Freezing temperature effects on photosystem II in Antarctic lichens evaluated by chlorophyll fluorescence.

This study explores and compares the limits for photosynthesis in subzero temperatures of six Antarctic lichens: Sphaerophorus globosus, Caloplaca regalis, Umbilicaria antarctica, Pseudephebe minuscula, Parmelia saxatilis and Lecania brialmontii combining linear cooling and chlorophyll fluorescence methods. The results revealed triphasic S-curves in the temperature response of the maximum quantum yield (FV/FM) and effective quantum yield of photosystem II (ΦPSII) for all species. All investigated species showed a high level of cryoresistance with critical temperatures (Tc) below -20 °C. However, record low Tc temperatures have been discovered for L. brialmotii (-54 °C for FV/FM and -40 °C for ΦPSII) and C. regalis (-52 °C for FV/FM and -38 °C for ΦPSII). Additionally, the yield differentials (FV/FM - ΦPSII) in functions of temperature revealed one or two peaks, with the larger one occurring for temperatures below -20 °C for the above-mentioned species. Finally, Kautsky kinetics were measured and compared at different temperatures (20 °C, 10 °C, 0 °C and -10 °C and then -10 °C after 1 h of incubation). This research serves as a foundation for further developing investigations into the biophysical mechanisms by which photosynthesis is carried out at subzero temperatures.

Resistance of Primary Photosynthesis to Photoinhibition in Antarctic Lichen Xanthoria elegans: Photoprotective Mechanisms Activated during a Short Period of High Light Stress.

The Antarctic lichen, Xanthoria elegans, in its hydrated state has several physiological mechanisms to cope with high light effects on the photosynthetic processes of its photobionts. We aim to investigate the changes in primary photochemical processes of photosystem II in response to a short-term photoinhibitory treatment. Several chlorophyll a fluorescence techniques: (1) slow Kautsky kinetics supplemented with quenching mechanism analysis; (2) light response curves of photosynthetic electron transport (ETR); and (3) response curves of non-photochemical quenching (NPQ) were used in order to evaluate the phenomenon of photoinhibition of photosynthesis and its consequent recovery. Our findings suggest that X. elegans copes well with short-term high light (HL) stress due to effective photoprotective mechanisms that are activated during the photoinhibitory treatment. The investigations of quenching mechanisms revealed that photoinhibitory quenching (qIt) was a major non-photochemical quenching in HL-treated X. elegans; qIt relaxed rapidly and returned to pre-photoinhibition levels after a 120 min recovery. We conclude that the Antarctic lichen species X. elegans exhibits a high degree of photoinhibition resistance and effective non-photochemical quenching mechanisms. This photoprotective mechanism may help it survive even repeated periods of high light during the early austral summer season, when lichens are moist and physiologically active.

Sulfate supplementation affects nutrient and photosynthetic status of Arabidopsis thaliana and Nicotiana tabacum differently under prolonged exposure to cadmium.

Hydroponic experiments were performed to examine the effect of prolonged sulfate limitation combined with cadmium (Cd) exposure in Arabidopsis thaliana and a potential Cd hyperaccumulator, Nicotiana tabacum. Low sulfate treatments (20 and 40 µM MgSO4) and Cd stress (4 µM CdCl2) showed adverse effects on morphology, photosynthetic and biochemical parameters and the nutritional status of both species. For example, Cd stress decreased NO3- root content under 20 µM MgSO4 to approximately 50% compared with respective controls. Interestingly, changes in many measured parameters, such as chlorophyll and carotenoid contents, the concentrations of anions, nutrients and Cd, induced by low sulfate supply, Cd exposure or a combination of both factors, were species-specific. Our data showed opposing effects of Cd exposure on Ca, Fe, Mn, Cu and Zn levels in roots of the studied plants. In A. thaliana, levels of glutathione, phytochelatins and glucosinolates demonstrated their distinct involvement in response to sub-optimal growth conditions and Cd stress. In shoot, the levels of phytochelatins and glucosinolates in the organic sulfur fraction were not dependent on sulfate supply under Cd stress. Altogether, our data showed both common and species-specific features of the complex plant response to prolonged sulfate deprivation and/or Cd exposure.

