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Euphausia superba (Antarctic krill) is a keystone species in the Southern Ocean, but little is known about how it will respond to climate change. Ocean acidification, caused by sequestration of carbon dioxide into ocean surface waters (pCO(2)), alters the lipid biochemistry of some organisms. This can have cascading effects up the food chain. In a year-long laboratory experiment adult krill were exposed to ambient seawater pCO(2) levels (400 mu atm), elevated pCO(2) levels mimicking near-future ocean acidification (1000, 1500 and 2000 mu atm) and an extreme pCO(3) level (4000 mu atm). Total lipid mass (mg g(-1) DM) of krill was unaffected by near-future pCO(2). Fatty acid composition (%) and fatty acid ratios associated with immune responses and cell membrane fluidity were also unaffected by near-future pCO(2), apart from an increase in 18:3n-3/18:2n-6 ratios in krill in 1500 mu atm pCO(2) in winter and spring. Extreme pCO(2) had no effect on krill lipid biochemistry during summer. During winter and spring, krill in extreme pCO(2) had elevated levels of 18:2n-6 (up to 1.2% increase), 20:4n-6 (up to 0.8% increase), lower 18:3n-3/18:2n-6 and 20:5n-3/20:4n-6 ratios, and showed evidence of increased membrane fluidity (up to three-fold increase in phospholipid/sterol ratios). These results indicate that the lipid biochemistry of adult krill is robust to near-future ocean acidification.