Lichens serve as important bioindicators of air pollution in cities. Here, we studied the diversity of epiphytic lichens in the urban area of Munich, Bavaria, southern Germany, to determine which factors influence species composition and diversity. Lichen diversity was quantified in altogether 18 plots and within each, five deciduous trees were investigated belonging to on average three tree species (range 1–5). Of the 18 plots, two were sampled in control areas in remote areas of southern Germany. For each lichen species, frequency of occurrence was determined in 10 quadrats of 100 cm² on the tree trunk. Moreover, the cover percentage of bryophytes was determined and used as a variable to represent potential biotic competition. We related our diversity data (species richness, Shannon index, evenness, abundance) to various environmental variables including tree traits, i.e. bark pH levels and species affiliation and air pollution data, i.e. NO₂ and SO₂ concentrations measured in the study plots. The SO₂ levels measured in our study were generally very low, while NO₂ levels were rather high in some plots. We found that the species composition of the epiphytic lichen communities was driven mainly by NO₂ pollution levels and all of the most common species in our study were nitrophilous lichens. Low NO₂ but high SO₂ values were associated with high lichen evenness. Tree-level lichen diversity and abundance were mainly determined by tree traits, not air pollution. These results confirm that ongoing NO₂ air pollution within cities is a major threat to lichen diversity, with non-nitrophilous lichens likely experiencing the greatest risk of local extinctions in urban areas in the future. Our study moreover highlights the importance of large urban green spaces for species diversity. City planners need to include large green spaces when designing urban areas, both to improve biodiversity and to promote human health and wellbeing.

Microplastics have been discovered ubiquitously in marine environments. While their accumulation is noted in seagrass ecosystems, little attention has yet been given to microplastic impacts on seagrass plants and their associated epiphytic and sediment communities. We initiate this discussion by synthesizing the potential impacts microplastics have on relevant seagrass plant, epiphyte, and sediment processes and functions. We suggest that microplastics may harm epiphytes and seagrasses via impalement and light/gas blockage, and increase local concentrations of toxins, causing a disruption in metabolic processes. Further, microplastics may alter nutrient cycling by inhibiting dinitrogen fixation by diazotrophs, preventing microbial processes, and reducing root nutrient uptake. They may also harm seagrass sediment communities via sediment characteristic alteration and organism complications associated with ingestion. All impacts will be exacerbated by the high trapping efficiency of seagrasses. As microplastics become a permanent and increasing member of seagrass ecosystems it will be pertinent to direct future research towards understanding the extent microplastics impact seagrass ecosystems.

Critical loads are thresholds of atmospheric deposition below which harmful ecological effects do not occur. Because lichens are sensitive to atmospheric deposition, lichen-based critical loads can foreshadow changes of other forest processes. Here, we derive critical loads of nitrogen (N) and sulfur (S) deposition for continental US and coastal Alaskan forests, based on nationally consistent lichen community surveys at 8855 sites. Across the eastern and western US ranges of 459 lichen species, each species' realized optimum was the N or S atmospheric deposition value at which it most frequently occurred. The mean of optima for all species at a site, weighted by their abundances, was defined as a community “airscore” indicative of species’ collective responses to atmospheric deposition. To determine critical loads for adverse community compositional shifts, we then modeled changes in airscores as a function of deposition, climate and forest habitat predictors in nonparametric multiplicative regression. Critical loads, indicative of initial shifts from pollution-sensitive toward pollution-tolerant species, occurred at 1.5 kg N ha⁻¹ y⁻¹ and 2.7 kg S ha⁻¹ y⁻¹. Importantly, these critical loads remain constant under any climate regime nationwide, suggesting both simplicity and nationwide applicability. Our models predict that preventing excess N deposition of just 0.2–2.0 kg ha⁻¹ y⁻¹ in the next century could offset the detrimental effects of predicted climate warming on lichen communities. Because excess deposition and climate warming both harm the most ecologically influential species, keeping conditions below critical loads would sustain both forest ecosystem functioning and climate resilience.

