New light on ‘bizarre phenomenon’ in photosynthesis

Roberta Croce, professor of biophysics of photosynthesis at Vrije Universiteit Amsterdam, describes the phenomenon upon which her group has shed new light as ‘a bizarre occurrence’, because it is rather counter-intuitive. ‘At excessively high light intensity, the efficiency of photosynthesis decreases,’ says Croce. ‘We know this process as photoinhibition. Under these conditions, you would expect a large proportion of the absorbed energy to be re-emitted as fluorescence, but the opposite happens: during photoinhibition, fluorescence actually decreases.’

‘Our work has been carried out largely in vivo and therefore has direct biological relevance’

Alga from Paris

This so-called photoinhibition quenching (q_I) had been known for some time, but nobody understood the molecular background. Wojciech Nawrocki, a postdoc in Croce’s group, wanted to investigate it on the basis of photosynthesis in the unicellular green alga Chlamydomonas reinhardtii. He received assistance with the modelling from Dallas (USA), and a necessary mutant of the alga came from Paris. Their findings were recently published in Science Advances.

A plant or other photosynthesising organism can, at high light intensity, implement various biochemical adaptations with remarkable speed, diverting a portion of the photon energy so that the chlorophyll pigments do not become overloaded. ‘However, this only works up to a certain point,’ says Croce. ‘Ultimately, the fact that a large proportion of the energy cannot be used by chlorophyll for photochemistry will lead to the formation of oxygen radicals.’

Devastating

These oxygen radicals have a particularly devastating effect on photosystem II (PSII), the complex that splits water (see box). As a result of this, and through direct damage from blue and UV light, photoinhibition occurs: the productivity of photosynthesis declines. ‘We discovered that this damage is probably caused by singlet oxygen (an extremely reactive variant of O₂, ed.) and occurs primarily in the reaction centre of PSII, where light is converted into chemical energy.’

‘The damaged reaction centre is simultaneously the quencher’

But that is not all. The damage in the reaction centre — the researchers suspect it specifically concerns chlorophyll Chl_D1, the primary donor of the system — also turns out to be the cause of photoinhibition quenching. ‘As soon as the damaged component has been completely broken down and cleared away by the cell, the quenching disappears as well,’ says Croce. ‘The damaged reaction centre is therefore simultaneously the quencher.’ That q_I is caused by the damaged PSII components is contrary to what everyone previously believed.

Protecting crops

The story is far from over. ‘It is of little surprise that photoinhibition, damaging one of the most complex enzymes in biology, is itself a complicated process,’ the authors write in their discussion. Croce: ‘Part of the beauty of the work in our publication is that it has been carried out largely in vivo and therefore has direct biological relevance. But our next experiments will be done in vitro, because that allows us to simplify the system and thereby measure far more precisely.’

A better understanding of photoinhibition and of the repair of damaged components of PSII resulting from excessive light intensity is enormously relevant, according to Croce. ‘Due to climate change, many crops that are important for food production are being exposed to greater levels of sunlight. They are not adapted to this, and it can result in lower yields. Perhaps in the future it will be possible to use precision DNA techniques to better protect plants against excessively intense sunlight, in order to maintain or even increase yields.’