Recovery of photosystem I and II activities during re-hydration of lichen Hypogymnia physodes thalli

Photosynthetic performance of Antarctic lichen Dermatocarpon polyphyllizum when affected by desiccation and low temperatures

Lichens are symbiotic organisms that are well adapted to desiccation/rehydration cycles. Over the last decades, the physiological background of their photosynthetic response-specifically activation of the protective mechanism during desiccation-has been studied at the level of photosystem II of the lichen photobiont by means of several biophysical methods. In our study, the effects of desiccation and low temperatures on chlorophyll fluorescence and spectral reflectance parameters were investigated in Antarctic chlorolichen Dermatocarpon polyphyllizum. Lichen thalli were collected from James Ross Island, Antarctica, and following transfer to a laboratory, samples were fully hydrated and exposed to desiccation at temperatures of 18, 10, and 4 °C. During the desiccation process, the relative water content (RWC) was measured gravimetrically and photosynthetic parameters related to the fast transient of chlorophyll fluorescence (OJIP) were measured repeatedly. Similarly, the change in spectral reflectance parameters (e.g., NDVI, PRI, G, NPCI) was monitored during thallus dehydration. The dehydration-response curves showed a decrease in a majority of the OJIP-derived parameters (e.g., maximum quantum yield of photosystem II photochemistry: F_V/F_M, and performance index: PI in D. polyphyllizum, which were more apparent at RWCs below 20%. The activation of protective mechanisms in severely dehydrated thalli was documented by increased thermal dissipation (DI₀/RC) and its quantum yield (Phi_D₀). Low temperature accelerated these processes. An analysis of the OJIP shape reveals the presence of K-bands (300 μs), and L-bands (80 μs), which can be attributed to dehydration-induced stress. Spectral reflectance indices decreased in a majority of cases with an RWC decrease and were positively related to the OJIP-derived parameters: F_V/F_M (capacity of photosynthetic processes in PSII), Phi_E₀ (effectiveness of electron transport), and PI_tot (total performance index), which was more apparent in NDVI. A negative relation was found for NPCI. These indices could be used in follow-up ecophysiological photosynthetic studies of lichens that are undergoing rehydration/dehydration cycles.

Photosynthetic recovery and acclimation to excess light intensity in the rehydrated lichen soil crusts

As an important successional stage and main type of biological soil crusts (BSCs) in Shapotou region of China (southeastern edge of Tengger Desert), lichen soil crusts (LSCs) often suffer from many stresses, such as desiccation and excess light intensity. In this study, the chlorophyll fluorescence and CO₂ exchange in the rehydrated LSCs were detected under a series of photosynthetically active radiation (PAR) gradients to study the photosynthetic acclimation of LSCs. The results showed that although desiccation leaded to the loss of photosynthetic activity in LSCs, the fluorescence parameters including Fo, Fv and Fv/Fm of LSCs could be well recovered after rehydration. After the recovery of photosynthetic activity, the effective photosynthetic efficiency Φ_PSII detected by Imaging PAM had declined to nearly 0 within both the lichen thallus upper and lower layers when the PAR increased to 200 μE m^-2 s^-1, however the net photosynthesis detected by the CO₂ gas analyzer in the LSCs still appeared when the PAR increased to 1000 μE m^-2 s^-1. Our results indicate that LSCs acclimating to high PAR, on the one hand is ascribed to the special structure in crust lichens, making the incident light into the lichen thallus be weakened; on the other hand the massive accumulation of photosynthetic pigments in LSCs also provides a protective barrier for the photosynthetic organisms against radiation damage. Furthermore, the excessive light energy absorbed by crust lichens is also possibly dissipated by the increasing non-photochemical quenching, therefore to some extent providing some protection for LSCs.

