Epilithic Lichens Research Articles

Will climate warming exceed lethal photosynthetic temperature thresholds of lichens in a southern African arid region?

AbstractPredicted elevated temperatures and a shift from a winter to summer rainfall pattern associated with global warming could result in the exposure of hydrated lichens during summer to more numerous temperature extremes that exceed their thermal thresholds. This hypothesis was tested by measuring lethal temperature thresholds under laboratory and natural conditions for four epilithic lichen species (Xanthoparmelia austro‐africana, X. hyporhytida, Xanthoparmelia sp., Xanthomaculina hottentotta) occurring on quartz gravel substrates at a hot arid inland site two epigeous lichen species (Teloschistes capensis, Ramalina sp.) occurring on gypsum‐rich topsoil at a warm humid coastal site. Extrapolated lethal temperatures for photosynthetic quantum yield under laboratory conditions were up to 4°C higher for lichens from a dry inland site than those from a humid coastal site. Lethal temperatures extrapolated for photosynthetic quantum yield at a saturating photosynthetic photon flux density of ≥11,000 μmol photons m−2 s−1 under natural conditions were up to 6°C higher for lichens from the dry inland site than the more humid coastal site. It is concluded that only under atypical conditions of lichen exposure in a hydrated state to temperature extremes at high midday solar irradiances during summer could lethal photosynthetic thresholds in sensitive lichen species be potentially exceeded, but whether the increased frequency of such conditions with climate warming would lead to increased likelihood of lichen mortality is debatable.

Weathering and particle entrapment at the rock–lichen interface in Italian volcanic environments

The present work investigated the biogeophysical and biogeochemical weathering associated to the growth of epilithic lichens on recent lava flows from Mt. Etna (Sicily) and Mt. Vesuvius (Campania). The chosen lichen species were the crustose Lecidea fuscoatra, the foliose Xanthoparmelia conspersa and the fruticose Stereocaulon vesuvianum. An integrated approach based on the study of both disturbed and undisturbed samples of lichenized rock was applied in order to allow for the complexity of the rock–lichen interface environment in terms of micromorphological, mineralogical and chemical properties. To this aim, a number of rock blocks colonised by intact thalli of different lichen species were collected in order to have at least 10 thalli for each species in each environment. XRD and XRF analyses coupled to microscopic (OM), submicroscopic (SEM) and microanalytical (EDS) observations were the used techniques. In both study environments, the chemical, mineralogical and micromorphological properties of the incoherent materials found at the rock–lichen interface suggested that they mostly consist of rock fragments eroded from the surroundings and accumulated in cavities and fissures of the rough lava flows. According to the thallus morphology, the lichens colonising the lava preserve the interface materials from further wind and water erosion, provide such materials with organic matter and moisture, and entrap allochthonous quartz, sea salts, iron oxides and clay minerals. Calcium oxalate production by L. fuscoatra and X. conspersa, Al enrichment around S. vesuvianum hyphae and iron accumulation at the interface constitute evidence of lichen interaction with the underlying sediments. Larger differences between rock and interface materials occur at Vesuvius vs Etna site. In line with the young age of the lavas, recent lichen colonisation only results in a physical reorganisation and slight chemical modification of the materials accumulated at the rock–lichen interface, whose abundance should not be considered an indicator of bioweathering. In volcanic environments, the ability of lichens to retain a considerable amount of unconsolidated material.

Antarctic epilithic lichens as niches for black meristematic fungi.

Sixteen epilithic lichen samples (13 species), collected from seven locations in Northern and Southern Victoria Land in Antarctica, were investigated for the presence of black fungi. Thirteen fungal strains isolated were studied by both morphological and molecular methods. Nuclear ribosomal 18S gene sequences were used together with the most similar published and unpublished sequences of fungi from other sources, to reconstruct an ML tree. Most of the studied fungi could be grouped together with described or still unnamed rock-inhabiting species in lichen dominated Antarctic cryptoendolithic communities. At the edge of life, epilithic lichens withdraw inside the airspaces of rocks to find conditions still compatible with life; this study provides evidence, for the first time, that the same microbes associated to epilithic thalli also have the same fate and chose endolithic life. These results support the concept of lichens being complex symbiotic systems, which offer attractive and sheltered habitats for other microbes.

