Effects of site-specific climatic conditions on the radial growth of the lichen biomonitor Xanthoria parietina

Should we hail the Red King? Evolutionary consequences of a mutualistic lifestyle in genomes of lichenized ascomycetes

The Red Queen dynamic is often brought into play for antagonistic relationships. However, the coevolutionary effects of mutualistic interactions, which predict slower evolution for interacting organisms (Red King), have been investigated to a lesser extent. Lichens are a stable, mutualistic relationship of fungi and cyanobacteria and/or algae, which originated several times independently during the evolution of fungi. Therefore, they represent a suitable system to investigate the coevolutionary effect of mutualism on the fungal genome. We measured substitution rates and selective pressure of about 2000 protein-coding genes (plus the rDNA region) in two different classes of Ascomycota, each consisting of closely related lineages of lichenized and non-lichenized fungi. Our results show that independent lichenized clades are characterized by significantly slower rates for both synonymous and non-synonymous substitutions. We hypothesize that this evolutionary pattern is connected to the lichen life cycle (longer generation time of lichenized fungi) rather than a result of different selection strengths, which is described as the main driver for the Red Kind dynamic. This first empirical evidence of slower evolution in lichens provides an important insight on how biotic cooperative interactions are able to shape the evolution of symbiotic organisms.

Effects of meteorological conditions and topography on the bioaccumulation of PAHs and metal elements by native lichen (Xanthoria parietina)

The bioaccumulation of PAHs and metal elements in the indigenous lichens Xanthoria parietina was monitored during two years at a quarterly frequency, in 3 sites of contrasted anthropic influence. The impact of the meteorological factors (temperature, relative humidity, rainfall, wind speed) was first estimated through principal component analysis, and then by stepwise multilinear regressions to include wind directions. The pollutants levels reflected the proximity of atmospheric emissions, in particular from a large industrial harbor. High humidity and mild temperatures, and in a lower extent low wind speed and rainfall, also favored higher concentration levels. The contributions of these meteorological aspects became minor when including wind direction, especially when approaching major emission sources. The bioaccumulation integration time towards meteorological variations was on a seasonal basis (1-2 months) but the wind direction and thus local emissions also relied on a longer time scale (12 months). This showed that the contribution of meteorological conditions may be prevalent in remote places, while secondary in polluted areas, and should be definitely taken into account regarding long-term lichen biomonitoring and inter-annual comparisons. In the same time, a quadruple sampling in each site revealed a high homogeneity among supporting tree species and topography. The resulting uncertainty, including sampling, preparation and analysis was below 30% when comfortable analytical conditions were achieved. Finally, the occurrence of unexpected events such as a major forest fire, permitted to evaluate that this type of short, although intense, events did not have a strong influence on PAH and metals bioaccumulation by lichen.

Enhanced culturing techniques for the mycobiont isolated from the lichen Xanthoria parietina

Lichens and their isolated symbionts are potentially valuable resources for biotechnological approaches. Especially mycobiont cultures that produce secondary lichen products are receiving increasing attention, but lichen mycobionts are notoriously slow-growing organisms. Sufficient biomass production often represents a limiting factor for scientific and biotechnological investigations, requiring improvement of existing culturing techniques as well as methods for non-invasive assessment of growth. Here, the effects of pH and the supplement of growth media with either D-glucose or three different sugar alcohols that commonly occur in lichens, D-arabitol, D-mannitol and ribitol, on the growth of the axenically cultured mycobiont isolated from the lichen Xanthoria parietina were tested. Either D-glucose or different sugar alcohols were offered to the fungus at different concentrations, and cumulative growth and growth rates were assessed using two-dimensional image analysis over a period of 8 weeks. The mycobiont grew at a pH range from 4.0 to 7.0, whereas no growth was observed at higher pH values. Varying the carbon source in Lilly-Barnett medium (LBM) by replacing 1% D-glucose used in the originally described LBM by either 1%, 2% or 3% of D-mannitol, or 3% of D-glucose increased fungal biomass production by up to 26%, with an exponential growth phase between 2 and 6 weeks after inoculation. In summary, we present protocols for enhanced culture conditions and non-invasive assessment of growth of axenically cultured lichen mycobionts using image analysis, which may be useful for scientific and biotechnological approaches requiring cultured lichen mycobionts.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11557-021-01707-7.

Element accumulation performance of living and dead lichens in a large-scale transplant application

In bioaccumulation studies, sample devitalization through acid washing or oven drying is commonly applied to enhance the element accumulation efficiency of moss sample. Such aspect, however, has never been considered in biomonitoring surveys using lichens. In this study, the trace element accumulation performance of living (L) and dead (D) samples of the lichen Pseudevernia furfuracea was compared by a side-by-side transplanting at 40 sites in a large, mixed land use area of NE Italy for 8 weeks. Devitalization was achieved without any physico-chemical treatments, by storing lichen samples in a dark cool room for 18 months. Health status of lichens was assessed before and after the sample exposure by chlorophyll fluorescence emission. Although elemental analysis of the two exposed sample sets revealed a similar trace element pollution scenario, the content of 13 out of the 24 selected elements was higher in D samples. By expressing results as exposed-to-unexposed (EU) ratio, D samples show a higher bioaccumulation signal in 80% of transplant sites for Al, Ca, Fe, Hg, Pb and Ti. Overall, the health status of lichen samples might lead to interpretational discrepancies when EU ratio is classified according to the recently proposed bioaccumulation scale.

