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New insights on the regulation of CO2 fixation in microalgae

​​​​​​Although ​essential for microalgal growth, light and CO2 are rarely available at optimal levels. To mitigate the effects of oxidative stress caused by too much light, microalgae activate a photoprotective mechanism that dissipates excess light energy in the form of heat, which is harmless to the algae. To ensure growth at low CO2 concentrations, microalgae activate a mechanism that increases the amount of CO2 available in their chloroplasts. Could there be a link between light and CO2?​

Published on 30 June 2023

​Quenching of energy (qE) and CO2 Concentrating Mechanism (CCM) are crucial for the survival of microalgae. Mutants deficient in qE cannot survive exposure to intense light, and mutants deficient in CCM cannot grow photoautotrophically (i.e. using light as an energy source and carbon dioxide as a carbon source) unless they are supplemented with high levels of CO2. While the two processes have traditionally been studied separately, researchers at IRIG have shown that they are largely co-regulated and share common regulatory elements (Figure).

While the presence of light was thought to be essential for the activation of qE and CMM, this study unexpectedly shows that the expression of genes involved in these processes is largely controlled by the availability of CO2. This finding suggests that the effect of light on the regulation of gene expression is often indirect and reflects changes in intracellular CO2 levels, which are determined by the balance between CO2 fixation in chloroplasts and CO2 generation by mitochondrial respiration. These data add a new facet to the role of CO2 in photosynthetic microalgae. In addition to being a substrate for photosynthesis, CO2 is also a signalling molecule that regulates gene expression in response to changes in light and CO2 availability. 

These discoveries provide a better understanding of the molecular mechanisms governing photosynthetic CO2 metabolism and can be used to enable the biosphere to respond more effectively to the regulation of atmospheric CO2 concentrations, but also to progress towards a sustainable low-carbon economy by fully exploiting the biotechnological potential of microalgae.

Figure caption:
An increase in respiration or a decrease in photosynthesis leads to high levels of intracellular CO2, which represses the CCM and qE genes by inactivating CIA5. Exposure to bright light increases the rate of CO2 fixation, leading to CO2​ depletion and activation of CIA5, which activates the genes associated with qE and CCM. The CIA5-dependent pathway is independent of light. Light affects the expression of the qE and CCM genes via phototropin. This regulatory protein operates in a signal transduction cascade, by acting as a repressor of the transcription factor QER7, which in turn represses the CCM and qE genes. The PHOT-QER7 signalling pathway is independent of CIA5.

The shared transcriptional regulators, between qE and CCM, identified in this study are:
- CIA5, initially established as a master regulator of CCM, has now been shown to be a crucial regulator of the expression of genes and proteins involved in photoprotection [1].
- the transcription factor LCR1, initially identified as a regulator of CCM, has also been shown to control the expression of photoprotection [2].
- QER7, a transcriptional factor whose function is as yet unknown in Chlamydomonas, acts as a repressor of the expression of genes linked to qE and CCM under the control of the blue light photoreceptor, phototropin [2].

Collaboration: international consortium of scientists led by Dimitris Petroutsos

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