TWENTY-THREE YEARS OF GROWTH MEASUREMENTS ON THE CRUSTOSE LICHEN CALOPLACA AURANTIA IN THE CENTRAL NEGEV DESERT

ABSTRACT Over a period of 23 years, growth rates of primarily circular thalli of the crustose epilithic lichen Caloplaca aurantia (Pers.) Hellb. var. aurantia Poelt were measured in the highlands of the Negev Desert near Avdat. Thalli with an area greater than 3 to 4 cm2have reached the “linear phase” of growth, i.e., they show an almost constant annual increment in their radius. In an average of 11 samples, this growth amounted to 0.371 mm yr−1. In order to determine the width of the marginal annulus which contributes to the extension growth of the lichen, different portions of the center of some of the thalli were removed. A remaining annulus with a width of 12 mm provides sufficient assimilates for unimpaired centrifugal growth, which was not the case with an annulus 6 mm in width. Thalli of Diploschistes calcareus (Mull. Arg.) Stein., another crustose lichen species, showed an annual radial growth increment of 0.17 mm yr−1; however, data from only two samples could be collected. From diameter and grow...

Desiccation tolerance of the epilithic lichen Lecanora muralis (Schreb.) Rabenh. in the temperate climate

Lecanora muralis (Schreb.) Rabenh. is a ubiquitous epilithic crustose lichen of the temperate climate. It is well studied in terms of diel and annual carbon budget and productivity with continuous long-term observations in the field in 1995/96 by Otto L. Lange, Würzburg, and collaborators. However, these earlier studies left open the question to which extent the lichen is desiccation tolerant and if desiccation tolerance might possibly limit photosynthetic activity. In present study measurements of chlorophyll fluorescence parameters were performed to assess photosynthetic activity under various daily weather conditions throughout the year and recovery from desiccation after various periods of dryness in ambient air. Under any weather conditions, including strong frost for several days with night-temperatures around −15°C and strong heat of several days with day-temperatures around 35°C, the lichen was fully photosynthetically competent after wetting the samples for 15min by submersion in water when they were dry in the field in the absence of actual incident precipitation. Chlorophyll fluorescence parameters were identical under all weather conditions sampled. A sample kept dry in ambient air for 37 days showed full recovery of chlorophyll fluorescence parameters after wetting for 30min. Samples desiccated for longer periods up to 155 days took longer wetting times of about 300min and recovered only partially but nevertheless showed active photosynthetic electron transport. Of 17 samples desiccated for 177–178 days only three recovered after rewetting for several days. It is concluded that the desiccation tolerance of L. muralis is sufficient to overcome dry spells of duration as it normally occurs in its natural environment. Desiccation tolerance is not likely to limit carbon budget and productivity.

A sub-aerial biofilms investigation and new approach in biocide application in cultural heritage conservation: Holy Virgin Church (Gradac Monastery, Serbia)

Sub-aerial biofilms (SAB) on frescoes and façade of the Holy Virgin Church (Gradac Monastery, Serbia) were investigated to study the effectiveness of benzalkonium chloride (BAC) as a biocide. Antifungal activity of Origanum vulgare essential oil as a natural product with confirmed high fungicidal potential was also tested against fungi causing alterations on the frescoes. Chemical and mineralogical composition of the church historical mortar showed the dominance of calcareous minerals. In large areas on the church façade, coloured biopatinas formed by different groups of coccoid and filamentous cyanobacteria, green algae and filamentous fungi were found. Epilithic lichens and bryophytes were also present on the church exterior suggesting long-term ecological succession. The presence on the frescoes, of human pathogenic fungus Aspergillus fumigatus were found in the internal environment. Aspergillus and Penicillium species isolated from painted walls were subject to an in vitro biocidal assay with the biocide BAC. An aqueous solution of 0.25% and 0.15% BAC was able to reduce the viability of all the tested fungal isolates after exposure to the solution for 5 min, while exposure to an aqueous solution of 0.05% for 30 min reduced conidial viability. Origanum vulgare essential oil also showed strong antifungal potential against selected fungal isolates.

