Thursday, 19 March 2015

On The Origin Of Photosynthesis

Photosynthesis drives the planet. No other process is responsible for capturing the input of energy from the sun and converting it into a form which all life can use, namely sugars. Without photosynthesis our planet would be a dull place, devoid of both the nutrients and oxygen needed to make the complex ecosystems formed by animals feasible. Ultimately, Photosynthesis is about reducing carbon dioxide with hydrogen to produce sugars. The hydrogen comes from water, the splitting of which produces the oxygen by-product or photolysis. This is catalysed by a protein containing a cage of manganese and calcium atoms.

The structure of the D1 protein responsible for obtaining
hydrogen for photosynthesis via the photolysis of water
Understanding when and how oxygenic photosynthesis evolved has been a goal of molecular biologists and a recent study of the D1 protein involved in photolysis - the first step in Photosystem II of oxygenic photosynthesis - has shed some light on the problem.

Many proteins exist in a number of different forms. Their amino acid sequences  can be compared and their ages correlated to build up a map of how the different forms have evolved from one another in terms of changes to the amino acid sequences. D1 is no different and a large number of different forms exist across various species of cyanobacteria and plants.

A research team, led by Dr Tanai Cardona from Imperial College in London, conducted a comparative study of the different forms of D1 to understand when D1 gained its water splitting ability. They found that some forms of the protein could have existed prior to the evolution of photolysis. What remained was to identify the structural changes leading to the reaction of the forms found in some of the most primitive forms of cyanobacteria, such as Gloeobacter, and then date when those changes occurred. Their results suggest that the form of photosynthesis we are familiar with evolved 3.2 to 2.7 billion years ago.

This gives cyanobacteria 100 million years to have produced enough oxygen to impact Earth's geochemistry, as seen in the geological record 2.6 billion years ago. However, the fossil record suggests that cyanobacteria are older. The upper limit of 3.2 billion years ago is a more likely figure when viewed in that context. Additionally, their results suggest that the transition from ancestral protein to forms of D1 capable of performing photolysis was a rapid process. The first cyanobacteria may have been better adapted for oxygenic photosynthesis. 

'I think the most significant implication of the paper is that now the evolution of biological water oxidation can be addressed experimentally,' said Cardona. 'It is quite possible that in extant cyanobacteria today Photosystem II, using these ancestral forms of D1, could display traits and perform chemistry that resemble those of transitional forms before the evolution of efficient water splitting as we understand it today. The study of these alternative photosystems will not only give insights into the evolution of the process but could also provide clues on the environmental conditions where oxygenic photosynthesis first arose billions of years ago in the early Earth.'