Figure 5.
Conceptual model of thermal limitation and functional optima (modified after Pörtner 2002a; Pörtner et al. 2005a). (a) Progressively enhanced thermal limitation occurs through the consecutive onset of a loss in aerobic scope beyond Tp, the onset of anaerobic metabolism at Tc and of molecular denaturation at Td. Antarctic stenotherms, especially among invertebrates, live close to their thermal optimum. The parallel shift of low and high thermal tolerance thresholds (Tp and Tc) during temperature adaptation occurs by adjustments of capacity at various functional levels. Td probably shifts with molecular modifications as well as the adjustment of molecular protection mechanisms like heat shock proteins or antioxidants (see text). Maximum scope in ATP generation at the upper Tp supports maximum capacity of organismic oxygen supply by circulatory and ventilatory muscles. It may also support (b) an asymmetric performance curve of the whole organism with optimal performance (e.g. growth, exercise) close to upper pejus temperature Tp.