Role of lichens in weathering of granodiorite in the Sila uplands (Calabria, southern Italy)

This paper explores the role of five recurrent epilithic lichen species (Aspicilia intermutans (Nyl.) Arnold, Xanthoparmelia pulla (Ach.) O. Blanco, A. Crespo, Elix, D. Hawksw. & Lumbsch, Rhizocarpon lecanorinum Anders, Tephromela atra (Huds.) Hafellner and Lecanora bolcana (Pollini) Poelt), which encrust granodiorite spheroidal boulders exposed in the Sila uplands (Calabria, southern Italy), in weathering of plutonic rocks in a typical mountainous Mediterranean environment. A detailed investigation was carried out on the lichen–rock interface of each species, by comparing them mutually and with lichen-free granodiorite samples. For this purpose, the lichen species were sampled together with the encrusted rock surface for detailed mineral-petrographic analyses performed in thin and ultra-thin sections. Optical and scanning electron microscopy of these sections and of bulk samples permitted us to highlight the peculiar modes of physical and chemical attacks of lichen thalli and hyphae on and into the substratum for each species. Crack systems often parallel to the outer rock surface appear often intruded by hyphae, which cause rupture of primary minerals, with detachment and progressive incorporation of their fragments into the thallus. In particular, the species L. bolcana and T. atra revealed an unexpected, partly endolithic behavior, presumably enhanced by the presence of rock fractures earlier generated by other physical breakage processes already affecting the spheroidal boulders in the Sila mountains. Dissolution features often affect primary minerals (even quartz), that may show very peculiar patterns which are suggestive of a biologically-induced control. Various phyllosilicate clay minerals were identified using SEM–EDS microprobe analyses and FT-IR spectroscopy, which also enabled the identification of possible amorphous silica (or quartz micrograins), rhizocarpic acid and carotenoid at the encrusted granodiorite interface. In contrast, neither oxalic acids nor oxalates were detected.

Epilithic lichens and their morphological adaptations to the conditions of the White and Barents Seas coast (Russian Arctic)

The main aim of our work was to investigate the biodiversity of coastal lichens, conditions of lichen cover formation, and study the structural and functional adaptations of Lecanora intricata (Ach.) Ach. and L. polytropa (Ehrh. ex Hoffm.) Rabenh. The investigation was carried out during 2008-2012 on cliffs both along the Murmansk (the Barents Sea) coast and the southern and western shores of the White Sea. For the evaluation of species composition, and ecotopic coenotical features of epilithic lichen growing on cliffs, the geobotanical methods have been used. In addition, the anatomical, morphological and biochemical studies of Lecanora intricata and L. polytropa have been made. 91 species have been included in the total list of lichens on the White Sea coast. On the Murmask coast of the Barents Sea, 36 lichen species had revealed. On the coastal territory, the epilithic lichens inhabit the upper littoral and supralittoral zone. The lichen cover is formed by two interacting factors: the water factor (sea) and the terrestrial vegetation. Four lichen zones were distinguished in the all studying territories. They differed by the lichen species composition and effect of the sea. The first lichen’s zone is the intrazonal structure in the complex coastal lichen cover. In Lecanora polytropa and L. intricata, structural and functional features of lichens for adaptation to unstable coastal conditions were identified. The crustose biomorphs were better adapted to temperature and degree of hydration of thalli. Formation of the smallest ascospores is reproductive strategy of epilithic lichens in extreme habitats. High content of usnic acid in the studied lichen thalli allows them to exist in the open areas exposed to solar radiation and provides the biotic regulation that affects the structure of lichen cover. Optimal ratio of algal to fungal components in the thalli of these species is necessary to maintain their life in extreme environments.

