Energy changes in photosynthetic electron transport: Probing photosynthesis by pulsed photoacoustics

Abstract

Pulsed photoacoustics (PPA) is the ideal method for determining the thermodynamics and energy-storage (E-S) efficiency of photosynthesis. In particular it is applicable to whole cells, and so allows direct study of in vivo energy storage by PSI and PSII, avoiding artifacts resulting from damage to isolated photosystems and disruption of linear electron flow. It is the in vivo efficiency that ultimately limits energy storage at limiting light intensities. In this chapter, after discussing advantages of separating the enthalpic and entropic contributions to free energy changes of photochemistry, we describe PPA methodology in detail, and discuss information available from the enthalpies it provides. To illustrate the use of PPA to obtain E-S efficiencies in vivo, we then describe a comprehensive study of cyanobacterial whole cells in the far-red spectral region. This study has resulted in the clear differentiation of the contributions of PSI and PSII to the observed efficiency; the first identification of the trap energies of both systems directly from enthalpy measurements; and quantification of the thermodynamically required decay in storage for excitations below these energies. This unique and elegant biophysical method can additionally provide such key information as volume changes resulting from electron transfer, optical cross sections, quantum yields, and turnover times.