Do secondary substances in the thallus of a lichen promote CO2 diffusion and prevent depression of net photosynthesis at high water content?

Many lichens show seriously depressed net photosynthesis (NP) at high thallus water contents due to increased carbon dioxide diffusion resistance through blockage of diffusion pathways by water. The soil lichen Diploschistes muscorum, however, shows no depression and NP is close to maximal even at the highest thallus water content. We investigated whether lichen substances (lecanoric and diploschistesic acids) in the cortex and medulla contributed to this ability to maintain high NP. Dry thalli were extracted with water-free acetone and, after this treatment, were found to be fully viable to the extent of continued growth after replanting in the field. No differences were found in the response of NP to thallus water content between the normal and extracted thalli, in fact the response curves were often nearly identical. Thus, in this species it seems that lichen substances did not maintain the water-free diffusion pathways and some other explanation, possibly structural, needs to be sought. Received: 5 April 1997 / Accepted: 26 April 1997     

Influence of sun irradiance and water availability on lichen photosynthetic pigments during a Mediterranean summer

This study was carried out to investigate changes in lichen photosynthetic pigments induced by different combinations of light irradiance and water availability during a Mediterranean summer. To this purpose, thalli of three epiphytic lichens with a markedly different ecology concerning photo-hygrophytism, namely Evernia prunastri (hygro-mesophytic), Flavoparmelia caperata (mesophytic) and Xanthoria parietina (xero-mesophytic), were transplanted for 30 days to N- and S-facing cardinal exposures in central Italy. To investigate the effect of thallus hydration, at each cardinal exposure 50% of thalli were hydrated daily with deionised water. The results showed that changes in the concentrations of photosynthetic pigments are species-specific and consist in a general depression of photosynthetic pigments only in the hygro-mesophytic species E. prunastri. The pattern of photosynthetic pigments was also investigated in spontaneous samples along contrasting aspects. In this case, X. parietina from S-facing slopes, adapted to direct solar influx, showed higher pigment content than N-facing thalli; F. caperata and E. prunastri avoid direct extreme solar radiation and assimilation pigments were influenced more by the shadowing of tree canopies than by the cardinal exposure. The influence of drought on lichen photosynthetic pigments in the Mediterranean area is discussed.     

Further reports on the brown Parmeliaceae of southern Africa

Three previously unknown members of the lichen genus Neofuscelia Essl. are described: N. esterhuyseniae Essl., N. nonreagens Essl., and N. pseudoloriloba Essl. New South African records for other members of the genus are given as well, and N. subimitatrix (Essl.) Essl. is reported from New Zealand for the first time. Apothecia of the recently described South African genus Almbornia Essl. have been observed for the first time and they are typically parmeliaceous.     

Surface hydrophobicity causes SO2 tolerance in lichens

BACKGROUND AND AIMS: The superhydrophobicity of the thallus surface in one of the most SO(2)-tolerant lichen species, Lecanora conizaeoides, suggests that surface hydrophobicity could be a general feature of lichen symbioses controlling their tolerance to SO(2). The study described here tests this hypothesis. METHODS: Water droplets of the size of a raindrop were placed on the surface of air-dry thalli in 50 lichen species of known SO(2) tolerance and contact angles were measured to quantify hydrophobicity. KEY RESULTS: The wettability of lichen thalli ranges from strongly hydrophobic to strongly hydrophilic. SO(2) tolerance of the studied lichen species increased with increasing hydrophobicity of the thallus surface. Extraction of extracellular lichen secondary metabolites with acetone reduced, but did not abolish the hydrophobicity of lichen thalli. CONCLUSIONS: Surface hydrophobicity is the main factor controlling SO(2) tolerance in lichens. It presumably originally evolved as an adaptation to wet habitats preventing the depression of net photosynthesis due to supersaturation of the thallus with water. Hydrophilicity of lichen thalli is an adaptation to dry or humid, but not directly rain-exposed habitats. The crucial role of surface hydrophobicity in SO(2) also explains why many markedly SO(2)-tolerant species are additionally tolerant to other (chemically unrelated) toxic substances including heavy metals.     