Antarctic Lichens under Long-Term Passive Warming: Species-Specific Photochemical Responses to Desiccation and Heat Shock Treatments.

Climate warming in the Antarctic tundra will affect locally dominant cryptogams. Being adapted to low temperatures and freezing, little is known about the response of the polar lichens' primary photochemistry to warming and desiccation. Since 2008, we have monitored the ecophysiological responses of lichens to the future warming scenario during a long-term warming experiment through open top chambers (OTCs) on Fildes Peninsula. We studied the primary photochemical response (potential Fv/Fm and effective efficiency of photosystem II YPSII) of different lichen taxa and morphotypes under desiccation kinetics and heat shock experiments. As lichens grow slowly, to observe changes during warming we methodologically focused on carbon and nitrogen content as well as on the stable isotope ratios. Endemic Himantormia lugubris showed the strongest effect of long-term warming on primary photochemistry, where PSII activity occurred at a lower %RWC inside the OTCs, in addition to higher Fv/Fm values at 30 °C in the heat shock kinetic treatment. In contrast, Usnea aurantiaco-atra did not show any effect of long-term warming but was active at a thallus RWC lower than 10%. Both Cladonia species were most affected by water stress, with Cladonia aff. gracilis showing no significant differences in primary photochemical responses between the warming and the control but a high sensibility to water deficiency, where, at 60% thallus RWC, the photochemical parameters began to decrease. We detected species-specific responses not only to long-term warming, but also to desiccation. On the other hand, the carbon content did not vary significantly among the species or because of the passive warming treatment. Similarly, the nitrogen content showed non-significant variation; however, the C/N ratio was affected, with the strongest C/N decrease in Cladonia borealis. Our results suggest that Antarctic lichens can tolerate warming and high temperature better than desiccation and that climate change may affect these species if it is associated with a decrease in water availability.

Inhibition of Primary Photosynthesis in Desiccating Antarctic Lichens Differing in Their Photobionts, Thallus Morphology, and Spectral Properties.

Five macrolichens of different thallus morphology from Antarctica (King George Island) were used for this ecophysiological study. The effect of thallus desiccation on primary photosynthetic processes was examined. We investigated the lichens' responses to the relative water content (RWC) in their thalli during the transition from a wet (RWC of 100%) to a dry state (RWC of 0%). The slow Kautsky kinetics of chlorophyll fluorescence (ChlF) that was recorded during controlled dehydration (RWC decreased from 100 to 0%) and supplemented with a quenching analysis revealed a polyphasic species-specific response of variable fluorescence. The changes in ChlF at a steady state (Fs), potential and effective quantum yields of photosystem II (FV/FM, ΦPSII), and nonphotochemical quenching (NPQ) reflected a desiccation-induced inhibition of the photosynthetic processes. The dehydration-dependent fall in FV/FM and ΦPSII was species-specific, starting at an RWC range of 22-32%. The critical RWC for ΦPSII was below 5%. The changes indicated the involvement of protective mechanisms in the chloroplastic apparatus of lichen photobionts at RWCs of below 20%. In both the wet and dry states, the spectral reflectance curves (SRC) (wavelength 400-800 nm) and indices (NDVI, PRI) of the studied lichen species were measured. Black Himantormia lugubris showed no difference in the SRCs between wet and dry state. Other lichens showed a higher reflectance in the dry state compared to the wet state. The lichen morphology and anatomy data, together with the ChlF and spectral reflectance data, are discussed in relation to its potential for ecophysiological studies in Antarctic lichens.

Chlorophyll a fluorescence and Raman spectroscopy can monitor activation/deactivation of photosynthesis and carotenoids in Antarctic lichens.

