Changes in macromolecular allocation in nondividing algal symbionts allow for photosynthetic acclimation in the lichen Lobaria pulmonaria

Heavy-metal pollution induces changes in the genetic composition and anatomical properties of photobionts in pioneer lichens colonising post-industrial habitats

Certain lichens are effective colonisers of polluted sites. However, little is known about the tolerance of photobionts and the degree of mycobiont selectivity to photobionts relative to metal pollution. The present study recognises the genetic and anatomical diversity of Asterochloris photobionts in epigeic lichens, i.e. Cladonia cariosa, C. rei, and Diploschistes muscorum, in relation to a wide spectrum of soil pollution. In accordance with phylogenetic analysis, photobionts were clustered in 7 moderately- to well-supported clades, including 19 haplotypes. The mycobionts of all studied lichens demonstrated a low level of selectivity and were capable of associating with various Asterochloris lineages. This tendency was also expressed by the frequent (~25%) occurrence of multiple algal genotypes in a single thallus. This indicates that identified Asterochloris lineages are generally tolerant to heavy-metal pollution, and the low level of selectivity of mycobionts enables them to select the most suitable and/or available partner. The trend of increasing incidence of certain Asterochloris lineages and decreasing frequency of others along with increasing soil pollution was observed. This proves the superior adaptation of some photobionts to polluted sites. Such symbiotic plasticity constitute an adaptive feature necessary for the successful colonisation. High number of haplotypes at polluted sites could be the result of multiple introduction events from different areas during the initial stages of spontaneous succession. Regardless of the genetic pattern, Asterochloris cells were considerably smaller, and the density and compaction of cells in the algal layer were higher, in lichen specimens from polluted sites, indicating that photobiont characteristics may be closely dependent on heavy-metal pollution.

Can Antarctic lichens acclimatize to changes in temperature?

The Antarctic Peninsula, a tundra biome dominated by lichens and bryophytes, is an ecozone undergoing rapid temperature shifts. Such changes may demand a high physiological plasticity of the local lichen species to maintain their role as key drivers in this pristine habitat. This study examines the response of net photosynthesis and respiration to increasing temperatures for three Antarctic lichen species with different ecological response amplitudes. We hypothesize that negative effects caused by increased temperatures can be mitigated by thermal acclimation of respiration and/or photosynthesis. The fully controlled growth chamber experiment simulated intermediate and extreme temperature increases over the time course of 6 weeks. Results showed that, in contrast to our hypothesis, none of the species was able to down-regulate temperature-driven respiratory losses through thermal acclimation of respiration. Instead, severe effects on photobiont vitality demonstrated that temperatures around 15°C mark the upper limit for the two species restricted to the Antarctic, and when mycobiont demands exceeded the photobiont capacity they could not survive within the lichen thallus. In contrast, the widespread lichen species was able to recover its homoeostasis by rapidly increasing net photosynthesis. We conclude that to understand the complete lichen response, acclimation processes of both symbionts, the photo- and the mycobiont, have to be evaluated separately. As a result, we postulate that any acclimation processes in lichen are species-specific. This, together with the high degree of response variability and sensitivity to temperature in different species that co-occur spatially close, complicates any predictions regarding future community composition in the Antarctic. Nevertheless, our results suggest that species with a broad ecological amplitude may be favoured with on-going changes in temperature.

Spatial organization of the three-component lichen Peltigera aphthosa in functional terms

The cephalolichen Peltigera aphthosa (L.) Willd. is characterized by lateral heterogeneity, which manifests itself in the presence of three thallus zones, referred to as the apical, basal and medial zone. These zones differ in terms of interaction between lichen bionts and their physiological activity. The apical thallus zone is more efficient in establishing a contact with cyanobacteria, because of a higher lectin content and a larger overall thallus surface area due to the presence of numerous mycobiont hyphae. Cephalodia are formed in this zone. The interaction between the mycobiont and cyanobiont is more intense in the medial zone. However, the establishment of the contact with cyanobacteria in this zone less probable. The spatial distribution of lectins in the thallus was determined. To reveal the differences in photosynthetic activity in three thallus zones, transient analysis of chlorophyll a fluorescence and the assessment of non-photochemical quenching of excited chlorophyll states were performed. Assimilation of absorbed light energy was more effective in the medial zone. The basal zone was characterized by decreased photosynthetic activity, lichen dissociation and thallus death.

