How Energy Is Transferred in Photosynthetic Bacteria

(Riken) A structural analysis reveals how energy harvested from sunlight is moved to where it can power photosynthesis -- RIKEN researchers have found out how light energy harvested by pigments besides chlorophyll is transferred to the molecular site where photosynthesis occurs in cyanobacteria1.

Green is the color that most people associate with plants, microalgae life and natural environments. That’s because most plants and microalgae use the green pigment chlorophyll to absorb sunlight. This absorbed visible light is converted into chemical energy, which plants use to power the process of photosynthesis.

Some microalgae such as cyanobacteria and red algae contain other pigments that complement chlorophyll by catching wavelengths that chlorophyll doesn’t absorb very well.

Chlorophyll is contained in a molecular structure known as photosystems I and II, which in turn is located in the thylakoid membrane of chloroplasts. Photosystem II absorbs solar energy, converts it into chemical energy, and splits water molecules to produce molecular oxygen.

In contrast, supplementary pigments are located in neighboring molecular structures called phycobilisomes (Fig. 1), which specialize only in energy capture. The captured energy then has to be transferred to photosystem II for use in photosynthesis.

But no one has yet discovered how this energy transfer occurs. While it is possible to determine the molecular structures of photosystem II and phycobilisomes separately, it has been impossible until now to determine their structures while they are connected to each other.

“Maintaining the structure of the phycobilisome–photosystem II megacomplex is more difficult than stabilizing phycobilisome or photosystem II alone,” notes Keisuke Kawakami of the RIKEN SPring-8 Center. “The main reason is that the interaction between phycobilisome and photosystem II is extremely fragile and easy to dissociate.”

Now, Kawakami and co-workers have found a way to isolate the phycobilisome–photosystem II megacomplex while preserving the interaction between them.

“I’ve been interested in understanding how and through which pathways the light energy absorbed by phycobilisome is transferred to photosystem II,” says Kawakami. “That motivated me to prepare megacomplex samples in which phycobilisome and photosystem II interact.”

Kawakami’s team took as their starting point a preparation method developed 40 years ago. But they found it hard to obtain consistent structural results using it.

By optimizing the sample preparation conditions, they were eventually able to establish conditions that enabled the preparation of a more stable phycobilisome–photosystem II megacomplex.

Using their method, the team was able to determine the rate of light-energy transfer from phycobilisome to photosystem II in a cyanobacterium and to identify two major pathways for this transfer.

The team intends to continue analyzing light-harvesting mechanisms and water-splitting and oxygen-evolving processes in a wide range of plants and algae, with the ultimate goal of elucidating the detailed molecular mechanisms of photosynthesis, Kawakami says. READ MORE