Experimental ocean acidification alters the allocation of metabolic energy

Energy is required to maintain physiological homeostasis in response to environmental change. Although responses to environmental stressors frequently are assumed to involve high metabolic costs, the biochemical bases of actual energy demands are rarely quantified. We studied the impact of a near-future scenario of ocean acidification [800 µatm partial pressure of CO ₂ ( p CO ₂)] during the development and growth of an important model organism in developmental and environmental biology, the sea urchin Strongylocentrotus purpuratus . Size, metabolic rate, biochemical content, and gene expression were not different in larvae growing under control and seawater acidification treatments. Measurements limited to those levels of biological analysis did not reveal the biochemical mechanisms of response to ocean acidification that occurred at the cellular level. In vivo rates of protein synthesis and ion transport increased ∼50% under acidification. Importantly, the in vivo physiological increases in ion transport were not predicted from total enzyme activity or gene expression. Under acidification, the increased rates of protein synthesis and ion transport that were sustained in growing larvae collectively accounted for the majority of available ATP (84%). In contrast, embryos and prefeeding and unfed larvae in control treatments allocated on average only 40% of ATP to these same two processes. Understanding the biochemical strategies for accommodating increases in metabolic energy demand and their biological limitations can serve as a quantitative basis for assessing sublethal effects of global change. Variation in the ability to allocate ATP differentially among essential functions may be a key basis of resilience to ocean acidification and other compounding environmental stressors. Significance Anthropogenic emission of CO ₂ is causing global ocean acidification. For many species, biological responses to acidification often show limited impact at the level of the whole animal. Our integrative studies of whole-organism growth and metabolic rates, rates of protein synthesis and ion transport, enzyme activity, and gene expression show that although the organismal-level impact of acidification on developing sea urchins was minimal, dramatic compensation occurred at the cellular level. Increased rates of synthesis and ion transport resulted in 84% of available energy being allocated to those processes under acidification. Defining the limits of differential energy a