New research reveals how proteins that harvest and use sunlight are organised by the most abundant organism in the world

New research from the Department of Molecular Biology and Biotechnology has imaged the machinery for solar energy capture and conversion in bacteria living in the oceans. Prochlorococcus 500

The paper, by the research group of Professor Neil Hunter in collaboration with the University of Illinois and the University of Warwick, is published this week in Nature Plants. This work reveals how the proteins that harvest and use sunlight are organised in the photosynthetic membranes of Prochlorococcus, the most abundant organism on the planet.

An estimated global abundance of 2.9 x 1027 Prochlorococcus cells fix 4 gigatonnes of carbon per year, and these microorganisms form the foundation of global food chains that sustain life on Earth. The photosynthetic machinery in Prochlorococcus is located within specialised membranes within the cell known as thylakoids that act as nanoscale solar panels. Globally Prochlorococcus thylakoids provide a gargantuan surface area, estimated to cover the Earth 28 times over, for the absorption of solar energy and its efficient conversion into chemical energy.

Despite the global scale and significance of these membranes, their underlying molecular organisation had not been elucidated. Using a combination of atomic force microscopy, mass spectrometry, pigment analysis and molecular modelling, Professor Hunter and his team were able to piece together the arrangement of the chlorophyll-containing photosystem complexes in thylakoid membranes for the first time.

Professor Hunter explains “The breathtaking images we obtained show how the thylakoid organisation of Prochlorococcus varies dramatically depending on their depth in the sea.”

Prochlorococcus ecotypes isolated from near the surface of the Mediterranean are optimised to make full use of the abundance of light in these waters, whereas the ecotypes from deeper parts of the sea have to work harder to scrape a living. We were able to understand these crucial adaptations in molecular detail showing how the ecotypes from deeper waters surround their photosystems with large rings of additional light harvesting proteins that are highly efficient at mopping up what little light penetrates.

“Understanding the detailed molecular organisation of the biological solar cells that power the planet is a crucial first step in constructing functional models that can be used to predict the contribution of these remarkable organisms to the oceanic ecosystem and global climate”

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