Do polyamines alter the sensitivity of lichens to nitrogen stress?

Distinguishing atmospheric nitrogen compounds (nitrate and ammonium) in lichen biomonitoring studies

Nitrogen speciation, i.e. distinguishing nitrate (NO₃^-) and ammonium (NH₄⁺), is commonly undertaken in soil studies, but has not been conducted extensively for lichens. Lichen total nitrogen contents (N wt%) reflect airborne atmospheric nitrogen loadings, originating from anthropogenic sources (e.g. vehicular and agricultural/livestock emissions). Albeit nitrogen being an essential lichen nutrient, nitrogen compound (i.e. NO₃^- and NH₄⁺) concentrations in the atmosphere can have deleterious effects on lichens. Moreover, N wt% do not provide information on individual nitrogen compounds, i.e. NO₃^- and NH₄⁺ which are major constituents of atmospheric particulate matter (e.g. PM₁₀ and PM_2.5). This study presents a novel method to separate and quantify NO₃^- and NH₄⁺ extracted from lichen material. An optimal approach was identified by testing different strengths and volumes of potassium chloride (KCl) solutions and variable extraction times, i.e. the use of 3% KCl for 6 hours can achieve a same-day extraction and subsequent ion chromatography (IC) analysis for reproducible lichen nitrate and ammonium concentration determinations. Application of the method was undertaken by comparing urban and rural Xanthoria parietina samples to investigate the relative importance of the two nitrogen compounds in contrasting environments. Findings presented showed that lichen nitrogen compound concentrations varied in rural and urban X. parietina samples, suggesting different atmospheric nitrogen loadings from potentially different sources (e.g. agricultural and traffic) and varied deposition patterns (e.g. urban layout impacts). Despite potential impacts of nitrogen compounds on lichen metabolism, the approach presented here can be used for quantification of two different nitrogen compounds in lichen biomonitoring studies that will provide specific information on spatial and temporal variability of airborne NO₃^- and NH₄⁺ concentrations that act as precursors of particulate matter, affecting air quality and subsequently human health.

The Effect of Foliar Putrescine Application, Ammonium Exposure, and Heat Stress on Antioxidant Compounds in Cauliflower Waste

This work has been focused on the study of how we can affect the short heat stress on the bioactive compounds content. Some recent investigations have observed that management of nitrogen fertilization can alleviate short-term heat effects on plants. Additionally, the short-term heat stress can be also ameliorated by using putrescine, a polyamine, due to its crucial role in the adaptation of plants to heat stress Therefore, different NO₃⁻/NH₄⁺ ratios and a foliar putrescine treatment have been used in order to increase tolerance to thermal stress in order to take advantage of the more frequent and intense heat waves and make this crop more sustainable. So, other objective of this work is to make the cauliflower waste more attractive for nutraceutical and pharmaceutical preparations. Thus, the effect of a thermal stress combined with a 50:50 NO₃⁻/NH₄⁺ ratio in the nutrient solution, and the foliar application of 2.5 mM putrescine increased in the content of various sugars (inositol, glucose, and fructose), total phenolic compounds and polyamines, as well as in the antioxidant activity. The greatest accumulation of these compounds was observed in young leaves. Our results show from a physiological and agronomic point of view, that the foliar application of putrescine and the 50:50 NO₃⁻/NH₄⁺ treatment managed to alleviate the negative effects of the abiotic stress suffered at high temperature, yielding plants with higher antioxidant compounds content.

Resilience of Epiphytic Lichens to Combined Effects of Increasing Nitrogen and Solar Radiation

Lichens are classified into different functional groups depending on their ecological and physiological response to a given environmental stressor. However, knowledge on lichen response to the synergistic effect of multiple environmental factors is extremely scarce, although vital to get a comprehensive understanding of the effects of global change. We exposed six lichen species belonging to different functional groups to the combined effects of two nitrogen (N) doses and direct sunlight involving both high temperatures and ultraviolet (UV) radiation for 58 days. Irrespective of their functional group, all species showed a homogenous response to N with cumulative, detrimental effects and an inability to recover following sunlight, UV exposure. Moreover, solar radiation made a tolerant species more prone to N pollution’s effects. Our results draw attention to the combined effects of global change and other environmental drivers on canopy defoliation and tree death, with consequences for the protection of ecosystems.