Lichen acclimation to changing environments: Photobiont switching vs. climate-specific uniqueness in Psora decipiens

Unraveling the complex relationship between lichen fungal and algal partners has been crucial in understanding lichen dispersal capacity, evolutionary processes, and responses in the face of environmental change. However, lichen symbiosis remains enigmatic, including the ability of a single fungal partner to associate with various algal partners. Psora decipiens is a characteristic lichen of biological soil crusts (BSCs), across semi-arid, temperate, and alpine biomes, which are particularly susceptible to habitat loss and climate change. The high levels of morphological variation found across the range of Psora decipiens may contribute to its ability to withstand environmental change. To investigate Psora decipiens acclimation potential, individuals were transplanted between four climatically distinct sites across a European latitudinal gradient for 2 years. The effect of treatment was investigated through a morphological examination using light and SEM microscopy; 26S rDNA and rbcL gene analysis assessed site-specific relationships and lichen acclimation through photobiont switching. Initial analysis revealed that many samples had lost their algal layers. Although new growth was often determined, the algae were frequently found to have died without evidence of a new photobiont being incorporated into the thallus. Mycobiont analysis investigated diversity and determined that new growth was a part of the transplant, thus, revealing that four distinct fungal clades, closely linked to site, exist. Additionally, P. decipiens was found to associate with the green algal genus Myrmecia, with only two genetically distinct clades between the four sites. Our investigation has suggested that P. decipiens cannot acclimate to the substantial climatic variability across its environmental range. Additionally, the different geographical areas are home to genetically distinct and unique populations. The variation found within the genotypic and morpho-physiological traits of P. decipiens appears to have a climatic determinant, but this is not always reflected by the algal partner. Although photobiont switching occurs on an evolutionary scale, there is little evidence to suggest an active environmentally induced response. These results suggest that this species, and therefore, other lichen species, and BSC ecosystems themselves may be significantly vulnerable to climate change and habitat loss.

Emissions of carotenoid cleavage products upon heat shock and mechanical wounding from a foliose lichen

Carotenoids constitute a major target of chloroplastic photooxidative reactions, leading to the formation of several oxidized derivatives and cleavage products, some of which are volatile (VCCPs). Among them, β-cyclocitral (β-CC), at least, is a retrograde signaling molecule that modulates the activity of many key physiological processes. In the present work, we aimed to study whether β-CC and other VCCPs are released into the atmosphere from photosynthetic tissues. To overcome stomatal limitations, the foliose chlorolichen Lobaria pulmonaria was used as the model system, and the emissions of biogenic volatiles, induced by heat and wounding stresses, were monitored by proton-transfer reaction time-of-flight mass-spectrometry (PTR-TOF-MS) and gas-chromatography (GC-MS). Prior to stress treatments, VCCPs were emitted constitutively, accounting for 1.3 % of the total volatile release, with β-CC being the most abundant VCCP. Heat and wounding stresses induced a burst of volatile release, including VCCPs, and a loss of carotenoids. Under heat stress, the production of β-CC correlated positively with temperature. However the enhancement of production of VCCPs was the lowest among all the groups of volatiles analyzed. Given that the rates of carotenoid loss were three orders of magnitude higher than the release rates of VCCPs and that these compounds only represent a minor fraction in the blend of volatiles, it seems unlikely that VCCPs might represent a global stress signal capable of diffusing through the atmosphere to different neighboring individuals.

