Thylakoids reorganization enables driving photosynthesis under far-red light in the microalga Nannochloropsis gaditana

Oxygenic photosynthesis is driven by visible light in most photosynthetic organisms, with exceptions in a few cyanobacteria and microalgae species, which can extend the light absorption to far-red (FR) wavelengths, by synthesizing new pigments or shifting the antennae absorption range beyond 700 nm. In this work, we describe a novel mechanism of acclimation in the marine microalga Nannochloropsis gaditana, which resulted capable of growth relying solely on FR light. Unexpectedly, the response did not involve the synthesis of red-shifted absorption forms but a peculiar reorganization of chloroplasts. The abundance of photosynthetic complexes changed, with an increased accumulation of all pigment-binding proteins and photosystem II. Chloroplasts became bigger and thylakoid membranes increased in number, occupying almost all the organelle volume, showing also newly observed structures, composed of a central superstack with perpendicular electron-dense interconnections, that we propose to name thylakoidal bodies. To the best of our knowledge, these results describe a novel acclimation strategy to FR light, overall highlighting that the biodiversity of responses to FR light is currently underestimated.