis." And that is great news, considering the economic and ecological value of this key marine species!

Posted 3 June 2020