Survival of Rock-Colonizing Organisms After 1.5 Years in Outer Space

Cryptoendolithic microbial communities and epilithic lichens have been considered as appropriate candidates for the scenario of lithopanspermia, which proposes a natural interplanetary exchange of organisms by means of rocks that have been impact ejected from their planet of origin. So far, the hardiness of these terrestrial organisms in the severe and hostile conditions of space has not been tested over extended periods of time. A first long-term (1.5 years) exposure experiment in space was performed with a variety of rock-colonizing eukaryotic organisms at the International Space Station on board the European EXPOSE-E facility. Organisms were selected that are especially adapted to cope with the environmental extremes of their natural habitats. It was found that some-but not all-of those most robust microbial communities from extremely hostile regions on Earth are also partially resistant to the even more hostile environment of outer space, including high vacuum, temperature fluctuation, the full spectrum of extraterrestrial solar electromagnetic radiation, and cosmic ionizing radiation. Although the reported experimental period of 1.5 years in space is not comparable with the time spans of thousands or millions of years believed to be required for lithopanspermia, our data provide first evidence of the differential hardiness of cryptoendolithic communities in space.

Differential effects of biocide treatments on saxicolous communities: Case study of the Segovia cathedral cloister (Spain)

This paper reports on the differential elimination of surface lichen thalli and their penetrating hyphae and epilithic and endolithic cyanobacteria by two biocide treatments applied on a small scale to the cloister of the Segovia cathedral. Both chemical treatments were followed by mechanical cleaning, and their effects were determined by scanning electron microscopy with backscattered electron imaging (SEM–BSE). Both treatments were effective at killing epilithic lichens and cyanobacteria, although they showed different effects on endolithic microbiota. The cleaning treatment involving Biotin T, dissolved in water, applied to areas extensively colonized by crustose lichens, resulted in a cleaner stone surface that was practically free of surface epilithic lichen remains, but the presence of living fungal hyphae was detected in fissures of the stone under these remnants in samples examined two months after treatment. The treatment using Biotin R, dissolved in White Spirit, left more superficial lichen remains but was better at eliminating mycobiont hyphae penetrating the stone. Endolithic cyanobacteria from zones showing green and black colourations were also more affected by Biotin R treatment. Our SEM–BSE observations highlight the need for a complete evaluation of treatment effects, including those produced on both epilithic lichen thalli and their penetrating components and on epilithic and endolithic microbiota. The causes and consequences of the different efficacy of treatments observed, as well as the potential risks of recolonization by viable cells left behind after treatment, are discussed.

An Investigation of the Role of Water (Rain and Dew) in Controlling the Growth Form of Lichens on Cobbles in the Negev Desert

Dew and rain were measured along the north facing slope (NFS, with epilithic lichens predominating) and the south-facing slope (SFS, with endolithic lichens predominating) in the Negev (P = 95 mm) in order to evaluate the relationships between the abiotic conditions and the lichen growth form. Although insignificant differences characterize the rain and dew amounts with the epilithic growth form, high correlation was obtained between dew duration (r2 = 0.73) and the combined duration of dew and rain (r2 = 0.79). The data imply that daylight wetness duration rather than the amount of precipitation may explain the predominance of epilithic lichens at the shaded NFS.

Ecological traits of the lichen flora of Khanty-Mansi Autonomous Okrug (Yugra)

A lichen biodiversity study conducted in the Khanty-Mansi Autonomous Okrug (district) reports 958 species in 199 genera and 72 families. Cetrelia alaskana (Llano) W. L. Culb. et C. F. Culb. is new to West Siberia; Graphis elegans (Borrer ex Sm.) Ach., Porpidia hydrophila (Fr.) Hertel et A. J. Schwab, and Sclerococcum sphaerale (Ach.) Fr. are new to Siberia; Acarospora fusca B. de Lesd., Cliostomum tenerum (Nyl.) Coppins et S. Ekman, Pertusaria corallina (L.) Arnold, and P. stalactiza Nyl. are new to Russia. High diversity of epilithic lichens is ecologically due to a variety of ecotopes that include mountain habitats on the eastern spurs of the Subarctic and Northern Urals in addition to plain ones.

