In nature, photosynthetic efficiency is often limited because of the inability of plants to maintain high photosynthetic rates at high intensity irradiation. In addition, light incident on photosynthetic organisms fluctuates rapidly between high and low intensity due to dynamic shading and cloud cover. Furthermore, much of the light energy absorbed during mid-day can be in excess of what can be used for CO₂ fixation, and ultimately cause cellular damage, especially to the photosynthetic apparatus. To protect the photosynthetic apparatus from photodamage, plants have evolved a set of dynamic processes, including nonphotochemical quenching, which dissipates much of the excess absorbed light energy as heat. While there is clearly biochemical/biophysical reorganization of the photosynthetic machinery within thylakoid membranes, our view of the the movement/re-organization of complexes within the membranes and their dynamic interactions would provide us with a new view of the networks that govern photosynthetic yields. 

It is difficult to study these dynamic changes in the organization of protein complexes within membranes using fluorescence-tagged live-cell imaging because the optical resolution is spatially limited and there is often an extensive fluorescence background generated by the chromophores that are required for photosynthetic function (e.g. chlorophyll). To overcome both of these issues, we are using Atomic Force Microscopy in liquid medium. At this point, we have resolved and characterized dimeric PS II complexes, each monomer having two lumenal protruding masses (of different heights); these protruding masses represent the PS II Oxygen-Evolving Complex (PSII-OEC) monomer (Figure 1). We have also been able to localize the cytochrome b₆f complex within the granal membranes. Some of this work was recently published [1]. We are now in the process of using AFM to observe the dynamics of these complexes, under different light conditions and redox states, in real time.