Interspecific diversity in PSII photoinhibition and photoprotection of benthic diatoms during consecutive light and heat stress

Diatoms inhabiting estuarine intertidal flats are regularly exposed to episodes of high irradiance and elevated temperature during daytime low tides, conditions prone to inducing strong photoinhibition of photosystem II (PSII). However, the extent to which different species cope with repeated exposure to these combined stressors, and how photoacclimation modulates these responses, remains poorly known. This study examined the interspecific diversity in PSII photoinactivation, photorepair and photoprotection of three estuarine pennate diatom species exposed to three consecutive daily periods of combined high light and heat: two benthic species, Navicula phyllepta and Entomoneis paludosa , and one tychoplanktonic species, Cylindrotheca closterium . Unialgal cultures of each species were also acclimated to contrasting low- (LL) and high-light (HL) regimes to further evaluate the influence of photoacclimation on the responses to light and heat stress. In the two benthic species, PSII photoinactivation was largely counterbalanced by efficient photorepair, leading to near-complete recovery after each stress event. In contrast, the tychoplanktonic C. closterium showed markedly higher photoinactivation rates and limited photorepair capacity, resulting in an overall poor recovery. The tested species also differed strongly in the operation of the xanthophyll cycle (XC). Although XC-mediated de-epoxidation of the pigment diadinoxanthin into diatoxanthin increased under stress in all LL-acclimated cultures, it was strongly reduced in the HL-acclimated benthic species. Despite pronounced short-term effects of each stress exposure, no significant accumulation of irreversible PSII damage occurred over the 3 days, likely due to extended recovery intervals that favored an effective PSII photorepair. Overall, the results show that species-specific physiological traits and photoacclimation state play a dominant role in determining the capacity of diatoms to cope with the extreme conditions found in the estuarine intertidal environments.