Desiccation tolerance and lichenization: a case study with the aeroterrestrial microalga Trebouxia sp. (Chlorophyta)

A comparative study of isolated vs. lichenized Trebouxia sp. showed that lichenization does not influence the survival capability of the alga to the photo-oxidative stress derived from prolonged desiccation. Coccoid algae in the Trebouxia genus are the most common photobionts of chlorolichens but are only sporadically found in soil or bark outside of a lichen. They all appear to be desiccation tolerant, i.e. they can survive drying to water contents of below 10%. However, little is known about their longevity in the dry state and to which extent lichenization can influence it. Here, we studied the longevity in the dry state of the lichenized alga (LT) Trebouxia sp. in the lichen Parmotrema perlatum, in comparison with axenically grown cultures (CT) isolated from the same lichen. We report on chlorophyll fluorescence emission and reactive oxygen species (ROS) production before desiccation, after 15-45 days in the dry state under different combinations of light and air humidity and after recovery for 1 or 3 days in fully hydrated conditions. Both the CT and the LT were able to withstand desiccation under high light (120 µmol photons m(-2) s(-1) for 14 h per day), but upon recovery after 45 days in the dry state the performance of the CT was better than that of the LT. By contrast, the quenching of excess light energy was more efficient in the LT, at high relative humidities especially. ROS production in the LT was influenced mostly by light exposure, whereas the CT showed an oxidative burst independent of the light conditions. Although lichenization provides benefits that are essential for the survival of the photobiont in high-light habitats, Trebouxia sp. can withstand protracted periods of photo-oxidative stress even outside of a lichen thallus.

Seasonal partitioning of growth into biomass and area expansion in a cephalolichen and a cyanolichen of the old forest genus Lobaria

Growth in two old forest lichens was studied to evaluate how temporal (seasonal) and spatial (aspect-wise) partitioning of biomass and area growth respond to seasonal changes in light and climate. We monitored relative growth rates during annual courses in the cephalolichen Lobaria pulmonaria and the cyanolichen Lobaria scrobiculata transplanted in boreal clear-cut to five fixed aspects in winter, spring, summer, and autumn. For each annual set, growth was quantified in January-March, April-June, July-September and October-December. Mean biomass and area increased in all seasons, but growth was highest in July-September. Mass growth did not follow area increment during a year. As a result, mass per area (specific thallus mass (STM)) declined (L. scrobiculata) or stayed constant (L. pulmonaria) in the dark, humid October-December season, whereas it strongly increased in the dry, sunny April-June season. Aspect influenced growth in species-specific ways. Seasonality in biomass growth mainly followed light availability, whereas area growth was strongest during humid seasons. The substantial STM changes across seasons, species, and aspects can be explained as passive responses to seasonal climate. However, as STM, according to the literature, is a driver of water storage, recorded changes probably improve fitness by prolonging hydration in places or during times with high evaporative demands.

Unravelling the roles of desiccation-induced xanthophyll cycle activity in darkness: a case study in Lobaria pulmonaria

Desiccation-tolerance ability in photosynthetic organisms is largely based on a battery of photoprotective mechanisms. Xanthophyll cycle operation induced by desiccation in the absence of light has been previously proven in the desiccation-tolerant fern Ceterach officinarum. To understand the physiological function of xanthophyll cycle induction in darkness and its implication in the desiccation tolerance in more detail, we studied its triggering factors and its photochemical effects in the lichen Lobaria pulmonaria. We found that both the drying rate and the degree of desiccation play a crucial role in the violaxanthin de-epoxidase activation. De-epoxidation of violaxanthin to zeaxanthin (Z) occurs when the tissue has lost most of its water and only after slow dehydration, suggesting that a minimum period of time is required for the enzyme activity induction. Fluorescence analysis showed that Z, synthesised during tissue dehydration in the absence of light, prevents photoinhibition when rewatered tissues are illuminated. This is probably due to Z implication in both non-photochemical quenching and/or antioxidative responses.

Annual variation in photo acclimation and photoprotection of the photobiont in the foliose lichen Xanthoria parietina

