Antarctic creatures are supremely adapted to their frozen habitat, but will they survive rising temperatures in the next millennium?
Antarctica is perceived as remote, hostile and desolate. Yet the Southern Ocean is dynamic, inhabited by strange and exotic creatures. The environment is extreme: spring ozone levels are the lowest anywhere; light and ice cover are intensely seasonal; winters are long and summers short. Sea temperatures around the continent have been low and stable for millions of years.
Ice dominates existence. The cold-blooded organisms living there have specific evolutionary adaptations, including slow growth and development rates, increased lifespans and low metabolic rates. With reduced metabolic rates come reduced abilities to raise metabolism. This results in limited capacities to cope with change, and most Antarctic marine invertebrates and fish die in temperatures above 5°C.
The ecological consequences of inhabiting extreme environments and the physiological mechanisms underlying reduced capacities to survive elevated temperature are research themes at the British Antarctic Survey. Most species solve the problems of low temperature and extreme seasonality by living a long time and reducing metabolic costs; small Antarctic shellfish may live 50 years compared with 10-15 for temperate species.
Mitochondria are the animal cell components that provide energy for work; they can be viewed as the body's power stations. They are particularly important in muscles, where they release the energy needed for contraction. Recent work has shown environmental temperature limits the rate at which mitochondria provide energy. This means each mitochondrion in a fish muscle in Antarctica produces about half the energy of one in Europe.
To provide sufficient energy for competent swimming, mitochondrial numbers increase at low temperatures, such that 35 per cent of a muscle is mitochondria in polar fish compared with 15-20 per cent in temperate species. Problems occur, however, when temperature rises. Mitochondria are a major metabolic cost in animals, and their maintenance requirements rise with temperature. For species with more mitochondria, elevations in temperature soon approach the limit for the animal's capacity to supply necessary resources, predominantly oxygen.
With German colleagues we have shown that blood oxygen content declines rapidly as upper lethal temperatures are approached. Eventually oxygen supply fails and they asphyxiate. Transfers to anaerobic states have been termed "critical temperatures" and define long-term limits. Even though cold seawater in Antarctica carries more oxygen than warm seawater, with rising temperature animals are doubly affected because requirements rise and availability decreases.
This year, with a Belgian collaborator, we have shown that oxygen availability sets upper size limits for marine amphipods, a group of crustaceans. Thus as seawater oxygen content increases at low temperatures their maximum size increases. This is probably why the Southern Ocean boasts giant species, with 30cm diameter sea spiders and isopods, relatives of woodlice, 13cm long.
Cold-blooded species inhabiting polar oceans are on a knife edge between the physiological adaptations needed to function in low-temperature, extremely seasonal environments and the problems of increased costs as temperatures rise. Our current state of understanding leaves us unable to predict whether these exotic temperature-limited organisms will survive the coming millennium.
Lloyd Peck is visiting professor of ecology at Sunderland University and principal investigator in the British Antarctic Survey's Life at the Edge: Stresses and Thresholds Programme at Cambridge.