Oxygenic photosynthesis is at the basis of most ecosystems on our planet, and it's functionning requires constant acclimation to various environmental changes (like light, CO2, temperature, drought) that are occuring on time scales ranging from milliseconds to years. The current global warming is quickly changing the extend and frequency at which CO2, temperature, drought and even light (by changing the cloud coverage) will fluctuate in the future. However, the mechanisms allowing dynamic acclimation of photosynthesis to these changes is yet quite poorly understood. Such knowledge would not only help us predict how a specific photosynthetic organisms will perform under a given climate, but it can be the basis of photosynthesis engineering for the development of a sustainable bio-based society.

Our group will take a scientific journey into the understanding of how photosynthetic organisms dynamically acclimate to light, CO2 and temperature changes; we will try using this knowledge for developping predictive models on how photosynthesis can be optimized depending on the environmental conditions they live in.

How photosynthesis acclimates to light fluctuations?

Light is the fuel of all photosynthetic life on our planet, however, its availability is highly fluctuating, forcing photosynthetic organisms like plants and algae to constantly juggle between low levels of light and very high light. While subsaturating levels of light will require plants or algae to absorb as much light as possible, they also face the necessity to protect against harmful saturating levels of light. All the light fluctuations they encounter in the environement (due to i.e. cloud coverage or the movement of a canopy of a forest) represent a real roller coaster to photosynthetic orgnaisms. Our current understanding of acclimation to light mostly encompasses what happens during long term acclimation to high light or low light. However, our knowledge on how plants and algae acclimate to the very dynamic light changes in the environment is only at its way out.

We use system engineering approaches in mutants of the green microalgae Chlamydomonas reinhardtii to characterize the molecular functionning of photosynthesis under various light fluctuations (Steen et al, 2022). We will further developp this approach on a large library of mutant (CLiP) to discover new genes involved in photosynthetic acclimation to light fluctuations.

Unravel the dynamic bioenergetic functionning of the algal CO2 concentration mechanism.

Microalgae are responsible for half of the photosynthetis-mediated CO2 fixation on our planet and are therefore a major actor of the CO2 cycle in the atmosphere. Their important productivity relies on their unique capacity of concentrating inorganic carbon at the active site of the CO2 fixing enzyme RuBisCO by using a Carbon Concentrating Mechanism (CCM). Such mechanism is crucial for growth in CO2 limited conditions, typical of what occurs in aquatic environments.
Such mechanism requires an important amount of energy but the bioenergetic of the CCM are not understood. We have recently unraveled one mechanistic link between photosynthesis activity and the CCM energization (
Burlacot et al, 2021), but we have yet just scratched the surface of a much more complex bioenergetic network allowing a proper CCM functionning. Using chlorophyll fluorescence screens on mutant libraries, we aim at unravelling the players involved in this bioenergetic network and further characterize their role under dynamic range of CO2 availability.

Model the functionning of algal photosynthesis

Although the main principles of photosynthesis are the same for all photosynthetic organisms, its functionning can varry from specie to specie. Modelling photosynthesis has had an important impact on our understanding of biogeochemical processes since it has allowed to calculate the rate of photosynthesis at the level of the Earth from remote measurements and follow its changes over time. However the current models are based on higher plant but microalgae, which represent half of photosynthesis worldwide have a rather different photosynthesis functionning. Building more models of photosynthesis would help assessing how global warming will affect microalgal-mediated CO2 fixation of Earth.

We will use the model algae Chlamydomonas reinhardtii to build green microalgal models on photosynthesis functionning.

Dissecting the molecular basis of photosynthetic proteins activity

The photosynthetic chain is generating energy that is used by the Calvin cycle to fix CO2 and generate the necessary sugars for the cell to grow. However, part of this energy can be redirect towards other purposes like nitrate reduction, O2 photoreduction or nitric oxide (NO) photoreduction. In algae, the main alternative electron flow is mediated by a couple of proteins that are part of a big family of proteins: the Flavodiiron proteins. We have previously discovered that their activity of O2 reduction had a major role for acclimation to environmental shifts (Chaux et al, 2017; Burlacot et al, 2018; Burlacot et al, 2021). Recently we also discovered that these proteins were capable of reducing NO (Burlacot et al, 2020), a crucial signaling molecule. While members of this family of proteins are usually able to do either O2 of NO reduction, the aminoacid arrangement that defines their activity remains a mystery.

We will try using high throughput protein modifications coupled to massive quantitative screenings to discover the amino acid changes that are crucial for changing the activity of this protein. This could set the path towards being able to design the tunning of photosynthetic proteins activity.

All drawings have been made by Solène Moulin