Studies on the impact of ocean acidification on marine organisms involve exposing organisms to future acidification scenarios which has limited relevance for coastal calcifiers living in a mosaic of habitats. Identification of tipping points beyond which detrimental effects are observed is a widely generalizable proxy of acidification susceptibility at the populational level. This approach is limited to a handful of studies that focus on only a few macro-physiological traits, thus overlooking the whole organism response. Here we develop a framework to analyze the broad macro-physiological and molecular responses over a wide pH range in juvenile oyster. We identify low tipping points for physiological traits at pH 7.3-6.9 that coincide with a major reshuffling in membrane lipids and transcriptome. In contrast, a drop in pH affects shell parameters above tipping points, likely impacting animal fitness. These findings were made possible by the development of an innovative methodology to synthesize and identify the main patterns of variations in large -omic datasets, fitting them to pH and identifying molecular tipping-points. We propose the broad application of our framework to the assessment of effects of global change on other organisms.

Abstract. Predicted ocean acidification and warming are likely to have major implications for marine organisms, especially marine calcifiers. However, little information is available on the response of marine benthic communities as a whole to predicted changes. Here, we experimentally examined the combined effects of temperature and partial pressure of carbon dioxide (pCO 2 ) increases on the response of maerl bed assemblages, composed of living and dead thalli of the free-living coralline alga Lithothamnion corallioides, epiphytic fleshy algae, and grazer species. Two 3-month experiments were performed in the winter and summer seasons in mesocosms with four different combinations of pCO 2 (ambient and high pCO 2 ) and temperature (ambient and +3 • C). The response of maerl assemblages was assessed using metabolic measurements at the species and assemblage scales. This study suggests that seasonal variability represents an important driver influencing the magnitude and the direction of species and community response to climate change. Gross primary production and respiration of assemblages was enhanced by high pCO 2 conditions in the summer. This positive effect was attributed to the increase in epiphyte biomass, which benefited from higher CO 2 concentrations for growth and primary production. Conversely, high pCO 2 drastically decreased the calcification rates in assemblages. This response can be attributed to the decline in calcification rates of living L. corallioides due to acidification and increased dissolution of dead L. corallioides. Future changes in pCO 2 and temperature are likely to promote the development of non-calcifying algae to the detriment of the engineer species L. corallioides. The development of fleshy algae may be modulated by the ability of grazers to regulate epiphyte growth. However, our results suggest that predicted changes will negatively affect the metabolism of grazers and potentially their ability to control epiphyte abundance. We show here that the effects of pCO 2 and temperature on maerl bed communities were weakened when these factors were combined. This underlines the importance of examining multifactorial approaches and community-level processes, which integrate species interactions, to better understand the impact of global change on marine ecosystems.

<p><strong>Abstract.</strong> Predicted ocean acidification and warming are likely to have major implications for marine organisms, especially marine calcifiers. However, little information is available on the response of marine communities as a whole to predicted changes. Here, we experimentally examined the combined effects of temperature and partial pressure of carbon dioxide (pCO<sub>2</sub>) increases on the response of maerl bed assemblages, composed of living and dead thalli of the free-living coralline alga <i>Lithothamnion corallioides</i>, epiphytic fleshy algae, and grazer species. Two three-month experiments were performed in the winter and summer seasons in mesocosms with four different combinations of pCO<sub>2</sub> (ambient and high pCO<sub>2</sub>) and temperature (ambient and +3 °C). The response of maerl assemblages was assessed using metabolic measurements at the species and assemblage scales. Gross primary production and respiration of assemblages were enhanced by high pCO<sub>2</sub> conditions in the summer. This positive effect was attributed to the increase in epiphyte biomass, which benefited from higher CO<sub>2</sub> concentrations for growth and primary production. Conversely, high pCO<sub>2</sub> drastically decreased the calcification rates in assemblages. This response can be attributed to the decline in calcification rates of living <i>L. corallioides</i> due to acidification as well as increased dissolution of dead <i>L. corallioides</i>. Future changes in pCO<sub>2</sub> and temperature are likely to promote the development of non-calcifying algae to the detriment of the engineer species <i>L. corallioides</i>. The development of fleshy algae may be modulated by the ability of grazers to regulate epiphyte growth. However, our results suggest that predicted changes will negatively affect the metabolism of grazers and potentially their ability to control epiphyte abundance. Here, we demonstrate that the response of marine communities to climate change will depend on the direct effects on species physiology and the indirect effects due to shifts in species interactions. This double, interdependent response underlines the importance of examining community-level processes, which integrate species interactions, to better understand the impact of global change on marine ecosystems.</p>