The enhancement of cyclic electron flow around photosystem I improves the recovery of severely desiccated Porphyra yezoensis (Bangiales, Rhodophyta)

Porphyra yezoensis, a representative species of intertidal macro-algae, is able to withstand periodic desiccation at low tide but is submerged in seawater at high tide. In this study, changes in photosynthetic electron flow in P. yezoensis during desiccation and re-hydration were investigated. The results suggested that the cyclic electron flow around photosystem I (PSI) increased significantly during desiccation, continued to operate at times of severe desiccation, and showed greater tolerance to desiccation than the electron flow around PSII. In addition, PSI activity in desiccated blades recovered faster than PSII activity during re-hydration. Even though linear electron flow was suppressed by DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea], cyclic electron flow could still be restored. This process was insensitive to antimycin A and could be suppressed by dibromothymoquinone (DBMIB). The prolonged dark treatment of blades reduced the speed in which the cyclic electron flow around PSI recovered, suggesting that stromal reductants, including NAD(P)H, played an important role in the donation of electrons to PSI and were the main cause of the rapid recovery of cyclic electron flow in desiccated blades during re-hydration. These results suggested that cyclic electron flow in P. yezoensis played a significant physiological role during desiccation and re-hydration and may be one of the most important factors allowing P. yezoensis blades to adapt to intertidal environments.

Heat stress and the photosynthetic electron transport chain of the lichen Parmelina tiliacea (Hoffm.) Ach. in the dry and the wet state: differences and similarities with the heat stress response of higher plants

Thalli of the foliose lichen species Parmelina tiliacea were studied to determine responses of the photosynthetic apparatus to high temperatures in the dry and wet state. The speed with which dry thalli were activated by water following a 24 h exposure at different temperatures decreased as the temperature was increased. But even following a 24 h exposure to 50 °C the fluorescence induction kinetics OJIP reflecting the reduction kinetics of the photosynthetic electron transport chain had completely recovered within 128 min. Exposure of dry thalli to 50 °C for 24 h did not induce a K-peak in the fluorescence rise suggesting that the oxygen evolving complex had remained intact. This contrasted strongly with wet thalli were submergence for 40 s in water of 45 °C inactivated most of the photosystem II reaction centres. In wet thalli, following the destruction of the Mn-cluster, the donation rate to photosystem II by alternative donors (e.g. ascorbate) was lower than in higher plants. This is associated with the near absence of a secondary rise peak (~1 s) normally observed in higher plants. Analysing the 820 nm and prompt fluorescence transients suggested that the M-peak (occurs around 2-5 s) in heat-treated wet lichen thalli is related to cyclic electron transport around photosystem I. Normally, heat stress in lichen thalli leads to desiccation and as consequence lichens may lack the heat-stress-tolerance-increasing mechanisms observed in higher plants. Wet lichen thalli may, therefore, represent an attractive reference system for the evaluation of processes related with heat stress in higher plants.

PSI-driven cyclic electron flow allows intertidal macro-algae Ulva sp. (Chlorophyta) to survive in desiccated conditions

Ulva sp. (Chlorophyta) is a representative species of the intertidal macro-algae responsible for the green tides that occurred along the shores of Qingdao in 2008 and had detrimental effects on the preparation for the 2008 Beijing Olympic Games sailing competition. In view of its significance, we have investigated the photosynthetic performance of the photosystems and the changes in photosynthetic electron transport that occur during desiccation and rehydration of Ulva sp. The PSII activity in Ulva sp. declined gradually during the course of desiccation, which was reflected by the decreased maximum quantum yield and effective quantum yield, whereas the PSI activity fluctuated significantly. In contrast, the electron transport rates of PSII approached zero at severe levels of desiccation, but the electron transport of PSI, which still operated, could be suppressed effectively by a specific inhibitor. Furthermore, the electron transport of PSI during rehydration of desiccated thalli was recovered faster than that of PSII. All these results implied that the linear electron flow was abolished in desiccated Ulva sp., whereas the cyclic PSI activity was significantly elevated, was still active at severe levels of desiccation and could be restored faster than PSII activity. Based on these results, we concluded the PSI-driven cyclic electron flow might provide desiccation tolerance and additional flexibility for the cell physiology of Ulva sp. under desiccation conditions, which might be one of the most important factors that make Ulva sp. well suited to experience daily cycles of desiccation at low tide and rehydration at high tide.

Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy

The mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715-740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns(-1) or 55.0 ns(-1) energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3-12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens.

Photosynthetic parameters of sexually different parts of Porphyra katadai var. hemiphylla (Bangiales, Rhodophyta) during dehydration and re-hydration

Physiological data from extreme habitat organisms during stresses are vital information for comprehending their survival. The intertidal seaweeds are exposed to a combination of environmental stresses, the most influential one being regular dehydration and re-hydration. Porphyra katadai var. hemiphylla is a unique intertidal macroalga species with two longitudinally separated, color distinct, sexually different parts. In this study, the photosynthetic performance of both PSI and PSII of the two sexually different parts of P. katadai thalli during dehydration and re-hydration was investigated. Under low-grade dehydration the variation of photosystems of male and female parts of P. katadai were similar. However, after the absolute water content reached 42%, the PSI of the female parts was nearly shut down while that of the male parts still coordinated well and worked properly with PSII. Furthermore, after re-hydration with a better conditioned PSI, the dehydrated male parts were able to restore photosynthesis within 1 h, while the female parts did not. It is concluded that in P. katadai the susceptibility of photosynthesis to dehydration depends on the accommodative ability of PSI. The relatively lower content of phycobiliprotein in male parts may be the cause for a stronger PSI after severe dehydration.

Investigation of water bound to photosystem I with multiquantum filtered 17 O nuclear magnetic resonance

A new analytical approach was developed to characterize the properties of water molecules bound to macromolecules in solution using 17 O nuclear magnetic resonance (NMR) relaxation. A combination of conventional (single-quantum) and triple-quantum filtered Hahn echo and inversion recovery measurements was employed. From measured relaxation rate constants, the fraction and the correlation time of bound H2 17 O molecules and the relaxation rate constant of bulk water in solution were calculated. This was done by solving analytically a set of nonlinear equations describing the overall relaxation rate constants in the presence of chemical exchange between bulk and bound water. The analytical approach shows the uniqueness of the solution for a given set of three relaxation rate constants. This important result sheds light on the data reduction problem from 17 O NMR experiments on biological systems. Water bound in photosystem I isolated from the wild type and rubA variant of the cyanobacterium Synechocystis species PCC 7002 was investigated for the first time. The analysis revealed that photosystem I isolated from the wild type binds 1720+/-110 water molecules, whereas photosystem I isolated from the rubA variant binds only 1310+/-170. The accuracy of the method proposed can be increased by further 17 O enrichment. The methodology, established for the first time in this work, allows the study of a diverse range of biological samples regardless of their size and molecular weight. Applied initially to photosystem I, this novel method has important consequences for the future investigation of the assembly of biological molecules.

Curling during desiccation protects the foliose lichen Lobaria pulmonaria against photoinhibition

This study aims to assess the photoprotective potential of desiccation-induced curling in the light-susceptible old forest lichen Lobaria pulmonaria by using chlorophyll fluorescence imaging. Naturally curled thalli showed less photoinhibition-induced limitations in primary processes of photosynthesis than artificially flattened specimens during exposures to 450 micromol m-2 s-1 in the laboratory after both 12- (medium dose treatment) and 62-h duration (high dose treatment). Thallus areas shaded by curled lobes during light exposure showed unchanged values of measured chlorophyll fluorescence parameters (FV/FM, PhiPS II), whereas non-shaded parts of curled thalli, as well as the mean for the entire flattened thalli, showed photoinhibitory limitation after light treatments. Furthermore, the chlorophyll fluorescence imaging showed that the typical small-scale reticulated ridges on the upper side of L. pulmonaria caused a spatial, small-scale reduction in damage due to minor shading. Severe dry-state photoinhibition readily occurred in flattened and light-treated L. pulmonaria, although the mechanisms for such damage in a desiccated and inactive stage are not well known. Natural curling is one strategy to reduce the chance for serious photoinhibition in desiccated L. pulmonaria thalli during high light exposures.

