Scientists 'hack' photosynthesis and discover novel route for renewables

Little did they know that we were about to unearth an entirely new photosynthetic electron transfer pathway.

"No one had properly studied how this molecule interplays with photosynthetic machineries at such an early point of photosynthesis: we thought we were just using a new technique to confirm what we already knew," said Zhang in a press release.

"Instead, we found a whole new pathway and opened the black box of photosynthesis a bit further."

The researchers discovered that the protein scaffold where the earliest chemical reactions of photosynthesis occur is "leaky," enabling electrons to escape. Key to this discovery was the use of ultrafast spectroscopy to watch the electrons.

"Since the electrons from photosynthesis are dispersed through the whole system, that means we can access them," said co-first author Dr. Laura Wey.

"The fact that we didn't know this pathway existed is exciting because we could be able to harness it to extract more energy for renewables," she explained.

According to the researchers, altering photosynthetic pathways to produce clean fuels from the Sun could be more efficient if charges could be extracted sooner in photosynthesis.

Besides this, the capacity to control photosynthesis might increase crops' tolerance for intense sunshine.

"Many scientists have tried to extract electrons from an earlier point in photosynthesis but said it wasn't possible because the energy is so buried in the protein scaffold," said Zhang.

"The fact that we can steal them at an earlier process is mind-blowing. At first, we thought we'd made a mistake: it took a while for us to convince ourselves that we'd done it."

The study was published in Nature on March 22.

Study abstract:

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to ‘re-wire’ photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H2 evolution or CO2 fixation.Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.