Physiological mechanisms linking climate to ecosystem change: oxygen and capacity limitation of thermal tolerance

Hans-Otto Pörtner

Alfred-Wegener-Institute, Bremerhaven, Germany

The recent concept of oxygen- and capacity-dependent thermal tolerance in aquatic ectotherms has been used successfully to explain climate-induced effects of rising temperatures on species abundance and survival in the field. From a general point of view, aerobic performance characters thus form the primary link between organismal fitness and its role and functioning at the ecosystem level. The thermal window of performance in water-breathers matches their window of aerobic scope. Thermal acclimatization between seasons or adaptation to a climate regime involves shifting thermal windows and adjusting window widths and thus reflects the need to specialize on a limited temperature range associated with trade-offs in molecular to whole-organism functioning. Loss of performance reflects the first level of thermal stress, in this case associated with hypoxemia caused by the progressive mismatch of oxygen supply and demand at the borders of the thermal envelope. This lack of respiratory capacity likely involves the accumulation of CO₂ and the evolution of systemic and cellular stress signals such as oxidative stress or hormonal responses. Various environmental factors would interact with these principle relationships, some supporting passive tolerance but leading the organism earlier to the limits of its acclimation capacity. The relationships between energy turnover, capacities of activity and other functions and the width of thermal windows may lead to a fundamental understanding of specialization on climate and, in turn, of sensitivity to climate change. Such functional relationships might also relate to climate-induced changes in species interactions and, thus, community responses at the ecosystem level.