Productivity gains do not compensate for reduced calcification under near‐future ocean acidification in the photosynthetic benthic foraminifer species Marginopora vertebralis

• Published in: Global Change Biology
• Main Authors: Uthicke, Sven, Fabricius, Katharina E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2012
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1111/j.1365-2486.2012.02715.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1365-2486.2012.02715.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2486.2012.02715.x

Abstract Changes in the seawater carbonate chemistry (ocean acidification) from increasing atmospheric carbon dioxide ( CO 2 ) concentrations negatively affect many marine calcifying organisms, but may benefit primary producers under dissolved inorganic carbon ( DIC ) limitation. To improve predictions of the ecological effects of ocean acidification, the net gains and losses between the processes of photosynthesis and calcification need to be studied jointly on physiological and population levels. We studied productivity, respiration, and abundances of the symbiont‐bearing foraminifer species Marginopora vertebralis on natural CO 2 seeps in Papua New Guinea and conducted additional studies on production and calcification on the Great Barrier Reef ( GBR ) using artificially enhanced pCO 2 . Net oxygen production increased up to 90% with increasing pCO 2 temperature, light, and pH together explaining 61% of the variance in production. Production increased with increasing light and increasing pCO 2 and declined at higher temperatures. Respiration was also significantly elevated (~25%), whereas calcification was reduced (16–39%) at low pH /high pCO 2 compared to present‐day conditions. In the field, M. vertebralis was absent at three CO 2 seep sites at pH Total levels below ~7.9 ( pCO 2 ~700 μatm), but it was found in densities of over 1000 m −2 at all three control sites. The study showed that endosymbiotic algae in foraminifera benefit from increased DIC availability and may be naturally carbon limited. The observed reduction in calcification may have been caused either by increased energy demands for proton pumping (measured as elevated rates of respiration) or by stronger competition for DIC from the more productive symbionts. The net outcome of these two competing processes is that M. vertebralis cannot maintain populations under pCO 2 exceeding 700 μatm, thus are likely to be extinct in the next century.