Effects of Different Nitrogen Forms and Exogenous Application of Putrescine on Heat Stress of Cauliflower: Photosynthetic Gas Exchange, Mineral Concentration and Lipid Peroxidation

This study examines the effect of the exogenous application of polyamine putrescine together with the application of different ratios of nitrate/ammonium (NO₃⁻/NH₄⁺), on the physiology of cauliflower subjected to heat stress. The 50:50 NO₃⁻/NH₄⁺ ratio was the best ratio against heat stress. As a result of the joint application of these compounds, a higher photosynthetic rate, a higher accumulation of both photosynthesis-related compounds and pigments, total proteins, and a change in the status of nutrients were obtained. Particularly, the decrease in content of calcium, chloride and sulphate in plants under heat stress is ameliorated by the ammonium effect. Additionally, it is important to highlight that cauliflower waste contains a higher content of mineral nutrients than floret cauliflower. These effects were more marked in young leaves. Furthermore, a synergistic effect for coping with heat stress between the polyamine and the nutritional treatment was observed. For this, both the application of putrescine and the feeding of plants with a 50:50 NO₃⁻/NH₄⁺ ratio before heat stress is proposed for the first time as an agricultural practice for increasing the thermotolerance of cauliflower cv Moonshine. On the other hand, due to the lower lipid peroxidation rate obtained in cauliflower leaves, these plants could be used for health purposes as ointments or other nutraceutical products, making the cultivation of this kind of cruciferous more sustainable.

Effects of wood distillate (pyroligneous acid) on sensitive bioindicators (lichen and moss)

Wood distillate (pyroligneous acid) can be successfully applied in agriculture to increase crop quality and productivity with a lower risk for the environment respect to synthetic chemical herbicides, pesticides or fertilizers. However, the effects of wood distillate on the environment and biota are still under investigation, depending on biological attributes of potentially influenced organisms. The potential toxicological effects of wood distillate on sensitive non-target organisms, lichens and mosses, are studied for the first time. The physiological parameters (chlorophyll a fluorescence emission F_V/F_M and PI_(ABS), chlorophyll content, spectral reflectance, antioxidant power, and dehydrogenase activity) and eventual bioaccumulation of selected elements (As, Ba, Cd, Cr, Cu, Fe, Ni, Pb, Zn) were investigated in the lichen Xanthoria parietina and the moss Hypnum cupressiforme after short-term treatments over a range of wood distillate solutions (1:300, 1:500, 1:700) to detect potential early stress responses. Overall, the lichen did not show changes after the treatments, while in the moss wood distillate caused only modest alterations in F_V/F_M and PI_(ABS) and progressive increasing of antioxidant activity according to the dose supplied. The bioaccumulation of toxic elements was low and did not show any pattern of uptake with increasing concentrations of wood distillate.

δ15N of lichens reflects the isotopic signature of ammonia source

Although it is generally accepted that δ¹⁵N in lichen reflects predominating N isotope sources in the environment, confirmation of the direct correlation between lichen δ¹⁵N and atmospheric δ¹⁵N is still missing, especially under field conditions with most confounding factors controlled. To fill this gap and investigate the response of lichens with different tolerance to atmospheric N deposition, thalli of the sensitive Evernia prunastri and the tolerant Xanthoria parietina were exposed for ten weeks to different forms and doses of N in a field manipulation experiment where confounding factors were minimized. During this period, several parameters, namely total N, δ¹⁵N and chlorophyll a fluorescence, were measured. Under the experimental conditions, δ¹⁵N in lichens quantitatively responded to the δ¹⁵N of released gaseous ammonia (NH₃). Although a high correlation between the isotopic signatures in lichen tissue and supplied N was found both in tolerant and sensitive species, chlorophyll a fluorescence indicated that the sensitive species very soon lost its photosynthetic functionality with increasing N availability. The most damaging response to the different N chemical forms was observed with dry deposition of NH₃, although wet deposition of ammonium ions had a significant observable physiological impact. Conversely, there was no significant effect of nitrate ions on chlorophyll a fluorescence, implying differential sensitivity to dry deposition versus wet deposition and to ammonium versus nitrate in wet deposition. Evernia prunastri was most sensitive to NH₃, then NH₄⁺, with lowest sensitivity to NO₃^-. Moreover, these results confirm that lichen δ¹⁵N can be used to indicate the δ¹⁵N of atmospheric ammonia, providing a suitable tool for the interpretation of the spatial distribution of NH₃ sources in relation to their δ¹⁵N signal.