Water Storage in the Lichen Family Umbilicariaceae

Quantitative relationships between thallus structure and water storage and retention capacities In 12 species of the lichen family Umbilicariaceae were explored using three recent techniques for plant structure analysis: stereology (3D quantification of microscopic Images), mercury Intrusion porosimetry (determination of pore size distribution of tissues) and low-temperature scanning electron microscopy (LTSEM). Water storage capacity of the thallus was related neither to thallus density nor surface area of the thallus; It was directly related to the total porosity of the thallus and inversely related to the proportion of thallus volume occupied by cell walls and gelatinous substances. Water retention capacity increased with increasing thallus density and was decreased by slight increases in the surface area of the upper side of the thallus. Water storage and retention capacities exhibited a positive correlation only when the storage capacity was expressed on a surface area basis. LTSEM study of fully hydrated specimens revealed that many intercellular spaces of the upper cortex and upper parts of the algal layer contained liquid water. Intercellular spaces of the lower part of the algal layer and medulla were in general either airfilled or partially occupied by gelatinous substances. Species with rhizinomorphs and substrate-hygrophytic (water uptake from surface run-offs) stored more water and retained it longer than aerohygrophytic species (water uptake from the atmosphere) lacking rhizinomorphs. Thallus structure of aerohygrophytic species seems to facilitate rapid gas exchange with the environment, improving water uptake and carbon gain when atmospheric moisture is available but accelerating dehydration when the atmosphere becomes dry.     

Exploring Phenotypic Plasticity in the LichenRamalina capitata: Morphology, Water Relations and Chlorophyll Content in North- and South-facing Populations

The present work analyses the morphology, anatomy, water relationsand chlorophyll content of thalli of the lichenRamalina capitatavar.protectafromtwo different populations exposed to contrasting microclimaticconditions due to differences in the orientation of the rocksurface. The population on the north-facing rock surface (NFS)was exposed to lower photosynthetic photon flux densities (PPFD),remained at high relative humidities for longer periods of timeand was exposed to lower temperatures than the population onthe south-facing surface (SFS). We proposed the hypothesis thatthe shadier the habitat the greater the ecological advantagefor enhanced light harvesting. Thalli from the SFS had shorterand wider lacinia, thicker thalli, mostly due to increased medullathickness, a higher water-retention capacity, a higher percentageof thallus volume occupied by the algal cells and a higher chlorophyllcontent than thalli from NFS. The phenotypic plastic responseof the traits studied inR. capitatavar.protectawas not directlyrelated to differences in the light availability, at least forthe range of PPFD experienced by the two populations studied,since the population exposed to higher PPFD exhibited largeramounts of light harvesting pigments. Both populations exhibitedthe same intrathalline distribution of algal cells and chlorophylls,which were more abundant in the apical than in the basal zonesof all thalli studied. Periods of water-induced metabolic activitywere shorter in the SFS than in the NFS, and structural andchlorophyll data indicated that thalli from the SFS were betterprepared for the photosynthetic exploitation of these brieferperiods and for maintaining thallus hydration into dry periods.These results suggest that differences in selective pressurebetween the two populations ofR. capitatavar.protectastudiedinvolved maximization of the photosynthetic exploitation ofthe periods of metabolic activity when they are brief, as hasbeen described for certain vascular plants from xeric environments. Ramalina capitatavar.protecta; algal cells; chlorophylls; water relations; microclimate; morphology; intrathalline variation; lichen; phenotypic plasticity     

Seasonal Trends in Nitrogen Status of Antarctic Lichens

The thallus nitrogen (N) concentration of two dominant macrolichensof continental Antarctica (Usnea sphacelata and Umbilicariadecussata) was estimated each month for 1 year on a low roundedknoll on Clark Peninsula, Windmill Islands, Wilkes Land, Antarctica.Thallus N was significantly higher in Umbilicaria decussatathan in Usnea sphacelata and varied according to site. Duringthe winter months, when the lichens were metabolically inactive,thalli gradually accumulated N. At the onset of warmer conditions,thallus N content fell dramatically in both species, with thetiming of the decline being related to microclimatic conditions.The strongly seasonal pattern of metabolic activity in thesespecies is reflected in their nitrogen relations. Copyright2000 Annals of Botany Company Lichen, nitrogen, Antarctica, season, Usnea sphacelata, Umbilicaria decussata     

