How are photosynthetic organisms able to protect themselves from energy-related damage?

Dissipation of excess excitation energy in the light harvesting antenna of Photosystem II (PSII) givesphotosynthetic organisms an evolutionary advantage by reducing damage in fluctuating light conditions. This dissipation, known as qE, only occurs when excitation energy cannot be used in productive photosynthesis. In order to understand the energetic rearrangement that takes place to switch on this pathway, we have measured the change in fluorescence lifetime of the PSII antenna as qE is turned on in intact cells of Chlamydomonas reinhardtii, a photosynthetic alga.

In order to dissipate excess excitation energy in the PSII antenna while retaining usable excitation energy, photosynthetic organisms have evolved a rapid nonphotochemical quenching mechanism, known as qE, that is rapidly turned on and off through a feedback loop. This feedback loop improves the plantʼs fitness in variable light conditions. Nonphotochemical quenching (NPQ) is the general name for quenching of chlorophyll excitations in the PSII supercomplex by a method other than photochemistry at the reaction center. In healthy plants, the majority of NPQ serves to protect the plant from damage caused by excess excitation energy that the plant cannot use.

Monitoring the Fluorescence Lifetime of chlorophylls in the PSII supercomplex enables us to determine the lifetime of excitations. The lifetime of chlorophyll excitations decreases when PSII is in a quenching configuration. We can measure the fraction of PSIIs in a quenching configuration, as well as the lifetime of an average trajectory to a quenching site. Using our apparatus, we can measure the reduction in fluorescence lifetime as qE is activated in Chlamydomonas reinhardtii. We see that there is a 300 ms offset before qE turns on, followed by a rise of qE with a time constant of 400 ms. This tool provides more information than conventional PAM fluorometers because it enables us to measure the amplitude of qE, rather than the bulk fluorescence yield. We can also simulate the effect of qE on the time-evolution of chlorophyll fluorescence yield. We can do this by modeling the relevant variables by using a series of coupled nonlinear differential equations that can be solved using a stiff differential equations solver.