Lichen as Micro-Ecosystem: Extremophilic Behavior with Astrobiotechnological Applications

This work demonstrates the tolerance of lichen Pleurosticta acetabulum under extreme conditions similar to those encountered in extraterrestrial environments. Specifically, the impact of three extreme Mars-like conditions-complete dehydration, extremely low temperature (-196°C/77K), and oxygen depletion-on lichens was investigated. The symbiosis of mycobiont and photobiont partners creates a micro-ecosystem that ensures viability of both symbiotic partners under prolonged desiccation and extremely low temperatures without any cultivation care. Changes in the molecular structure and function of the photosynthetic apparatus, in the level of chlorophylls, polyamines, fatty acids, carbohydrates, ergosterol, efflux of K⁺, and DNA methylation ensure the ecological integrity of the system and offer resistance of lichens to above-mentioned extreme environmental conditions. For the first time, we also demonstrate that the unprecedented polyextremophilic characteristic of lichens could be linked to biotechnological applications, following exposure to these extreme conditions, such that their ability to produce a high yield of hydrogen was unchanged. All these support that lichens are (a) ideal model systems for a space mission to inhabit other planets, supporting also the aspect that the panspermia theory could be extended to incorporate in the traveling entities not only single organisms but micro-ecosystems like lichens, and (b) ideal model systems for astrobiotechnological applications (hydrogen production), such as in the development of bioregeneration systems for extraterrestrial environments.

Intra- and inter-specific variations in chitin in lichens along a N-deposition gradient

The mechanisms of nitrogen (N) tolerance in lichens are not yet fully understood. Here, we investigated how the increase of chitin content is related with N excess at inter- and intra-specific levels, by using species with differing ecological N tolerances (the tolerant Xanthoria parietina and Parmotrema hypoleucinum and the sensitive Evernia prunastri and Usnea sp.) and thalli of X. parietina and P. hypoleucinum from sites with different availabilities of N of agricultural origin (livestock), as confirmed by lichen N content and δ¹⁵N. Nitrogen, chitin (N-containing compound), and ergosterol contents were measured in lichen thalli. Nitrogen and chitin contents were higher in tolerant species than those in sensitive ones (inter-specific level) and in thalli collected from the N-polluted site than in thalli from the clean site (intra-specific level). We suggest that chitin contributes to N stress tolerance in lichens, and that excess N can be partially stored as chitin (non-toxic form) in the cell walls of tolerant species.

The influence of growth form and substrate on lichen ecophysiological responses along an aridity gradient

In this paper, we investigated whether growth form and substrate in lichens influence their physiological responses along an aridity gradient. Thalli of the foliose lichen Parmotrema perlatum and the fruticose lichen Ramalina canariensis were transplanted in selected rural/forested sites of Southern Portugal characterized by a different aridity index. Physiological parameters including photosynthetic performances, assimilation pigments, ergosterol content and sample viability were measured prior to exposure (winter) and after 6-month exposure (summer). Photosynthetic performances were also investigated in common native foliose and fruticose epiphytic lichens and in fruticose terricolous species. Both transplanted and native lichens showed a decrease in photosynthetic performances in summer and lower performances in sites classified as drier and higher performances in humid forested sites. No relevant differences occurred between epiphytic foliose and fruticose growth forms. However, terricolous fruticose samples showed a significant difference in humid and drier sites and between winter and summer, probably due to microclimatic conditions similarly to other biological crusts.