Lichens survive harsh weather of Antarctica as well as of other hostile environments worldwide. Therefore, this investigation is important to understand the evolution of life on Earth in relation to their stress tolerance strategy. We have used chlorophyll a fluorescence (ChlF) and Raman spectroscopy, respectively, to monitor the activation/deactivation of photosynthesis and carotenoids in three diverse Antarctic lichens, Dermatocarpon polyphyllizum (DP), Umbilicaria antarctica (UA), and Leptogium puberulum (LP). These lichens, post 4 h or 24 h of hydration, showed differences in their ChlF transients and values of major ChlF parameters, e.g., in the maximum quantum efficiency of PSII photochemistry (Fv/Fm), and yields of fluorescence and heat dissipation (Φf,d), of effective quantum efficiency of PSII photochemistry (ΦPSII) and of non-photochemical quenching (Φnpq), which may be due to quantitative and/or qualitative differences in the composition of their photobionts. For understanding the kinetics of hydration-induced activation of photosynthesis, we screened ΦPSII of these lichens and reported its non-linear stimulation on a minute time scale; half of the activation time (t1/2) was fastest ~4.05 ± 0.29 min for DP, which was followed by 5.46 ± 0.18 min for UA, and 13.95 ± 1.24 min for LP. Upon drying of fully activated lichen thallus, there was a slow decay, in hours, of relative water content (RWC) as well as of Fv/Fm. Raman spectral signatures were different for lichens having algal (in DP and UA) and cyanobacteria (in LP) photobionts, and there was a significant shift in ν1(C=C) Raman band of carotenoids post 24 h hydration as compared to their value at a dry state or post 4 h of hydration; this shift was decreased, when drying, in DP and LP but not in UA. We conclude that hydration nonlinearly activated photosynthetic apparatus/reactions of these lichens in minute time range but there was a de-novo synthesis of chlorophylls as well as of carotenoids post 24 h. Their dehydration-induced deactivation, however, was comparatively slow, in hours range, and there seemed a degradation of synthesized chlorophylls and carotenoids post dryness. We conclude that in extremophilic lichens, their photosynthetic partners, in particular, possess a complex survival and photoprotective strategy to be successful in the extreme terrestrial environments in Antarctica.

Eco-Physiological Screening of Different Tomato Genotypes in Response to High Temperatures: A Combined Field-to-Laboratory Approach.

High temperatures represent a limitation for growth and development of many crop species. Several studies have demonstrated that the yield reduction of tomato under high temperatures and drought is mainly due to a photosynthetic decline. In this paper, a set of 15 tomato genotypes were screened for tolerance to elevated temperatures by cultivating plants under plastic walk-in tunnels. To assess the potential tolerance of tomato genotypes to high temperatures, measurements of chlorophyll fluorescence, pigments content and leaf functional traits have been carried out together with the evaluation of the final yields. Based on the greenhouse trials, a group of eight putative heat-sensitive and heat-tolerant tomato genotypes was selected for laboratory experiments aimed at investigating the effects of short-term high temperatures treatments in controlled conditions. The chlorophyll fluorescence induction kinetics were recorded on detached leaves treated for 60 min at 35 °C or at 45 °C. The last treatment significantly affected the photosystem II (PSII) photochemical efficiency (namely maximum PSII quantum efficiency, Fv/Fm, and quantum yield of PSII electron transport, ΦPSII) and the non-photochemical quenching (NPQ) in the majority of genotypes. The short-term heat shock treatments also led to significant differences in the shape of the slow Kautsky kinetics and its significant time points (chlorophyll fluorescence levels minimum O, peak P, semi-steady state S, maximum M, terminal steady state T) compared to the control, demonstrating heat shock-induced changes in PSII functionality. Genotypes potentially tolerant to high temperatures have been identified. Our findings support the idea that chlorophyll fluorescence parameters (i.e., ΦPSII or NPQ) and some leaf functional traits may be used as a tool to detect high temperatures-tolerant tomato cultivars.

Post rapid freezing growth of Antarctic strain of Heterococcus sp. monitored by cell viability and chlorophyll fluorescence.

