The Non-effects of Ocean Acidification on a Marine Microalgae

Hu, S., Wang, Y., Wang, Y., Zhao, Y., Zhang, X., Zhang, Y., Jiang, M. and Tang, X. 2018. Effects of elevated pCO2 on physiological performance of marine microalgae Dunaliella salina (Chlorophyta, Chlorophyceae). Journal of Oceanology and Limnology 36: 317-328.

Dunaliella salina is a green microalgae that is typically found in high saline waters. It is known for its antioxidant and carotenoid production and is used as a nutritional supplement and cosmetic additive. To date, however, little is known about how this halotolerant marine species might respond to predicted changes in oceanic pH (i.e., ocean acidification) over the course of the next century or two. Consequently, Hu et al. (2018) set out to examine the physiological responses of D. salina to three different pCO2 levels.

The work was conducted in a controlled-environment setting in which the eight Chinese researchers measured (among other things) the growth, photosynthesis and dark respiration of D. salina under current and predicted seawater pH. The three pCO2 levels examined included a present (390 µatm, pH 8.10) and two potential future levels (1,000 and 2,000 µatm, corresponding to seawater pH values of 7.78 and 7.49, respectively). Culture media were renewed every 24 hours to ensure the cells remained in the exponential growth phase and harvested after 4-6 weeks of semi-continuous incubation when their growth rates remained relatively constant for three or more days, at which time the authors considered the microalgae to be “fully acclimated to their respective experimental treatments.”

In reporting their findings, Hu et al. say that elevated pCO2 “significantly enhanced photosynthesis (in terms of gross photosynthetic O2 evolution, effective quantum yield, photosynthetic efficiency, maximum relative electron transport rate and ribulose-1.5-biphosphate carboxylase/oxygenase (Rubisco) activity).” However, it also enhanced dark respiration, leading to “insignificant effects on growth” among the three pCO2 treatment levels. Additional findings revealed there was a down-regulation of the microalgae’s carbon concentrating mechanism at high pCO2, suggesting less energy was required for photosynthesis under ocean acidification conditions. Other noted observations included no change in chlorophyll a, chlorophyll b or carotenoid contents at the different pCO2 levels and other measurements revealed a pCO2-induced improvement in light use efficiency.

Considering all of the findings presented above, it would appear that so-called ocean acidification does not negatively impact the growth of this green microalgae species. And, therefore, this study is another in a long line of studies (see the many reviews we have posted on our website under the headings Ocean Acidification and Ocean Acidification and Warming in our Subject Index) showing ocean acidification to be a non-problem.

This article appeared on the CO2 Science website at http://www.co2science.org/articles/V22/mar/a4.php