The lower limit of photon fluence rate for phototrophic growth: the significance of ‘slippage’ reactions

Abstract. The role of ‘slippage’ reactions, in the form of passive H⁺ uniport through CF₀‐CF₁, ATP synthetase and breakdown of the S₂ and S₃ intermediates of O₂ evolution, is considered in relation to the growth of phototrophic organisms at low photon fluence rates. Analysis of the limited data available suggests that adaptation (phenotypic or genotypic) to low photon fluence rates is accompanied by an increase in the ratio of light‐absorbing pigments to the (potentially slippage‐inducing) photosystem two units and CF₀‐CF₁ ATP synthetases. Furthermore, organisms which are genotypically adapted to high photon fluence rates do not, when grown at low photon fluence rates, achieve the same low ratio of reaction centres to total light‐harvesting pigments as is found in phototrophs genotypically adapted to low photon fluence rates. The limits to, and energy costs of, such a mechanism of adaptation to low photon fluence rates are also discussed.