The soil microalgae of the genus Heterococcus are found in cold environments and have been reported for the terrestrial ecosystems of several Sub-Antarctic and Antarctic Islands. This study focused on resistance of Heterococcus sp. to sub-zero temperature. Heterococcus sp. was isolated from soil samples from James Ross Island, Antarctica. Culture of Heterococcus sp. grown in liquid medium were used to study ribitol effects at sub-zero temperatures on the species resistance to rapid freezing (RF, immersion of a sample into liquid nitrogen) and consequent cultivation on agar. Before the experiment, Heterococcus sp. was cultured in liquid medium for 11 months and then treated in ribitol concentrations of 32 or 50 mM for 2 h. Then, 1 ml samples were frozen to -196 °C in liquid nitrogen (day 0) and inoculated on BBM agar after thawing. Number of living and dead cells was evaluated and the cell viability (Pν) was calculated repeatedly using the optical microscopy approach. The addition of ribitol caused a noticable increase in Pν on days 9, 12, 14 (with a Pν of 25-45% in ribitol-treated samples compared to 10% in the untreated control). In the following period (d 16-19), the positive effect of ribitol on Pν was less pronounced but still statistically significant. To evaluate the negative effects of RF on chlorophyll fluorescence parameters, the potential yield of photochemical reactions in PS II (FV/FM), and the effective quantum yield of photochemical reactions in PS II (ФPSII) were measured immediately before and after RF. Consequently, FV/FM and ФPSII of agar inoculates were measured repeatedly for 30 d cultivation in 3 d interval. Both the 32 and the 50 mM addition of ribitol caused earlier detection of the parameters (d 16) compared to the control measurements (d 23) as well as reaching the maximum values of the chlorophyll fluorescence parameters earlier (d 23 in ribitol-treated samples compared to d 25 in control samples). Heterococcus sp. proved to be a species resistant to rapid freezing. The ability may help the species to survive in harsh Antarctic environments typified by rapid fluctuations in temperature that may bring a rapid freezing of the alga.

Sensitivity of photosynthetic processes to freezing temperature in extremophilic lichens evaluated by linear cooling and chlorophyll fluorescence.

Extremophilic lichens and their photosynthesizing photobionts from the cold regions of Earth are adapted to perform photosynthesis at subzero temperatures. To evaluate interspecific differences in the critical temperature for primary photochemical processes of photosynthesis, we exposed lichen thalli of Usnea antarctica, Usnea aurantiaco-atra, and Umbilicaria cylindrica to linear cooling from +20 to -50 °C at a constant rate of 2 °C min-1. Simultaneously, two chlorophyll fluorescence parameters (FV/FM - potential yield of photosynthetic processes in photosystem II, ΦPSII - effective quantum yield of PS II) evaluating a gradual subzero temperature-induced decline in photosynthetic processes were measured by a modulated fluorometer. For the studied species, the response of FV/FM and ΦPSII to declining temperature showed an S-curve shape. The decline in FV/FM and ΦPSII at low temperatures started at -5 and +5 °C, respectively in the majority of cases. The decline was, however, species-specific. U. aurantiaco-atra showed a constant-rate decline of ΦPSII from the physiological temperature 20 °C. U. antarctica exhibited the first sign of FV/FM decline at -12 °C. The critical temperature related to full inhibition of the photosynthetic processes in PSII (FV/FM), was found at -20 °C. However, this occurred at -30 °C for U. cylindrica. In an individual sample, the critical temperature for FV/FM was typically lower than for ΦPSII. The method of linear cooling combined with simultaneous measurements of chlorophyll fluorescence parameters proved to be an efficient tool in the estimation of extremophilic species sensitivity/resistance to freezing.

The ice nucleation activity of extremophilic algae.

Differences in the level of cold acclimation and cryoprotection estimated as ice nucleation activity in snow algae (Chlamydomonas cf. nivalis and Chloromonas nivalis), lichen symbiotic algae (Trebouxia asymmetrica, Trebouxia erici and Trebouxia glomerata), and a mesophilic strain (Chlamydomonas reinhardti) were evaluated. Ice nucleation activity was measured using the freezing droplet method. Measurements were performed using suspensions of cells of A750 (absorbance at 750 nm) ~ 1, 0.1, 0.01 and 0.001 dilutions for each strain. The algae had lower ice nucleation activity, with the exception of Chloromonas nivalis contaminated by bacteria. The supercooling points of the snow algae were higher than those of lichen photobionts. The supercooling points of both, mesophilic and snow Chlamydomonas strains were similar. The lower freezing temperatures of the lichen algae may reflect either the more extreme and more variable environmental conditions of the original localities or the different cellular structure of the strains examined.