Morphological and Physiological differences between Epilithic and Epiphytic populations of the Lichen Parmelia pastillifera

CO₂ exchanges, morphology and chlorophyll content of epiphyticand epilithic populations of Parmelia pastillifera growing inthe same locality (Bristol, UK) were investigated. Epilithicsamples had reduced net photosynthetic rates and a lower chlorophyllcontent, apparently caused by the high levels of irradianceof the exposed habitat, not completely buffered by the increasedthickness of the upper cortex. These samples also had much moredeveloped rhizinae; this modification is interpreted as a mechanismto increase the water storage capacity of the thallus. Physiologicaldifferences between the two populations seem to be due to themore extreme environmental conditions of the epilithic population.Copyright1995, 1999 Academic Press Chlorophyll content, CO₂-exchange rates, habitat, lichen, morphology, Parmelia pastillifera     

Natural and Experimentally-induced Zinc and Copper Resistance in the Lichen Genus Peltigera

Using a tolerance index based on metal-induced reductions inphotosynthesis, members of the lichen genus Peltigera, sampledfrom a range of sites with contrasting metal status, were demonstratedto possess Zn tolerance directly proportional to the Zn contentof the thallus. Tolerance was not related to the concentrationof other cations present in the thallus or weight of thallusper unit surface area. Although differences in Cu tolerancewere found, these were not related to the Cu content of thethallus but rather to the Zn content. In the laboratory, pretreatmentof P. membranacea with low concentrations of Zn significantlydecreased the inhibition of photosynthesis caused when subsequentlysupplied with high levels of Zn compared to water pretreatedplants. Increased resistance to Cd but not Cu was also found,although it was not possible to increase either Cu or Zn resistanceby pretreatment with Cu. Peltigera membranacea, lichen, heavy metal tolerance, zinc, copper, cadmium     

Juvenile Development and Diaspore Survival in the Threatened Epiphytic Lichen Species Sticta fuliginosa, Leptogium saturninum and Menegazzia terebrata: Conclusions for in situ Conservation

Abstract: Development and growth of the three threatened epiphytic lichen species Sticta fuliginosa (Hoffm.) Ach., Leptogium saturninum (Dicks.) Nyl. and Menegazzia terebrata (Hoffm.) Massal. was investigated by low temperature scanning electron microscopy and macro‐photography. Small cotton gauze discs acting as artificial substrata were fixed with aluminium staples on the bark of selected trees and vegetative diaspores (isidia or soredia) were transferred onto these discs. The subsequent development into small thalli of up to 3 mm length was observed within the 32‐month study period. All three species produced anchoring hyphae within the first month after transplantation. Two months later 52 % of the S. fuliginosa diaspores were still on the gauze discs and after 16 months 29 % remained attached. For L. saturninum, the corresponding percentages were 46 % and 19 %, respectively. First lobes resembling adult thalli were observed after 8 to 12 months in S. fuliginosa and L. saturninum but only after 16 months in M. terebrata. All three species developed usually more than one thallus primordium (pseudomeristematic growth zone) per isidium or soredial cluster. Transplanted thallus fragments were able to fix themselves on the new substratum but in all three species large parts degenerated and fell off during the first year, particularly in S. fuliginosa. The results show that the juvenile development of the investigated species is not restricted by microclimatic factors at the study site. We therefore conclude that the juvenile development is not the restricting factor in regard to growth and population survival. Other factors, such as the competition with bryophytes, insufficient diaspore dispersal or forest management practice must account for the small population sizes. The described transplantation technique of vegetative diaspores has proved to be very useful for the augmentation of small populations without damaging the existing thalli and we suggest use of this method for in situ conservation of endangered lichen species.     

