Stable Photosymbiotic Relationship under CO2-Induced Acidification in the Acoel Worm Symsagittifera Roscoffensis

As a consequence of anthropogenic CO2 emissions, oceans are becoming more acidic, a phenomenon known as ocean acidification. Many marine species predicted to be sensitive to this stressor are photosymbiotic, including corals and foraminifera. However, the direct impact of ocean acidification on the relationship between the photosynthetic and nonphotosynthetic organism remains unclear and is complicated by other physiological processes known to be sensitive to ocean acidification (e. g. calcification and feeding). We have studied the impact of extreme pH decrease/pCO(2) increase on the complete life cycle of the photosymbiotic, non-calcifying and pure autotrophic acoel worm, Symsagittifera roscoffensis. Our results show that this species is resistant to high pCO(2) with no negative or even positive effects on fitness (survival, growth, fertility) and/or photosymbiotic relationship till pCO(2) up to 54 K mu atm. Some sub-lethal bleaching is only observed at pCO(2) up to 270 K mu atm when seawater is saturated by CO2. This indicates that photosymbiosis can be resistant to high pCO(2). If such a finding would be confirmed in other photosymbiotic species, we could then hypothesize that negative impact of high pCO(2) observed on other photosymbiotic species such as corals and foraminifera could occur through indirect impacts at other levels (calcification, feeding).