Snail herbivory decreases cyanobacterial abundance and lichen diversity along cracks of limestone pavements

Herbivores are known to decrease plant species diversity in ecosystems with low productivity. Limestone pavements are low-productive habitats harboring specialized communities of cyanobacteria, and endo- and epilithic lichens exposed to extreme temperature and humidity fluctuations. Pavements of the Great Alvar (O¨ land, Sweden) are covered by free-living cyanobacteria giving the rock surface a dark color. Based on cyanobacterial abundance along the edges, two types of cracks intersecting the pavements have been described: Type one with abundant cyanobacteria and type two without cyanobacteria resulting in light-colored edges. Erosion and different lengths of inundation by melt water have been suggested to cause the conspicuous differences in community composition and hence color between cracks. We hypothesized that this pattern results from the grazing activity of the cyanobacteria- and lichen-feeding snail Chondrina clienta, which reduces cyanobacterial cover along light-colored cracks and facilitates endolithic lichens. Three dark and three light-colored cracks were investigated at each of three localities. Crack characteristics (i.e., aspect, width, depth and erosion) and snail density were assessed at the crack level. Cyanobacterial cover and lichen diversity were recorded in 1-cm sections, sampled every 5 cm along eight 160-cm-long transects per crack. Model selection was applied to estimate effects of snail density and distance from crack edges on cyanobacterial abundance and lichen diversity. Crack characteristics explained no differences in cyanobacterial cover or lichen diversity. However, cyanobacterial cover decreased towards the edges of cracks with high snail densities. A transplant experiment supported the correlational evidence. The abundant cyanobacterial cover on pieces of stone placed close to cracks with high snail densities was completely grazed within 19 months. By contrast, cyanobacteria recolonized initially completely grazed pieces of stone when fixed near cracks without snails. Abundance and diversity of endolithic lichens increased along cracks with high compared to low snail densities but decreased in epilithic lichens and lichens with cyanobacterial symbionts. However, the presence of the gastropod herbivore decreased overall lichen diversity. Comparing presence-absence matrices with null models revealed that species co-occurred less frequently than expected. Taken together, we provide evidence that herbivory indirectly released endolithic lichens from competition for light by reducing cyanobacterial cover. (Less)

Lichen Bioerosion on Fossil Vertebrates from the Cenozoic of Patagonia and Antarctica

Different traces occur on fossil bones and teeth coming from the Early Miocene Gaiman Formation (Patagonia, Argentina). Most traces were attributed to the action of terrestrial and marine predators and scavengers. However, other traces on bones and teeth from this unit and one tooth from the Eocene La Meseta Formation (Antarctica) are attributed to chemical corrosion by lichens in recent times, that is, in a very late diagenetic time. The living lichens and calcium oxalate deposits occurring on the traces and their particular pattern indicates that they were not produced by vegetal roots. The lichens include reproductive structures which allowed a proper determination. A kind of corrosion pattern (Type 1) on bones and teeth from Patagonia is associated to Sarcogyne orbicularis Körber, Verrucaria sp. Schrad, and Buellia aff. punctiformis (Hoff.) Massal. The lichen Aspicilia aff. aquatica produced rounded holes on an Antarctic tooth (Type 2). On the same tooth, the epilithic lichen Caloplaca sp. Th. Fries did not leave any kind of mark on the enameloid.