The Effect of Phosphate Buffer on the Physiology of the Lichen Evernia prunastri

The physiological consequences of incubating either fresh ordesiccated thalli of Evernia prunastri in phosphate buffer orwater, in the presence or absence of added urea, was investigated.Phosphate buffer, with or without added urea, induced an immediateand sustained inhibition of photosynthesis. This was enhancedby prior desiccation. Urea in water also caused a reductionin photosynthesis but had little effect on respiration, whichwas initially enhanced by phosphate buffer but subsequentlydeclined. Release of intracellular K indicated a slower butsubstantial loss of membrane integrity in the presence of phosphatebuffer or, to a lesser extent, urea. Intracellular Na concentrationsrose initially on incubation in sodium phosphate buffer andthen declined, implying the occurrence of membrane damage. Urea-inducedurease activity was sustained in the presence of dithiothreitolwhen expressed on a unit protein basis. However, a decline wasobserved when results were calculated on a thallus dry weightbasis. The previously reported loss of urease activity on prolongedincubation in phosphate buffer is now suggested to be a consequenceof general buffer-induced damage rather than a specific urea-inducedsynthesis of inhibitory phenolic compounds. Evernia prunastri, cation location, lichen phenols, phosphate buffer, photosynthesis, respiration, urease activity     

Light screening in lichen cortices can be quantified by chlorophyll fluorescence techniques for both reflecting and absorbing pigments

Lichens, representing mutualistic symbioses between photobionts and mycobionts, often accumulate high concentrations of secondary compounds synthesized by the fungal partner. Light screening is one function for cortical compounds being deposited as crystals outside fungal hyphae. These compounds can non-destructively be extracted by 100% acetone from air-dry living thalli. Extraction of atranorin from Physcia aipolia changed the lichen colour from pale grey to green in the hydrated state, whereas acetone-rinsed and control thalli were all pale grey when dry. Removal of parietin from Xanthoria parietina changed the colour of desiccated thalli from orange to grey. Colour changes were quantified by reflectance measurements. By a new chlorophyll fluorescence method, screening was assessed as the decrease in incident irradiance (PAR) necessary to reach identical effective quantum yields of PSII (Φ_PSII) in acetone-rinsed and control thalli. Thereby, we estimated a screening efficiency due to cortical atranorin crystals at 61, 38, and 40% of blue, green and red light, respectively, whereas parietin screened 81, 27 and 1% of these wavelength ranges. Removal of atranorin caused similar levels of increased photoinhibition for P. aipolia in blue, green and red light, whereas parietin-deficient thalli of X. parietina exhibited increased photoinhibition with decreasing wavelengths. Atranorin possibly prevents water from entering the spaces between the hyphae in the cortex. The air-filled cavities with white atranorin crystals reflect excess light, whereas the yellow compound parietin absorbs excess light. Thereby, both atranorin and parietin play significant photoprotective roles for symbiotic green algae, but with compound-specific screening mechanisms.     

A Fourier Transform-Raman Spectroscopic Study of Lichen Strategies on Granite Monuments

The biodeterioration of granite by Lecidea fuscoatra (L.) Ach., Porpidia cinereoatra (Ach.) Hertel & Knoph, and P. macrocarpa (DC.) Hertel & Schwab growing in the same environmental conditions has been studied by using Fourier transform-Raman spectroscopy. Results were significantly different for the three species, with P. cinereoatra being the most active biodeteriorative lichen. This lichen was also the only one in which calcium oxalate and gypsum were identified spectroscopically. Physical disturbances to the substratum were evidenced in all lichens studied by the incoporation of material, such as quartz and feldspar, into their thalli, but this phenomenon varied considerably from species to species. The results indicate that lichen species can adopt different strategies and can have different biodeteriorative effects on granite, independently of environmental conditions and substrate.     