How dry is dry? Molecular mobility in relation to thallus water content in a lichen

Lichens can withstand extreme desiccation to water contents of ≤ 0.1 g H2O g-1 DW, and in the desiccated state are among the most extremotolerant organisms known. Desiccation-tolerant life-forms such as seeds, mosses and lichens survive 'vitrification', that is the transition of their cytoplasm to a 'glassy' state, which causes metabolism to cease. However, our understanding of the mechanisms of desiccation tolerance is hindered by poor knowledge of what reactions occur in the desiccated state. Using Flavoparmelia caperata as a model lichen, we determined at what water contents vitrification occurred upon desiccation. Molecular mobility was assessed by dynamic mechanical thermal analysis, and the de- and re-epoxidation of the xanthophyll cycle pigments (measured by HPLC) was used as a proxy to assess enzyme activity. At 20 °C vitrification occurred between 0.12-0.08 g H2O g-1 DW and enzymes were active in a 'rubbery' state (0.17 g H2O g-1 DW) but not in a glassy state (0.03 g H2O g-1 DW). Therefore, desiccated tissues may appear to be 'dry' in the conventional sense, but subtle differences in water content will have substantial consequences on the types of (bio)chemical reactions that can occur, with downstream effects on longevity in the desiccated state.

Validation of particulate dispersion models by native lichens as point receptors: a case study from NE Italy

Atmospheric dispersion model (ADM) simulations are increasingly used as management tools in air pollution monitoring programs, even in the absence of proper validation. Biomonitors can provide important information for ADM validation, but an open question is their temporal frame of application, particularly when native organisms are used. In this study, we tested two alternative ADM simulating the total suspended particulate (TSP) released by a coal power station, against the element content of two native lichens collected at 40 sites, integrated by soil samples. The ADM simulations differed by the time references: the 6-month period preceding lichen sampling, approximately corresponding to the estimated age of the samples (Mod. A), and the whole year 2005, representative of the local average conditions and used in the plant authorization processes (Mod. B). A generalized regression model analysis clearly showed that the Cr, Pb and V content of lichen samples was spatially associated to the outcomes of Mod. A, but not with Mod. B. Interestingly, the Cr content of lichen samples consistently correlated to TSP concentration predicted by Mod. A along two transects placed downwind from the coal power station. This result was corroborated by an air particulate matter sampling which pointed out that air Cr concentrations increased during the operative period of the source. Overall, our results suggest that lichen bioaccumulation data can proficiently be used to validate ADM simulations if the exposure time of the biological samples is consistent with the temporal domain of the ADM simulations.

Congruence Evaluation of Mercury Pollution Patterns Around a Waste Incinerator over a 16-Year-Long Period Using Different Biomonitors

To date, there has been an ever-increasing interest in complementary air monitoring techniques, which may fill the deficiencies of air quality networks. The present work reports the results concerning five biomonitoring surveys (BSs) performed in the proximity of a waste incinerator (WI) over a 16-year period. Hg emission related to the WI activity was monitored by means of both active and passive BSs based on three photosynthetically-active biomonitors (i.e., two epiphytic lichens: Pseudevernia furfuracea and Xanthoria parietina; one vascular plant: Robinia pseudoacacia) collected or exposed before and/or after the WI installation, and after a four-month period of inactivity. Hg concentration values observed in biomonitor samples varied according to the implemented species and to the status of WI plant (active vs. inactive). Our data demonstrate that, in the same pollution scenario, P. furfuracea accumulates three times more Hg than both X. parietina and R. pseudoacacia. The results are discussed in the context of the actual European Union directives concerning air pollution monitoring and assessment, revealing that both active and passive BSs are efficient tools to provide a reliable estimation of the spatial changes of Hg concentrations in the environment.

New Interpretative Scales for Lichen Bioaccumulation Data: The Italian Proposal

The interpretation of lichen bioaccumulation data is of paramount importance in environmental forensics and decision-making processes. By implementing basic ideas underlying previous interpretative scales, new dimensionless, species-independent “bioaccumulation scales” for native and transplanted lichens are proposed. Methodologically consistent element concentration datasets were populated with data from biomonitoring studies relying on native and transplanted lichens. The scale for native lichens was built up by analyzing the distribution of ratios between element concentration data and species-specific background concentration references (B ratios), herein provided for Flavoparmelia caperata and Xanthoria parietina (foliose lichens). The scale for transplants was built up by analyzing the distribution of ratios between element concentration in exposed and unexposed samples (EU ratio) of Evernia prunastri and Pseudevernia furfuracea (fruticose lichens). Both scales consist of five percentile-based classes; namely, “Absence of”, “Low”, “Moderate”, “High”, and “Severe” bioaccumulation. A comparative analysis of extant interpretative tools showed that previous ones for native lichens suffered from the obsolescence of source data, whereas the previous expert-assessed scale for transplants failed in describing noticeable element concentration variations. The new scales, based on the concept that pollution can be quantified by dimensionless ratios between experimental and benchmark values, overcome most critical points affecting the previous scales.