The cost of surviving nitrogen excess: energy and protein demand in the lichen Cladonia portentosa as revealed by proteomic analysis

Different nitrogen forms affect different metabolic pathways in lichens. In particular, the most relevant changes in protein expression were observed in the fungal partner, with NO ₃^- mostly affecting the energetic metabolism and NH ₄⁺ affecting transport and regulation of proteins and the energetic metabolism much more than NO ₃^- did. Excess deposition of reactive nitrogen is a well-known agent of stress for lichens, but which symbiont is most affected and how, remains a mystery. Using proteomics can expand our understanding of stress effects on lichens. We investigated the effects of different doses and forms of reactive nitrogen, with and without supplementary phosphorus and potassium, on the proteome of the lichen Cladonia portentosa growing in a 'real-world' simulation of nitrogen deposition. Protein expression changed with the nitrogen treatments but mostly in the fungal partner, with NO₃^- mainly affecting the energetic metabolism and NH₄⁺ also affecting the protein synthesis machinery. The photobiont mainly responded overexpressing proteins involved in energy production. This suggests that in response to nitrogen stress, the photobiont mainly supports the defensive mechanisms initiated by the mycobiont with an increased energy production. Such surplus energy is then used by the cell to maintain functionality in the presence of NO₃^-, while a futile cycle of protein production can be hypothesized to be induced by NH₄⁺ excess. External supply of potassium and phosphorus influenced differently the responses of particular enzymes, likely reflecting the many processes in which potassium exerts a regulatory function.

Mechanisms of nitrogen deposition effects on temperate forest lichens and trees

We review the mechanisms of deleterious nitrogen (N) deposition impacts on temperate forests, with a particular focus on trees and lichens. Elevated anthropogenic N deposition to forests has varied effects on individual organisms depending on characteristics both of the N inputs (form, timing, amount) and of the organisms (ecology, physiology) involved. Improved mechanistic knowledge of these effects can aid in developing robust predictions of how organisms respond to either increases or decreases in N deposition. Rising N levels affect forests in micro- and macroscopic ways from physiological responses at the cellular, tissue, and organism levels to influencing individual species and entire communities and ecosystems. A synthesis of these processes forms the basis for the overarching themes of this paper, which focuses on N effects at different levels of biological organization in temperate forests. For lichens, the mechanisms of direct effects of N are relatively well known at cellular, organismal, and community levels, though interactions of N with other stressors merit further research. For trees, effects of N deposition are better understood for N as an acidifying agent than as a nutrient; in both cases, the impacts can reflect direct effects on short time scales and indirect effects mediated through long-term soil and belowground changes. There are many gaps on fundamental N use and cycling in ecosystems, and we highlight the most critical gaps for understanding potential deleterious effects of N deposition. For lichens, these gaps include both how N affects specific metabolic pathways and how N is metabolized. For trees, these gaps include understanding the direct effects of N deposition onto forest canopies, the sensitivity of different tree species and mycorrhizal symbionts to N, the influence of soil properties, and the reversibility of N and acidification effects on plants and soils. Continued study of how these N response mechanisms interact with one another, and with other dimensions of global change, remains essential for predicting ongoing changes in lichen and tree populations across North American temperate forests.

Biological effects of ammonia released from a composting plant assessed with lichens

In this study, we investigated whether ammonia emissions from industrial composting of organic waste may influence the surrounding environment, using lichens as bioindicators. To this purpose, samples of N-tolerant and N-sensitive lichens, namely Xanthoria parietina and Evernia prunastri, were transplanted for 1-3 months along transects at increasing distance (0-400 m) from a composting facility in Tuscany, Italy. Atmospheric concentrations of ammonia were measured using passive samplers. The physiological response of lichen transplants was investigated by means of the photosynthetic efficiency (measured as chlorophyll a fluorescence emission), the integrity of cell membranes (measured as electrolyte leakage), and sample viability (measured as enzymatic activity of dehydrogenase). Epiphytic lichen communities were investigated using biodiversity indices. The results showed decreasing concentrations of ammonia, from 48.7 μg/m(3) at the composting facility to 2.7 μg/m(3) at 400 m. The N-tolerant X. parietina was not affected and some physiological parameters even showed a higher performance, while the N-sensitive E. prunastri showed a reduced performance with increasing atmospheric concentrations approaching the source. A shift from lichen communities composed by meso-acidophilous species (actual condition) to more nitrophilous communities in the near future, approaching the composting facility is suggested. It is concluded that lichens can provide useful data for decision-makers to establish correct science-based environmentally sustainable waste management policies.