Microbial metacommunities in the lichen–rock habitat

Lichens are common as colonizers of bare rocks and contribute to weathering, but their associated bacterial communities have been poorly studied. In this study Hydropunctaria maura, Ophioparma ventosa, Pertusaria corallina and Rhizocarpon geographicum were analysed to determine the influence of lichens on lichen-rock-associated microbial metacommunities. For the first time, Archaea were documented to be associated with rock-inhabiting lichens. All the archaeal sequences obtained were affiliated with Crenarchaeota. The Bacteria detected in the lichen-rock samples were affiliated with the major lineages Acidobacteria, Actinobacteria, Alpha-, Beta-, Gammaproteobacteria, Bacteriodetes, Chloroflexi, Deinococcus, Firmicutes, Planctomycetes, Tenericutes and Cyanobacteria. The microbial communities of O. ventosa, P. corallina and R. geographicum were more similar to each other both terms of the number and types of different sequences, than to H. maura. A higher bacterial diversity was observed endolithically than within the epilithic lichen thalli directly above. The abundance of Archaea were also generally higher endolithically than in the epilithic lichen thalli, while the abundance of Bacteria was higher in the lichen thalli compared with within the rock. These results demonstrated that the lichen-rock interfaces are complex habitats, where the macroscopic lichens influence the composition of microbial metacommunities.

Microbial colonization of Ca-sulfate crusts in the hyperarid core of the Atacama Desert: implications for the search for life on Mars

The scarcity of liquid water in the hyperarid core of the Atacama Desert makes this region one of the most challenging environments for life on Earth. The low numbers of microbial cells in the soils suggest that within the Atacama Desert lies the dry limit for life on our planet. Here, we show that the Ca-sulfate crusts of this hyperarid core are the habitats of lithobiontic micro-organisms. This microporous, translucent substrate is colonized by epilithic lichens, as well as endolithic free-living algae, fungal hyphae, cyanobacteria and non photosynthetic bacteria. We also report a novel type of endolithic community, "hypoendoliths", colonizing the undermost layer of the crusts. The colonization of gypsum crusts within the hyperarid core appears to be controlled by the moisture regime. Our data shows that the threshold for colonization is crossed within the dry core, with abundant colonization in gypsum crusts at one study site, while crusts at a drier site are virtually devoid of life. We show that the cumulative time in 1 year of relative humidity (RH) above 60% is the best parameter to explain the difference in colonization between both sites. This is supported by controlled humidity experiments, where we show that colonies of endolithic cyanobacteria in the Ca-sulfate crust undergo imbibition process at RH >60%. Assuming that life once arose on Mars, it is conceivable that Martian micro-organisms sought refuge in similar isolated evaporite microenvironments during their last struggle for life as their planet turned arid.

Cesium-137 monitoring using lichens from W. Macedonia, N. Greece

137Cs content in lichens and substrate (soil, bark) collected from W. Macedonia, Greece was measured 20 years after the Chernobyl reactor accident. Archive material from previous collections was also used for comparison and a diachronic estimation of the radio-contamination status. A gradual decrease was detected which depended on various factors such as the collected species, location, growth rate and substrate. Maximum accumulation capacity of 137Cs was observed in epilithic lichens in comparison to epigeic or epiphytic ones. Furthermore, foliose or crustose lichens such as Parmelia sulcata, Xanthoria parietina, Xanthoria calcicola, Xanthoparmelia somloensis were more contaminated than filamentose at the same biotope. Among filamentose or fruticose species those with large surface area to biomass ratio e.g. Usnea sp. showed also greater accumulation capacity. Autoradiography revealed an amount of 137Cs distributed more or less uniformly in lichen thalli. The high 137Cs activities found in lichens 20 years after Chernobyl suggest that these primitive plants are effective, suitable and inexpensive biological detectors of the distribution and burden of fallout radionuclide fallout pattern.

Lichen Colonization on Cobbles in the Negev Desert Following 15 Years in the Field

Rock slabs were placed from 1991 through 2006 at the bottom of a north- (NFS) and a south-facing (SFS) slope of a first order drainage basin in the Negev Desert Highlands, Israel (P = 95 mm) where natural cobbles have >95% cover of endolithic and epilithic lichens. Periodic measurements of rain, dew and chlorophyll content took place. Lichen cover and the regeneration time were aspect-dependent. Lichen cover was 98% and 61% at NFS and SFS, respectively, while the lichen's recovery time was respectively ∼45 and ∼80 years. The fast regeneration time may have important implications regarding weathering and soil formation.

