# Acclimation to high and low diurnal light is flexible in Chlamydomonas reinhardtii

## TL;DR

Chlamydomonas reinhardtii can quickly adjust to sudden changes in light intensity, maintaining its photosynthetic function and fitness.

## Contribution

The study reveals the rapid and flexible acclimation mechanisms of Chlamydomonas reinhardtii to diurnal changes in light intensity.

## Key findings

- LL-acclimated cells recover and divide after a sudden HL day despite initial photodamage.
- HL-acclimated cells struggle during an LL day, failing to complete cell division.
- Transcriptomic and proteomic analyses show rapid gene induction for photoprotection and protein homeostasis.

## Abstract

Photosynthetic microbes are major contributors to the global carbon cycle, and they have strategies to succeed in the dynamic light environment of our planet. When it experiences repeated limiting or excess light days, the model alga Chlamydomonas reinhardtii maintains optimal thylakoid membrane architecture, photosynthetic complexes, and photoprotective capacity for the routine light intensity. Here, we investigate how maintenance of these phenotypes into the night impacts algal fitness upon a change in daylight intensity. We find that cells accustomed to dim days experience severe photoinhibition during a surprising bright day and their chloroplast integrity suffers. Nevertheless, they grow, divide, and flexibly adjust their photoprotective strategy in just 1 d. We chart the transcriptome and proteome as Chlamydomonas acclimates anew.

Chlamydomonas acclimates to repeated low (LL) or high light (HL) days by changing the abundance of photosynthetic complexes and the ultrastructure of its thylakoid membranes. These phenotypes persist through the night phases, suggesting a readiness for the daylight environment that is routinely experienced despite the intervening dark periods [S. Dupuis et al., Plant Cell
37, koaf086 (2025), 10.1093/plcell/koaf086]. Here, we investigate how prior acclimation impacts algal fitness upon a change in daylight intensity and how quickly Chlamydomonas can reprogram its photoprotective strategy in a diurnal context. We performed a systems analysis of synchronized populations acclimated to diurnal LL when subjected to HL days and of populations acclimated to diurnal HL when subjected to LL days. In the latter case, diurnal photoacclimation decreased fitness during the first day at a new light intensity: HL-acclimated cells barely increased in size over the first LL period, and they failed to complete a cell cycle. However, although LL-acclimated cells showed severe photodamage after 6 h of HL, they recovered chloroplast form and function later that afternoon and successfully divided at nightfall. These cells rapidly altered their thylakoid membrane ultrastructure, increased their photoprotective quenching capacity, and decreased their inventory of photosystem and antenna proteins by the end of the first HL day. Transcriptomic and proteomic analyses revealed rapid induction of thousands of genes, including those encoding proteases, chaperones, and other proteins involved in the chloroplast unfolded protein response. These results show that the alga is highly flexible and competent to rapidly acclimate to changes in diurnal light intensity.