Ocean acidification interacts with growth light to suppress CO2 acquisition efficiency and enhance mitochondrial respiration in a coastal diatom

Diatom responses to ocean acidification have been documented with variable and controversial results. We grew the coastal diatom Thalassiosira weissflogii under 410 (LC, pH 8.13) vs 1000 mu atm (HC, pH 7.83) pCO(2) and at different levels of light (80, 140, 220 mu mol photons m(-2) s(-1)), and found that light level alters physiological responses to OA. CO2 concentrating mechanisms (CCMs) were down-regulated in the HC-grown cells across all the light levels, as reflected by lowered activity of the periplasmic carbonic anhydrase and decreased photosynthetic affinity for CO2 or dissolved inorganic carbon. The specific growth rate was, however, enhanced significantly by 9.2% only at the limiting low light level. These results indicate that rather than CO2 fertilization, the energy saved from down-regulation of CCMs promoted the growth rate of the diatom when light availability is low, in parallel with enhanced respiration under OA to cope with the acidic stress by providing extra energy.

Automated summary

The study demonstrates that coastal diatom Thalassiosira weissflogii's physiological responses to ocean acidification vary with different light levels. Under high pCO2 conditions, CO2 concentrating mechanisms are down-regulated, leading to enhanced growth rate only at low light levels.