A dip in the chlorophyll fluorescence induction at 0.2-2 s in Trebouxia-possessing lichens reflects a fast reoxidation of photosystem I. A comparison with higher plants

An unusual dip (compared to higher plant behaviour under comparable light conditions) in chlorophyll fluorescence induction (FI) at about 0.2-2 s was observed for thalli of several lichen species having Trebouxia species (the most common symbiotic green algae) as their native photobionts and for Trebouxia species cultured separately in nutrient solution. This dip appears after the usual O(J)IP transient at a wide range of excitation light intensities (100-1800 micromol photons m(-2) s(-1)). Simultaneous measurements of FI and 820-nm transmission kinetics (I(820)) with lichen thalli showed that the decreasing part of the fluorescence dip (0.2-0.4 s) is accompanied by a decrease of I(820), i.e., by a reoxidation of electron carriers at photosystem I (PSI), while the subsequent increasing part (0.4-2 s) of the dip is not paralleled by the change in I(820). These results were compared with that measured with pea leaves-representatives of higher plants. In pea, PSI started to reoxidize after 2-s excitation. The simultaneous measurements performed with thalli treated with methylviologen (MV), an efficient electron acceptor from PSI, revealed that the narrow P peak in FI of Trebouxia-possessing lichens (i.e., the I-P-dip phase) gradually disappeared with prolonged MV treatment. Thus, the P peak behaves in a similar way as in higher plants where it reflects a traffic jam of electrons induced by a transient block at the acceptor side of PSI. The increasing part of the dip in FI remained unaffected by the addition of MV. We have found that the fluorescence dip is insensitive to antimycin A, rotenone (inhibitors of cyclic electron flow around PSI), and propyl gallate (an inhibitor of plastid terminal oxidase). The 2-h treatment with 5 microM nigericin, an ionophore effectively dissipating the pH-gradient across the thylakoid membrane, did not lead to significant changes either in FI nor I(820) kinetics. On the basis of the presented results, we suggest that the decreasing part of the fluorescence dip in FI of Trebouxia-lichens reflects the activation of ferredoxin-NADP(+)-oxidoreductase or Mehler-peroxidase reaction leading to the fast reoxidation of electron carriers in thylakoid membranes. The increasing part of the dip probably reflects a transient reduction of plastoquinone (PQ) pool that is not associated with cyclic electron flow around PSI. Possible causes of this MV-insensitive PQ reduction are discussed.

Activation of photosynthesis and resistance to photoinhibition in cyanobacteria within biological desert crust

Filamentous cyanobacteria are the main primary producers in biological desert sand crusts. The cells are exposed to extreme environmental conditions including temperature, light, and diurnal desiccation/rehydration cycles. We have studied the kinetics of activation of photosynthesis during rehydration of the cyanobacteria, primarily Microcoleus sp., within crust samples collected in the Negev desert, Israel. We also investigated their susceptibility to photoinhibition. Activation of the photosynthetic apparatus, measured by fluorescence kinetics, thermoluminescence, and low temperature fluorescence emission spectra, did not require de novo protein synthesis. Over 50% of the photosystem II (PSII) activity, assembled phycobilisomes, and photosystem I (PSI) antennae were detected within less than 5 min of rehydration. Energy transfer to PSII and PSI by the respective antennae was fully established within 10 to 20 min of rehydration. The activation of a fraction of PSII population (about 20%-30%) was light and temperature-dependent but did not require electron flow to plastoquinone [was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea]. The cyanobacteria within the crusts are remarkably resistant to photoinhibition even in the absence of protein synthesis. The rate of PSII repair increased with light intensity and with time of exposure. Consequently, the extent of photoinhibition in high-light-exposed crusts reached a constant, relatively low, level. This is in contrast to model organisms such as Synechocystis sp. strain PCC 6803 where PSII activity declined continuously over the entire exposure to high illumination. Ability of the crust's organisms to rapidly activate photosynthesis upon rehydration and withstand photoinhibition under high light intensity may partly explain their ability to survive in this ecosystem.