The real limits to marine life: a further critique of the Respiration Index

[摘要] The recently proposed "Respiration Index"(RI = log PO₂/PCO₂) suggests that aerobic metabolism islimited by the ratio of reactants (oxygen) to products (carbondioxide) according to the thermodynamics of cellular respiration. Here, wedemonstrate further that, because of the large standard free energy changefor organic carbon oxidation (ΔG° = −686 kcal mol⁻¹), carbon dioxide can never reach concentrations that would limitthe thermodynamics of this reaction. A PCO₂ to PO₂ ratio of10⁵⁰³ would be required to reach equilibrium (equilibrium constant,K_eq = 10⁵⁰³), where ΔG = 0. Thus, a Respiration Indexof −503 would be the real thermodynamic limit to aerobic life. Such aRespiration Index is never reached, either in the cell or in the environment.Moreover, cellular respiration and oxygen provision are kinetically controlledsuch that, within limits, environmental oxygen and CO₂ concentrationshave little to do with intracellular concentrations. The RI is fundamentallydifferent from the aragonite saturation state, a thermodynamic index used toquantify the potential effect of CO₂ on calcification rates, because ofits failure to incorporate the equilibrium constant of the reaction. Not onlyis the RI invalid, but its use leads to incorrect and misleading predictions ofthe threat of changing oxygen and carbon dioxide to marine life. We provide aphysiological framework that identifies oxygen thresholds and allows forsynergistic effects of ocean acidification and global warming.

[学科分类] 地球化学与岩石