CHANGES IN WATER ECONOMY IN RELATION TO ANATOMICAL AND MORPHOLOGICAL CHARACTERISTICS DURING THALLUS DEVELOPMENT INPARMELIA ACETABULUM

The influence on uptake and water loss of the structural changes experienced by Parmelia acetabulum during thallus development were investigated. Small specimens were characterized by flat lobes and a thin thallus and cortex. Large specimens appeared strongly rugose, imbricate and irregularly folded, and had a significantly thicker cortex and medulla than small thalli. Maximum water storage capacity did not differ between large and small thalli, although water retention was much higher in large thalli, presumably due to the interaction of structural characteristics and a higher boundary layer resistance. This translated into a longer duration of the period of thallus hydration in large thalli compared to small thalli. Incubation of thalli in water-vapour-saturated atmospheres induced full recovery of photosynthetic electron transport to the values before desiccation in small thalli but only induced a partial recovery in large thalli. The close correlation between photosynthetic electron transport and net photosynthesis during desiccation found in this species suggested that carbon-fixation activity could be regained to a larger extent by incubation of thalli in water vapour in small compared to large thalli. The higher ability for water vapour uptake of small thalli might allow them to efficiently use small amounts of intermittently available water or periods of high relative humidity. The significance of this differential ability to utilize water is discussed with regard to the known ecological preferences of the species.     

Secondary chemistry of cultured mycobionts: formation of a complete chemosyndrome by the lichen fungus of Lobaria spathulata

Abstract:The study aimed to optimize culture conditions and nutrient requirements for the production of secondary metabolites by the cultured mycobiont Lobaria spathulata. This species proved to be an excellent model system for such studies, as the complete chemosyndrome found in the natural lichen thallus was repeatedly formed in the cultured mycobiont with differentiated, aerial mycelia. Nutrient media containing the disaccharide, sucrose, were found to favour both rapid growth and the production of typical lichen substances. Higher proportions of the secondary compounds were detected in the developing mycobiont than in mature lichen thalli.     

The occurrence of biomineralization products in four lichen species growing on sandstone in western Norway

High performance thin-layer chromatography/thin-layer chromatography, X-ray diffraction, and scanning electron microscopy analysis of thallus and lichen-rock interface samples, were undertaken to characterize biomineralization products in Fuscidea cyathoides, Ochrolechia tartarea, Ophioparma ventosa, and Pertusaria corallina, growing on sandstone in western Norway. Whewellite (monohydrate form of Ca oxalate) was found in the thallus of all species, but not in any of the weathering rinds beneath the species. A significantly higher amount of whewellite was detected in the thalli of F. cyathoides and O. ventosa than in the other two species. There were only a few differences in whewellite occurrence between the thallus edge and centre samples in the four species. HPTLC/TLC and SEM analysis indicate that lichen compounds occur within the rock beneath some of the lichen specimens. Only divaricatic acid was observed within the weathering rind beneath O. ventosa. No lichen substances were found in the weathering rind beneath F. cyathoides and P. corallina, whereas gyrophoric and lecanoric acids were found in the weathering rind beneath O. tartarea.     

Parietin in the tolerant lichen Xanthoria parietina (L.) Th. Fr. increases protection of Trebouxia photobionts from cadmium excess

Secondary metabolites of lichens can be involved in production of chelates with heavy metals. We hypothesized that parietin plays important role in protection of photobiont cells in Xanthoria parietina from an excess of cadmium ions. Two types of X. parietina lichen thalli, natural with presence of secondary metabolite parietin (p+) as well as without parietin (p−) were exposed to different doses of cadmium (up to 300 μmol g⁻¹ dw). Based on determination of the total and intracellular Cd-accumulation, ergosterol and thiobarbituric acid reactive substances (TBARS) content did not show statistically significant differences in the response of both types of thalli (p+ and p−). However, a stronger toxic effect of the highest Cd-dose on photosynthetic pigment content and chlorophyll a fluorescence was observed in the parietin-depleted thalli. The protective role of parietin against Cd excess was better supported and concluded from the differences observed in the production of non-protein thiol compounds (cysteine, glutathione and phytochelatins) involved in Cd detoxification. In the p+ thalli Cys content was stable but GSH content slightly decreased in the studied Cd range, while in the p− thalli these compounds were completely absent at high Cd doses. At Cd doses higher than 37.5 μmol Cd g⁻¹ dw, toxic to both types of X. parietina thalli, Cys and GSH contents were significantly higher in p+ than in p− thalli. Also, the photobiont partner in the p+ thalli was better protected of the metal exposition, and able to produce phytochelatins (PCs) over the whole range of metal, while in the p− thalli the production was completely inhibited at 75 μmol Cd g⁻¹ dw and higher concentrations, together with the inhibition of cysteine (Cys) and reduced glutathione (GSH) production. The obtained results indicate that the parietin layer is a natural barrier decreasing Cd access to algal cells in X. parietina. Comparison of PCs production appeared to be the most sensitive marker for estimation of Cd availability to photobiont in the symbiotic system.     

