Unraveling the Photoprotective Response of Lichenized and Free-Living Green Algae (Trebouxiophyceae, Chlorophyta) to Photochilling Stress

Abstract

Lichens and free-living terrestrial algae are widespread across many habitats and develop successfully in ecosystems where a cold winter limits survival. With the goal of comparing photoprotective responses in free-living and lichenized algae, the physiological responses to chilling and photochilling conditions were studied in three lichens and their isolated algal photobionts together as well as in a fourth free-living algal species. We specifically addressed the following questions: (i) Are there general patterns of acclimation in green algae under chilling and photochilling stresses? (ii) Do free-living algae exhibit a similar pattern of responses as their lichenized counterparts? (iii) Are these responses influenced by the selection pressure of environmental conditions or by the phylogenetic position of each species? To answer these questions, photosynthetic fluorescence measurements as well as pigment and low molecular weight carbohydrate pool analyses were performed under controlled laboratory conditions. In general, photochemical efficiency in all free-living algae decreased with increasing duration of the stress, while the majority of lichens maintained an unchanged photochemical activity. Nevertheless, these patterns cannot be generalized because the alga Trebouxia arboricola and the lichen Ramalina pollinaria (associated with Trebouxia photobionts) both showed a similar decrease in photochemical efficiency. In contrast, in the couple Elliptochloris bilobata-Baeomyces rufus, only the algal partner exhibited a broad physiological performance under stress. This study also highlights the importance of the xanthophyll cycle in response to the studied lichens and algae to photochilling stress, while the accumulation of sugars was not related to cold acclimation, except in the alga E. bilobata. The differences in response patterns detected among species can be mainly explained by their geographic origin, although the phylogenetic position should also be considered, especially in some species.

Ecophysiological changes and spore formation: two strategies in response to low‐temperature and high‐light stress in Klebsormidium cf. flaccidum (Klebsormidiophyceae, Streptophyta)1

“…Photosynthesis/irradiance curves (P/I curves) were determined as described by Míguez et al. (,b). Briefly, samples filtered on filter discs were measured in a PAM‐2500 fluorometer (Walz, Effeltrich, Germany) after 30 min of dark adaptation.…”

“…Pigments were extracted from freeze‐dried algal material as described by Míguez et al. (,b) and were separated by HPLC with a reverse‐phase C18 column (Waters Spherisorb ODS1, 4.6 x 250 mm; Milford, MA, USA). Pigments were detected and quantified with a photodiode array detector, following the method of García‐Plazaola and Becerril () modified after Garcia‐Plazaola and Esteban ().…”

“…The photochemistry of terrestrial algae under LT is insufficiently studied (Míguez et al. ,b). Terrestrial green algae are a heterogeneous group and originate from several independent transitions from aquatic to terrestrial habitats.…”

“…Although Míguez et al. (,b) recently investigated photochilling in Chlorophyta, available information about early‐branching Streptophyta is very sparse. Overall, down‐regulation of photochemical processes is seen as a controlled mechanism that favors the dissipation of excess energy to protect the photosynthetic apparatus under LT conditions.…”

“…Míguez et al. (,b) found that under LT stress, algae (as well as other photosynthetic groups) not only undergo changes in pigment/antioxidant content, but also accumulate soluble sugars.…”