12 March 2020

Evolutionary compromise inherent to photosynthesis discovered

Scientists have discovered that only certain carotenoids are capable of fulfilling both photoprotective and light harvesting functions in photosynthesis.

Carotenoid molecule

Scientists have unravelled the a key evolutionary compromise photosynthetic organisms make between optimal light harvesting and photoprotection.

Carotenoids play a number of important roles in photosynthesis, primarily providing light-harvesting and photoprotective energy dissipation functions within pigment–protein complexes.

The ability of carotenoid molecules to absorb light energy relies on the long, generally between 9 and 15, series of carbon–carbon double bonds in their structure. Although carotenoids with fewer than 9 double bonds should, in principle, be able to transfer energy efficiently to bacteriochlorophyll molecules for photosynthesis, they are not found in photosynthetic organisms.

Dr Andy Hitchcock's group in the Department of Molecular Biology and Biotechnology investigated this issue by transferring a carotenoid biosynthesis gene from a cyanobacterium, Synechocystis, into the purple bacterium Rhodobacter to create a mutant that makes zeta-carotene with only 7 double bonds, which is never found in natural light-harvesting complexes. Spectroscopic studies showed why this is the case.

Light-harvesting complexes from the mutant and native bacteria were compared by collaborators at Washington University, St Louis in the United States using ultrafast (femtosecond) transient absorption spectroscopy to understand the effect of the new carotenoid on light harvesting and photoprotection. Remarkably, the shorter carotenoid zeta-carotene transferred energy even more efficiently to bacteriochlorophyll than the native carotenoid neurosporene. However, there was a price to pay: zeta-carotene was unable to function in photoprotection leaving the light-harvesting complex vulnerable to photo-damage in unfavourable conditions.

Commenting on the discovery Dr Andy Hitchcock said "Our results reveal a fundamental compromise made by photosynthetic organisms during the course of evolution. They must use the right kinds of carotenoids that strike a balance between optimal photosynthesis and retaining the ability to protect themselves in adverse conditions."

The study was published in Proceedings of the National Academy of Sciences (PNAS), 5th March 2020.

Read the paper

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