Green microalgae, the ancestors of land plants, are microscopic organisms mainly living in the planet waters. Like plants, they are able of performing photosynthesis, a complex metabolic process that converts CO2 into biomass and oxygen using the energy of sunlight. Light is not only the source of energy that fuels photosynthesis but also acts as source of information. Thanks to specific sensors, called photoreceptors, algae respond to the light colour moving towards or against the light (phototaxis), regulating their cell division and other fundamental processes. When light intensity becomes very strong (i.e. under full sunlight) it can become toxic, producing reactive oxygen species and cell damage. To avoid this, photosynthetic organisms can actively decrease their light absorption capacity via photoprotective mechanisms that dissipate the excess light as heat, avoiding cell damage.
In a collaborative work with Japanese and German, we revealed a link between these three functions of the light. We shown that the green alga Chlamydomonas reinhardtii uses a specific blue light photoreceptor (phototropin, PHOT) not only to sense the light color but also to integrate information about the light intensity, ultimately activating the photoprotective mechanism that dissipates the excess light for photosynthesis in the form of heat (Figure). We propose that the mechanism that brings together light sensing, utilisation and dissipation evolved in the ocean, the ancestral environment where photosynthetic organisms evolved, there where blue light dominates the available spectrum. It then contributed to the capacity of photosynthetic organism to colonize the land, and eventually disappeared in plants, which are exposed to the full visible spectrum.
Schematic representation of the molecular links between the perception of light, its use for photosynthesis, and protection against the light stress in the microalgae Chlamydomonas reinhardtii.