DIATOMS LIVING INSIDE THE THALLUS OF THE GREEN ALGAL LICHEN COENOGONIUM LINKII IN NEOTROPICAL LOWLAND RAIN FORESTS1

Coenogonium linkii Ehrenb. is a very common filamentous lichen, growing on stems, hanging roots, and lianas in the understory of neotropical lowland rain forests. We investigated several thalli of this species from locations in Panama and French Guiana. All thalli were inhabited by various species of terrestrial diatoms, which were found between the thallus filaments on extracellular material of the mycobiont. We identified 18 species of diatoms belonging to nine genera: Diadesmis, Eunotia, Hantzschia, Luticola, Melosira, Nitzschia, Orthoseira, Pinnularia, and Stauroneis. The potential benefits both diatoms and lichens could derive from symbiosis in relation to water, irradiance, and nitrogen availability are discussed.     

Lichenochemical Screening and Antioxidant Capacity of Four Tunisian Lichen Species

The phenolic composition and antioxidant capacity of four Tunisian lichen species, Cladonia rangiformis, Flavoparmelia caperata, Squamarina cartilaginea and Xanthoria parietina, were determined in order to provide a better understanding of their lichenochemical composition. Powdered material of F. caperata was the richest in total phenolic content (956.68 μg GAE g⁻¹ DW) and S. cartilaginea in proanthocyanidin content (77.31 μg CE g⁻¹ DW), while the acetone extract of X. parietina showed the highest flavonoid content (9.56 μg CE g⁻¹ DW). The antioxidant capacity of all lichen extracts and crude material was evaluated by DPPH^. scavenging, iron-chelating, and iron-reducing powers. Results showed that methanol extracts of S. cartilaginea had the highest DPPH^. antioxidant capacity (IC₅₀=0.9 μg mL⁻¹) and the highest iron-reducing power was attributed to the acetone extract of this species. All extracts of all species were further screened by Fourier-transform infrared spectroscopy (FT-IR) and nuclear resonance spectroscopy (NMR); results showed an abundance of phenols, aromatic compounds, and fatty acids. Overall, our results showed that the investigated species are a rich source of potentially bioactive compounds with valuable properties.     

Texture and Hygroscopic Features of the Upper Surface of the Thallus in the Lichen Family Umbilicariaceae

The texture of the upper surface of the thallus and its relationshipto certain aspects of water relations were studied in 20 lichentaxa of the family Umbilicariaceae. The anatomy and thicknessof the upper cortex were studied by light microscopy of transversesections and the texture of the upper surface of the thalluswas observed by scanning electron microscopy. The hygroscopicfeatures of the upper surface of the thallus were measured with20 µl drops of a water-soluble ink. Both the maximum surfacearea covered by the absorbed drop and the time elapsed untilits complete absorption were recorded. The degree of wettabilitywas closely related to surface texture but not to cortical thickness.The lichens which possessed the most hygroscopic upper surface(in some cases more so than the filter paper) were those specieswhich usually obtain water as vapour from the air, whereas thosespecies dependent on the substrate for their water had a relativelyimpermeable upper surface.Copyright 1994, 1999 Academic Press Umbilicariaceae, upper surface texture, hygroscopic features, water relations