Photoacclimation and photoprotection strategies in siphonous green algae of the order Bryopsidales (Codium tomentosum and Bryopsis plumosa)

Several species of Bryopsidales (Chlorophyta) are known for displaying functional absence of the xanthophyll cycle, a common photoprotection mechanism responsible for qE component of NPQ. To cope with the extreme variability of their natural environment, these algae must be able to avoid photodamage. Previous works reported significant accumulation of all-trans-neoxanthin and violaxanthin under high light acclimation in different Bryopsidales, and speculated that these xanthophylls might control the amount of energy that reaches the photosystems, causing photoprotection. In this work, we investigated photoacclimation and photoprotection strategies in two species of Bryopsidales (Codium tomentosum and Bryopsis plumosa). We first characterised the acclimation state of algae exposed for 7 days to low light or high light (respectively 20 and 1000 μmol photons m2 s−1) in terms of pigment content (HPLC) and chlorophyll a variable fluorescence (PAM). We confirmed that high light triggers significant alteration of pigment content with accumulation of trans-Neoxanthin and Violaxanthin, and for the first time we characterised thoroughly how the pigment pool is altered during acclimation. We also confirmed that no evidence of any xanthophyll cycle is present in high light acclimated cultures. On a second note we tried to answer another major question: are trans-neoxanthin and violaxanthin photoprotective? Using a novel chlorophyll a variable fluorescence approach (pNPQ assessment) and performing quantification of PSII repair capacity (via lincomycin treatment) we were not able to give a clear answer to this question. Nonetheless, we concluded that despite trans-Neoxanthin and Violaxanthin might contribute to photoprotection, this process in Bryopsidales algae is likely given by the coordination between different mechanisms that deserve to be further investigated, including chloroplast movement, PSII repair/modulation, state transitions, and PSI cyclic electron transport.