Culturable bacteria associated with Antarctic lichens: affiliation and psychrotolerance

Antarctic habitats harbour yet unexplored niches for microbial communities. Among these, lichen symbioses are very long-living and stable microenvironments for bacterial colonization. In this work, we present a first assessment of the culturable fraction of bacteria associated with Antarctic lichens. A phylogenetic analysis based on 16S rRNA gene sequence of 30 bacterial strains isolated from five epilithic lichens belonging to four species (Lecanora fuscobrunnea, Umbilicaria decussata, Usnea antarctica, Xanthoria elegans) shows that these represent the main bacterial lineages Actinobacteria, Firmicutes, Proteobacteria and Deinococcus-Thermus. Within the Actinomycetales, two strains group in the genera Arthrobacter and Knoellia, respectively. Most of the other Actinobacteria form well-supported groups, but could be assigned with certainty only at the family level, and one is in isolated position in the Mycobacteriaceae. The strains in Firmicutes and Proteobacteria belong to the genera Paenibacillus,Bacillus and Pseudomonas, which were already reported from lichen thalli. Some genera such as Burkholderia and Azotobacter, reported in the literature as also associated with lichens, have not been detected in this study. One strain represents the first record of Deinococcus in epilithic lichens; it is related to the species Deinococcus alpinitundrae from Alpine environments and may represent a new species. Further separated and well-supported clades indicate the presence of possibly new entities. Some of the examined strains are related to known psychrophilic bacteria isolated from ice and other extreme environments, others with bacteria distributed worldwide even in temperate climates. Most of the strains tested were able to grow at low temperatures, but tolerated a wider range of temperature. Ecological and evolutionary implications of these lichen-associated bacteria are discussed.

Diel and seasonal courses of ambient carbon dioxide concentration and their effect on productivity of the epilithic lichen Lecanora muralis in a temperate, suburban habitat

Abstract:Ambient CO2 concentration (together with CO2 exchange and microclimate) was recorded every 30 min for 15 months for Lecanora muralis growing in the Botanical Garden Würzburg (Germany, northern Bavaria), a habitat on the outskirts of the city. Annual mean CO2 was around 17 ppm higher than the global average reported for the time of measurement (361 ppm; 1995/96), and daily values ranged from 317 to 490 ppm. Diel courses of CO2 could be classified into three different types. Type A, when CO2 levels rose overnight and then fell strongly to below global levels during the day, which predominated in the summer (about 75 of days); Type B, irregular diel courses occurred during all seasons with often very rapid changes apparently due to advective CO2 transport; Type C, CO2 concentration was typically almost stable at generally between c. 330 and 430 ppm which predominated in the winter (63 of days).Under controlled conditions, CO2 saturation of net photosynthesis (NP) of L. muralis at optimal hydration and light occurred at around 1000 ppm. NP was also affected by low CO2 at limiting light and thallus water contents. Based upon these data, we estimated the improvement of NP of L. muralis due to transient increase of ambient CO2 (as compared with the global average) for one selected combination of environmental factors (nocturnal dew or frost). This combination is an important source of water for the lichen, resulting in 40 of its annual production and, especially in these situations, photosynthesis was increased by high ambient CO2 in the early morning under prevailing Type A conditions. After dew activation, light compensation point of NP occurred at an average concentration of 413 ppm and diel maxima of NP at 402 ppm. This allows a rough estimate that the transiently elevated CO2 increased the photosynthetic gain of the lichen after dew of 7, or an improvement to its annual carbon balance of about 3. Conditions, especially interrelationships between lichen hydration, light and CO2 are so complex that we are not yet able to extend our estimates to other environmental situations of photosynthetic activity of L. muralis.