A surprising photosynthetic activity

In the photosynthetic process which is so well described in green organisms, CO₂ fixation requires the production of chemical energy in the chloroplast (the molecule ATP – adenosine triphosphate and the molecule NADPH – nicotinamide adenine dinucleotide phosphate) in well-defined proportions.  The light phase of photosynthesis which consists of the conversion of light energy into chemical energy (ATP and NADPH) produces an ATP/NADPH ratio that is less than that required for the dark phase of photosynthesis where CO₂ is converted into sugar.

During a post-doctorate at the Laboratory of Genetics and Physiology of Microalgae at the University of Liege, Benjamin Bailleul – the first author of the article published in Nature, with the assistance of Nicolas Berne and Pierre Cardol – was involved  in a research project conducted simultaneously in France, Italy and the United States between 2009 and 2015. Benjamin Bailleul and his colleagues focussed on studying the photosynthetic behaviour of diatoms and the conversion of light energy.

While searching for the reasons why diatoms dominate the phytoplankton community, the scientists discovered an unexpected level of interaction between the chloroplasts and the mitochondria, two types of cellular compartments dedicated respectively to photosynthesis and respiration. The molecular mechanisms of diatoms involved in ensuring that ATP and NADPH are in an adequate ratio for CO₂ fixation to take place, depend on sustained exchanges between the chloroplast and the mitochondria. This process enables the exportation of NADPH to the mitochondria and in return, the importation of ATP into the chloroplast.

The particular contribution made to this project by the team from Liege, was to demonstrate the conservation of this interaction between photosynthesis and respiration in different species of diatoms. “The mechanism demonstrated, is, in fact, a mechanism that has already been discovered in terrestrial plants but which only manifests itself under particular conditions”, says Pierre Cardol. “Terrestrial plants have systems for regulating (or optimising) photosynthesis which are essentially located in the chloroplast.  These mechanisms, described in green terrestrial plants, were probably lost during the evolution of diatoms which acquired their chloroplast in a secondary way. This could be a lead for finding an explanation as to why diatoms need to resort to a system that works on the basis of interaction between the two energy centres of the cell (the chloroplast and the mitochondria). Optimisation of this interaction is likely to be responsible for the efficiency of photosynthesis in diatoms”. This process of photosynthesis optimisation has certainly contributed to the ecological success of diatoms in all the seas of the planet.

“But before being able to affirm this with greater certainty, it will certainly be necessary to study the other sixty percent of marine-based photosynthesis, explains Pierre Cardol. It would indeed be interesting to see how the rest of the photosynthetic organisms live in the oceans in order to better understand their ways of producing oxygen and fixing carbon dioxide”.

Finally, this particular interaction between respiration and photosynthesis could inspire promising developments in terms of biotechnology, “this interaction suggests the idea that increasing the respiratory process by adding sources of organic carbon could stimulate the growth of diatoms”. Apart from their important ecological interest, diatoms are currently being looked at by industry for their great capacity to produce poly-unsaturated fatty acids and other molecules of interest like natural pigments.