Climate change and temperature-dependent biogeography: oxygen limitation of thermal tolerance in animals

Recent years have shown a rise in mean global temperatures and a shift in t he geographical distribution of ectothermic animals. For a cause and effect analysis the present paper discusses those physiological processes limitin g thermal tolerance. The lower heat tolerance in metazoa compared with unic ellular eukaryotes and bacteria suggests that a complex systemic rather tha n molecular process is limiting in metazoa. Whole-animal aerobic scope appe ars as tho first process limited at low and high temperatures, linked to th e progressively insufficient capacity of circulation and ventilation. Oxyge n levels in body fluids may decrease, reflecting excessive oxygen demand at high temperatures or insufficient aerobic capacity of mitochondria at low temperatures. Aerobic scope falls at temperatures beyond the thermal optimu m and vanishes at low or high critical temperatures when transition to an a naerobic mitochondrial metabolism occurs. The adjustment of mitochondrial d ensities on top of parallel molecular or membrane adjustments appears cruci al for maintaining aerobic scope and for shining thermal tolerance. In conc lusion, the capacity of oxygen delivery matches full aerobic scope only wit hin the thermal optimum. At temperatures outside this range, only time-limi ted survival is supported by residual aerobic scope, then anaerobic metabol ism and finally molecular protection by heat shock proteins and antioxidati ve defence. In a cause and effect hierarchy, the progressive increase in ox ygen limitation at extreme temperatures may even enhance oxidative and dena turation stress. As a corollary, capacity limitations at a complex level of organisation, the oxygen delivery system, define thermal tolerance Limits before molecular functions become disturbed.