Can ammonia tolerance amongst lichen functional groups be explained by physiological responses?

Ammonia (NH3) empirical critical levels for Europe were re-evaluated in 2009, based mainly on the ecological responses of lichen communities without acknowledging the physiological differences between oligotrophic and nitrophytic species. Here, we compare a nitrogen sensitive lichen (Evernia prunastri) with a nitrogen tolerant one (Xanthoria parietina), focussing on their physiological response (Fv/Fm) to short-term NH3 exposure and their frequency of occurrence along an NH3 field gradient. Both frequency and Fv/Fm of E. prunastri decreased abruptly above 3 μg m(-3) NH3 suggesting direct adverse effects of NH3 on its photosynthetic performance. By contrast, X. parietina increased its frequency with NH3, despite showing decreased capacity of photosystem II above 50 μg m(-3) NH3, suggesting that the ecological success of X. parietina at ammonia-rich sites might be related to indirect effects of increased nitrogen (NH3) availability. These results highlight the need to establish NH3 critical levels based on oligotrophic lichen species.

The effect of aluminium-stress and exogenous spermidine on chlorophyll degradation, glutathione reductase activity and the photosystem II D1 protein gene (psbA) transcript level in lichen Xanthoria parietina

In this study, the effects of short-term aluminium toxicity and the application of spermidine on the lichen Xanthoria parietina were investigated at the physiological and transcriptional levels. Our results suggest that aluminium stress leads to physiological processes in a dose-dependent manner through differences in lipid peroxidation rate, chlorophyll content and glutathione reductase (EC 1.6.4.2) activity in aluminium and spermidine treated samples. The expression of the photosystem II D1 protein (psbA) gene was quantified using semi-quantitative RT-PCR. Increased glutathione reductase activity and psbA mRNA transcript levels were observed in the X. parietina thalli that were treated with spermidine before aluminium-stress. The results showed that the application of spermidine could mitigate aluminium-induced lipid peroxidation and chlorophyll degradation on lichen X. parietina thalli through an increase in psbA transcript levels and activity of glutathione reductase (GR) enzymes.

Nitrogen tolerance in the lichen Xanthoria parietina: the sensitive side of a resistant species

To investigate the mechanisms of nitrogen (N) tolerance in lichens, we examined the physiological responses to increased N availability in different functional groups. Thalli of the nitrophytic Xanthoria parietina (L.) Th.Fr. previously grown both in an N-poor environment (~2kgNha-1year-1) and in an N-rich environment (~52kgNha-1year-1) were compared with the oligotrophic species Evernia prunastri (L.) Ach. and Usnea sp. Lichens were submitted to ammonium treatments. Maximum PSII efficiency, redistribution of the ions between the intra- and extracellular compartments and potassium and magnesium concentrations were the parameters used to check for the effects of N supply. The buffering capacity of the lichen extracts was also determined in untreated lichen thalli to check if different lichen behaviours were due to their ability to maintain the pH. The results showed a more similar response between X. parietina from the N-poor environment and the N-sensitive species than between X. parietina from the N-poor and N-rich environments, suggesting that X. parietina achieved N-tolerance after long-term exposure to N-rich environment. These results are important in understanding the effects of chronic ammonium pollution on one of the most sensitive components of the ecosystem, linking physiological response and ecological consequences.

