text: While the photothermal mechanism is universal, there could exist  additional other mechanisms, superimposed on the photothermal mechanism, which may contribute significantly to the photoacoustic signal. These mechanisms are generally related to photophysical processes and photochemical reactions following light absorption: (1) change in the material balance of the sample or the gaseous phase around the sample; (2) change in the molecular organization, which results in molecular volume changes. Most prominent examples for these two kinds of mechanisms are in photosynthesis.
  The first mechanism above is mostly conspicuous in a photosynthesizing plant leaf. There,  the light induced oxygen evolution causes pressure changes in the air phase,  resulting in a photoacoustic signal, which is comparable in magnitude to that caused by the photothermal mechanism. This mechanism was tentatively named "photobaric". The second mechanism shows up in photosynthetically active sub-cell complexes in suspension (e.g. photosynthetic reaction centers). There, the electric field which is formed in the reaction center, following the light induced electron transfer process,  causes a micro electrostriction effect  with a change in the molecular volume. This, in turn, induces a pressure wave which propagates in the macroscopic medium. Another case for this mechanism is Bacteriorhodopsin proton pump. Here the light induced change in the molecular volume is caused by  conformational changes that occur in this protein following light absorption.