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Antarctic macroalgal communities of the upper intertidal zone are particularly poor compared with other coastal regions. Exposure to desiccation and freezing combined with the abrasive effect of ice threatens the life of sessile organisms. One of the few species able to colonize this environment is the rhodophyte Pyropia endiviifolia. It belongs to the Bangiales, one of the oldest extant clades of photosynthetic eukaryotes, which has occurred for more than one billion years with basically the same morphological structure. Considering that the extent of Antarctic glaciation is a geologically recent event, we hypothesized that pre-adaptations to desiccation in bangiophytes may have contributed to the success of P. endiviifolia in Antarctica. To test this, we compared its photosynthetic performance and tolerance to desiccation and freezing with those from a related intertidal species, the temperate Atlantic species Porphyra linearis. As evidenced by gas exchange and chlorophyll fluorescence, P. endiviifolia is more adapted to high irradiances than P. linearis. The former species was also more desiccation-tolerant, and showed a higher glass transition temperature. Both species did not differ in chlorophyll content per dry weight, and tolerance to −20°C, even though the ice-nucleation temperature was much higher in P. endiviifolia. Membrane integrity may depend on fatty acid composition in P. endiviifolia, while on enhanced tocopherol in P. linearis. Overall, both species show different strategies to deal with freezing temperatures: supercooling in P. linearis vs. freezing-tolerance in P. endiviifolia. This matches with the probability of being subjected to sub-zero temperatures in their natural environments (lower in the case of P. linearis). Surprisingly, the higher risk of dehydration in the natural habitat of P. linearis is not matched by a higher desiccation tolerance. This observation does not support the initial hypothesis of the study but suggests the opposite: the acclimation to a cold environment results in higher desiccation tolerance. HIGHLIGHTS ● Porphyra linearis and Pyropia endiviifolia are remarkably tolerant to desiccation and freezing.● Antarctic P. endiviifolia is remarkably tolerant to desiccation and freezing.● Mechanisms of freezing tolerance could induce a higher tolerance to desiccation. [ABSTRACT FROM AUTHOR]

Summary: Desiccation tolerant plants can survive extreme water loss in their vegetative tissues. The fern Anemia caffrorum produces desiccation tolerant (DT) fronds in the dry season and desiccation sensitive (DS) fronds in the wet season, providing a unique opportunity to explore the physiological mechanisms associated with desiccation tolerance.Anemia caffrorum plants with either DT or DS fronds were acclimated in growth chambers. Photosynthesis, frond structure and anatomy, water relations and minimum conductance to water vapour were measured under well‐watered conditions. Photosynthesis, hydraulics, frond pigments, antioxidants and abscisic acid contents were monitored under water deficit.A comparison between DT and DS fronds under well‐watered conditions showed that the former presented higher leaf mass per area, minimum conductance, tissue elasticity and lower CO2 assimilation. Water deficit resulted in a similar induction of abscisic acid in both frond types, but DT fronds maintained higher stomatal conductance and upregulated more prominently lipophilic antioxidants.The seasonal alternation in production of DT and DS fronds in A. caffrorum represents a mechanism by which carbon gain can be maximized during the rainy season, and a greater investment in protective mechanisms occurs during the hot dry season, enabling the exploitation of episodic water availability. [ABSTRACT FROM AUTHOR]

Resurrection plants are vascular species able to sustain extreme desiccation in their vegetative tissues. Despite its potential interest, the role of leaf anatomy in CO2 diffusion and photosynthesis under non-stressed conditions has not been explored in these species. Net CO2 assimilation (A n) and its underlying diffusive, biochemical, and anatomical determinants were assessed in 10 resurrection species from diverse locations, including ferns, and homoiochlorophyllous and poikilochlorophyllous angiosperms. Data obtained were compared with previously published results in desiccation-sensitive ferns and angiosperms. A n in resurrection plants was mostly driven by mesophyll conductance to CO2 (g m) and limited by CO2 diffusion. Resurrection species had a greater cell wall thickness (T cw) than desiccation-sensitive plants, a feature associated with limited CO2 diffusion in the mesophyll, but also greater chloroplast exposure to intercellular spaces (S c), which usually leads to higher g m. This combination enabled a higher A n per T cw compared with desiccation-sensitive species. Resurrection species possess unusual anatomical features that could confer stress tolerance (thick cell walls) without compromising the photosynthetic capacity (high chloroplast exposure). This mechanism is particularly successful in resurrection ferns, which display higher photosynthesis than their desiccation-sensitive counterparts. [ABSTRACT FROM AUTHOR]

Background and Aims Lichens represent a symbiotic relationship between at least one fungal and one photosynthetic partner. The association between the lichen-forming fungus Mastodia tessellata (Verrucariaceae) and different species of Prasiola (Trebouxiophyceae) has an amphipolar distribution and represents a unique case study for the understanding of lichen symbiosis because of the macroalgal nature of the photobiont, the flexibility of the symbiotic interaction and the co-existence of free-living and lichenized forms in the same microenvironment. In this context, we aimed to (1) characterize the photosynthetic performance of co-occurring populations of free-living and lichenized Prasiola and (2) assess the effect of the symbiosis on water relations in Prasiola , including its tolerance of desiccation and its survival and performance under sub-zero temperatures. Methods Photochemical responses to irradiance, desiccation and freezing temperature and pressure–volume curves of co-existing free-living and lichenized Prasiola thalli were measured in situ in Livingston Island (Maritime Antarctica). Analyses of photosynthetic pigment, glass transition and ice nucleation temperatures, surface hydrophobicity extent and molecular analyses were conducted in the laboratory. Key Results Free-living and lichenized forms of Prasiola were identified as two different species: P. crispa and Prasiola sp. respectively. While lichenization appears to have no effect on the photochemical performance of the alga or its tolerance of desiccation (in the short term), the symbiotic lifestyle involves (1) changes in water relations, (2) a considerable decrease in the net carbon balance and (3) enhanced freezing tolerance. Conclusions Our results support improved tolerance of sub-zero temperature as the main benefit of lichenization for the photobiont, but highlight that lichenization represents a delicate equilibrium between a mutualistic and a less reciprocal relationship. In a warmer climate scenario, the spread of the free-living Prasiola to the detriment of the lichen form would be likely, with unknown consequences for Maritime Antarctic ecosystems. [ABSTRACT FROM AUTHOR]