Seasonal variation in maximal photochemical quantum yield (F(V)/F(M)) of photosystem II (PS II), light adapted quantum yield (Phi(II)) of PS II, non-photochemical quenching (NPQ), contents of chlorophylls, and xanthophyll cycle pigments (VAZ) was studied in Xanthoria parietina repeatedly sampled in one location in S Norway during one year. The seasonal course in the susceptibility to photoinhibition was evaluated as high light-induced changes (1,800 micromol photons m(-2) s(-1) for 24h) in F(V)/F(M), Phi(II), and NPQ, measured as the ability to recover after 2 and 20 h at low light in control thalli with a natural cortical parietin screen, and in thalli from which parietin had been removed prior to high light exposures. F(V)/F(M), Phi(II), chlorophyll content, and the conversion state of VAZ (DEPS) reached minimum in spring. At the same time, yearly maxima of VAZ content and NPQ were recorded. Thereafter, F(V)/F(M), Phi(II), and chlorophyll content increased gradually, reaching maximum values in late autumn. DEPS peaked already in summer. Similarly, VAZ and NPQ decreased from early summer until winter. All data show that the X. parietina photobiont acclimates to seasonal changes in solar radiation, consistent with the lichen's preference for well-lit habitats. However, a comparison with a study of seasonal acclimation in the X. parietina mycobiont shows that in order to understand the seasonal photobiont acclimation, one has to consider the seasonal variation in internal screening caused by the fungal regulation of the PAR-absorbing parietin. A joint effort of both bionts seems to be required to avoid serious photoinhibition.

Growth and ecophysiological acclimation of the foliose lichen Lobaria pulmonaria in forests with contrasting light climates

This study aims to assess biomass and area growth of 600 thalli of the old forest lichen, Lobaria pulmonaria, transplanted to three successional boreal forest stands with (1) natural rainfall regime, (2) additional moistening during dry days, and (3) additional moistening with added nutrients. Mean biomass growth during 100 days varied from 8.3% in the dark young spruce forest to 23.1% in the clear-cut area, with the old forest in between (16.0%). Additional moistening did not enhance lichen growth, probably because the transplantation period was wet. Nutrient additions slightly increased area growth compared to artificial water additions only. Growth was determined by a combination of external (forest stand, site factors) and internal factors (chlorophyll content, biomass per area). Transplants acclimated to high light by increasing thickness and chlorophyll a/b-ratio. Some visible bleaching and a strong positive correlation between chlorophyll content per area and lichen growth in clear-cuts suggest some high light-induced chlorophyll degradation. We believe that biomass growth and natural occurrence of L. pulmonaria is controlled by a delicate balance between light availability and desiccation risk, and that the species is confined to old forests due to a physiological trade-off between growth potential and fatal desiccation damage, both of which increase with increasing light. The discrepancy between potential and realized ecological niches is probably caused by a long-term risk to be killed in open habitats by high light during long periods with no rain.

High-light stress and photoprotection in Umbilicaria antarctica monitored by chlorophyll fluorescence imaging and changes in zeaxanthin and glutathione

The effect of high light on spatial distribution of chlorophyll (Chl) fluorescence parameters over a lichen thallus (Umbilicaria antarctica) was investigated by imaging of Chl fluorescence parameters before and after exposure to high light (1500 micro mol m (-2) s (-1), 30 min at 5 degrees C). False colour images of F (V)/F (M) and Phi (II) distribution, taken over thallus with 0.1 mm (2) resolution, showed that maximum F (V)/F (M) and Phi (II) values were located close to the thallus centre. Minimum values were typical for thallus margins. After exposure to high light, a differential response of F (V)/F (M) and Phi (II) was found. The marginal thallus part exhibited a loss of photosynthetic activity, manifested as a lack of Chl fluorescence signal, and close-to-centre parts showed a different extent of F (V)/F (M) and Phi (II) decrease. Subsequent recovery in the dark led to a gradual return of F (V)/F (M) and Phi (II) to their initial values. Fast (30 min) and slow (1 - 22 h) phase of recovery were distinguished, suggesting a sufficient capacity of photoprotective mechanisms in U. antarctica to cope with low-temperature photoinhibition. Glutathione and xanthophyll cycle pigments were analyzed by HPLC. High light led to an increase in oxidized glutathione (GSSG), and a conversion of violaxanthin to zeaxanthin, expressed as their de-epoxidation state (DEPS). The responses of GSSG and DEPS were reversible during subsequent recovery in the dark. GSSG and DEPS were highly correlated to non-photochemical quenching (NPQ), indicating involvement of these antioxidants in the resistance of U. antarctica to high-light stress. Heterogeneity of Chl fluorescence parameters over the thallus and differential response to high light are discussed in relation to thallus anatomy and intrathalline distribution of the symbiotic alga Trebouxia sp.