Physiological response of the epiphytic lichen Evernia prunastri (L.) Ach. to ecologically relevant nitrogen concentrations

This study investigated the physiological response of the epiphytic lichen Evernia prunastri to ecologically relevant concentrations of nitrogen compounds. Lichen samples were sprayed for 4 weeks either with water or 50, 150 and 500 μM NH(4)Cl. The integrity of cell membranes and chlorophyll a fluorescence emission (F(V)/F(M) and PI(ABS)) were analyzed. No membrane damage occurred after the exposure period. F(V)/F(M), a classical fluorescence indicator, decreased during the second week of treatment with 500 μM NH(4)Cl and the third week with 50 and 150 μM NH(4)Cl. PI(ABS), an overall index of the photosynthetic performance, was more sensitive and decreased already during the first week with 500 μM NH(4)Cl and the second week with 150 μM NH(4)Cl. Since E. prunastri has been exposed to ammonium loads corresponding to real environmental conditions, these findings open the way to an effective use of this species as early indicators of environmental nitrogen excess.

Physiological responses of root-less epiphytic plants to acid rain

Selected physiological responses of Tillandsia albida (Bromeliaceae) and two lichens (Hypogymnia physodes and Xanthoria parietina) exposed to simulated acid rain (AR) over 3 months were studied. Pigments were depressed in all species being affected the most in Tillandsia. Amounts of hydrogen peroxide and superoxide were elevated and soluble proteins decreased only in AR-exposed Hypogymnia. Free amino acids were slightly affected among species and only glutamate sharply decreased in AR-exposed Xanthoria. Slight increase in soluble phenols but decrease in flavonoids in almost all species suggests that the latter are not essential for tolerance to AR. Almost all phenolic acids in Tillandsia leaves decreased in response to AR and activities of selected enzymes (phenylalanine ammonia-lyase, polyphenol oxidase, ascorbate- and guaiacol-peroxidase) were enhanced by AR. In lichens, considerable increase in metabolites (physodalic acid, atranorin and parietin) in response to AR was found but amount of ergosterol was unchanged. Macronutrients (K, Ca, Mg) decreased more pronouncedly in comparison with micronutrients in all species. Xanthoria showed higher tolerance in comparison with Hypogymnia, suggesting that could be useful for long-term biomonitoring.

Do lichens have "memory" of their native nitrogen environment?

This study aimed to deepen the knowledge about intraspecific mechanisms regulating nitrogen tolerance in lichens to wet nitrogen deposition. Thalli of the nitrophilous lichen Xanthoria parietina were collected from environments with different nitrogen availabilities and immersed in 80 mL of ammonium sulphate (NH₄)₂SO₄ solutions with distinct concentrations (0, 0.025, 0.05 and 0.25 M) for 5 h per day during 3 days in a week. After each soaking event, lichens were air dried. After each treatment, maximal PSII efficiency, localization of ammonium ions, concentrations of K+ and Mg²+ and thalli buffer capacity were determined. Our results show that lichens are marked by their native nitrogen environment, since there were important differences between the physiological responses of X. parietina thalli previously grown in an area with high nitrogen deposition (nitrogen emissions of ca. 13,000 t/year) and those previously grown in an unpolluted area (nitrogen emissions of ca. 500 t/year). Greater N availability seems to enable X. parietina to cope better with the effects of nitrogen pollution.

Time- and dose-dependency of the effects of nitrogen pollution on lichens

The present work aims at testing if exposure time and dose play a role in the response of lichen species to nitrogen (N) pollution. To this purpose, samples of the N-sensitive Evernia prunastri and the N-tolerant Xanthoria parietina were treated for 5 weeks either with solutions of NH(4)NO(3) 0.05 and 1 M, or (NH(4))(2)SO(4) 0.025 and 0.5 M. Photosynthetic efficiency was measured as an indicator of sample vitality. The results showed that the lowest concentrations were ineffective at the beginning, but after several supplies both compounds inhibited photosynthetic activity of E. prunastri. The highest concentrations had a deleterious effect, but with a temporal trend. For X. parietina no effect was found for the lowest concentrations, while the same trend shown by E. prunastri was instead observed following treatments with the highest concentrations. It was concluded that the response of lichens to N supply is not only species-specific, but also time- and dose-dependent. The results give a clue on field studies on the relationships between lichens and N pollution.

Effects of ammonia from livestock farming on lichen photosynthesis

This study investigated if atmospheric ammonia (NH3) pollution around a sheep farm influences the photosynthetic performance of the lichens Evernia prunastri and Pseudevernia furfuracea. Thalli of both species were transplanted for up to 30 days in a semi-arid region (Crete, Greece), at sites with concentrations of atmospheric ammonia of ca. 60 microg/m3 (at a sheep farm), ca. 15 microg/m3 (60 m from the sheep farm) and ca. 2 microg/m3 (a remote area 5 km away). Lichen photosynthesis was analysed by the chlorophyll a fluorescence emission to identify targets of ammonia pollution. The results indicated that the photosystem II of the two lichens exposed to NH(3) is susceptible to this pollutant in the gas-phase. The parameter PI(ABS), a global index of photosynthetic performance that combines in a single expression the three functional steps of the photosynthetic activity (light absorption, excitation energy trapping, and conversion of excitation energy to electron transport) was much more sensitive to NH3 than the FV/FM ratio, one of the most commonly used stress indicators.

Ammonium and nitrate tolerance in lichens

Since lichens lack roots and take up water, solutes and gases over the entire thallus surface, these organisms respond more sensitively to changes in atmospheric purity than vascular plants. After centuries where effects of sulphur dioxide and acidity were in the focus of research on atmospheric chemistry and lichens, recently the globally increased levels of ammonia and nitrate increasingly affect lichen vegetation and gave rise to intense research on the tolerance of lichens to nitrogen pollution. The present paper discusses the main findings on the uptake of ammonia and nitrate in the lichen symbiosis and to the tolerance of lichens to eutrophication. Ammonia and nitrate are both efficiently taken up under ambient conditions. The tolerance to high nitrogen levels depends, among others, on the capability of the photobiont to provide sufficient amounts of carbon skeletons for ammonia assimilation. Lowly productive lichens are apparently predisposed to be sensitive to excess nitrogen.

Mixotrophic and photoheterotrophic metabolism in Cyanothece sp. ATCC 51142 under continuous light

The unicellular diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 (Cyanothece 51142) is able to grow aerobically under nitrogen-fixing conditions with alternating light-dark cycles or continuous illumination. This study investigated the effects of carbon and nitrogen sources on Cyanothece 51142 metabolism via (13)C-assisted metabolite analysis and biochemical measurements. Under continuous light (50 mumol photons m(-2) s(-1)) and nitrogen-fixing conditions, we found that glycerol addition promoted aerobic biomass growth (by twofold) and nitrogenase-dependent hydrogen production [up to 25 mumol H(2) (mg chlorophyll)( -1) h(-1)], but strongly reduced phototrophic CO(2) utilization. Under nitrogen-sufficient conditions, Cyanothece 51142 was able to metabolize glycerol photoheterotrophically, and the activity of light-dependent reactions (e.g. oxygen evolution) was not significantly reduced. In contrast, Synechocystis sp. PCC 6803 showed apparent mixotrophic metabolism under similar growth conditions. Isotopomer analysis also detected that Cyanothece 51142 was able to fix CO(2) via anaplerotic pathways, and to take up glucose and pyruvate for mixotrophic biomass synthesis.

The integrity of lichen cell membrane as a suitable parameter for monitoring biological effects of acute nitrogen pollution

This study aimed at testing the suitability of cell membrane integrity in the lichen Evernia prunastri (L.) Ach. as sensitive indicator of nitrogen (N) stress, to set up a rapid and effective method for monitoring biological effects of acute N pollution. Lichen samples were incubated in solutions of potassium nitrate, ammonium nitrate and ammonium sulphate at different concentrations, and cell membrane damage, expressed in terms of increased electrolyte leakage, was measured after 0, 24, 48 and 96 h. Cell membrane damage was observed in E. prunastri in the presence of high or very high N concentrations, irrespective of the compound supplied. Since the mycobiont represents the large majority of a lichen biomass, it is reasonable to assume that ion leakage mainly occurred from fungal cells. Although in biomonitoring studies the photobiont is usually regarded as the most sensitive partner of the lichen symbiosis, our findings suggest that the mycobiont is most affected in the case of N-excess, and that this feature can be used as suitable indicator of acute N stress episodes.