Multi-elemental isotopic approach associated with a land-use characteristic sampling strategy may be relevant for conducting biomonitoring studies to determine the spatial extent of atmospheric contamination sources. In this work, we investigated how the combined isotopic signatures in epiphytic lichens of two major metallic pollutants, lead (²⁰⁶Pb/²⁰⁷Pb) and mercury (δ²⁰²Hg, Δ¹⁹⁹Hg), together with the isotopic composition of nitrogen and carbon (δ¹⁵N, δ¹³C), can be used to better constrain atmospheric contamination inputs. To this end, an intensive and integrated sampling strategy based on land-use characteristics (Geographic information system, GIS) over a meso-scale area (Pyrénées-Atlantiques, SW France) was applied to more than 90 sampling stations. To depict potential relationships between such multi-elemental isotopic fingerprint and land-use characteristics, multivariate analysis was carried out. Combined Pb and Hg isotopic signatures resolved spatially the contribution of background atmospheric inputs from long range transport, from local legacy contamination (i.e. Pb) or actual industrial inputs (i.e. Pb and Hg from steel industry). Application of clustering multivariate analysis to all studied isotopes provided a new assessment of the region in accordance with the land-use characteristics and anthropogenic pressures.

Highways are major sources of nitrogen dioxide (NO2) and ammonia (NH3). In this study, springtime NO2 and NH3 concentrations were measured at 17 Ontario Forest Biomonitoring Network (OFBN) plots using passive samplers. Average springtime NO2 concentrations were between 1.3 μg m−3 and 27 μg m−3, and NH3 concentrations were between 0.2 μg m−3 and 1.7 μg m−3, although concentrations measured in May (before leaf out) were typically twice as high as values recorded in June. Average NO2 concentrations, and to a lesser extent NH3, could be predicted by road density at all radii (around the plot) tested (500 m, 1000 m, 1500 m). Springtime NO2 concentrations were predicted for a further 50 OFBN sites. Normalized plant/lichen N concentrations were positively correlated with estimated springtime NO2 and NH3 concentrations. Epiphytic foliose lichen richness decreased with increasing NO2 and NH3, but vascular plant richness was positively related to estimated springtime NO2 and NH3.

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 NH₄Cl. The integrity of cell membranes and chlorophyll a fluorescence emission (FV/FM and PIABS) were analyzed. No membrane damage occurred after the exposure period. FV/FM, a classical fluorescence indicator, decreased during the second week of treatment with 500 μM NH₄Cl and the third week with 50 and 150 μM NH₄Cl. PIABS, an overall index of the photosynthetic performance, was more sensitive and decreased already during the first week with 500 μM NH₄Cl and the second week with 150 μM NH₄Cl. 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.

We investigated lichen diversity in temperate oak forests using standardized protocols. Forty-eight sites were sampled in the Czech Republic, Slovakia and Hungary. The effects of natural environmental predictors and human influences on lichen diversity (lichen diversity value, species richness) were analysed by means of correlation tests. We found that lichen diversity responded differently to environmental predictors between two regions with different human impact. In the industrial region, air pollution was the strongest factor. In the agricultural to highly forested regions, lichen diversity was strongly influenced by forest age and forest fragmentation. We found that several natural factors can in some cases obscure the effect of human influences. Thus, factors of naturality gradient must be considered (both statistically and interpretively) when studying human impact on lichen diversity. We detected the different responses of lichens to ecological predictors in polluted and unpolluted areas.

Submersed macrophytes accumulate large amounts of macro- and trace elements from the environment and, therefore, are frequently used as indicators of water pollution and tools to remove pollutants from contaminated waters. This study provides evidences that the quantity of macro- and trace elements accumulated in the macrophyte Ceratophyllum demersum depends strongly on the seasonality, on the vertical position of the plant material and on the biofilm cover. Element contents of macrophytes with and without biofilm cover and that of vertical plant sections were investigated by an ICP-MS technique in three different habitats, at the beginning and at the end of the vegetation period. Results demonstrated that the element concentrations of Ceratophyllum demersum dropped to one-half and one-eighth by the end of the summer; and the amount of certain elements in the lower part of plants were up to six times higher than in the upper and in plants with well-developed epiphytic microbial community 2-5-fold higher than in plants without biofilm.These results help in phytoremediation practice and in setting up future biomonitoring studies. When it is necessary to calculate the exact amount of elements which can be accumulated by plants in a polluted environment or should be removed from a contaminated water by harvesting macrophytes, it is of high importance to consider the month of the study, the plant parts harvested and the biofilm cover.

Microcystins (MCs) are toxins produced during cyanobacterial blooms. They reach soil and translocated to plants through irrigation of agricultural land with water from MC-impacted freshwater systems. To date we have good understanding of MC effects on plants, but not for their effects on plant-associated microbiota. We tested the hypothesis that MC-LR, either alone or with other stressors present in the water of the Karla reservoir (a low ecological quality and MC-impacted freshwater system), would affect radish plants and their rhizospheric and phyllospheric microbiome. In this context a pot experiment was employed where radish plants were irrigated with tap water without MC-LR (control) or with 2 or 12 μg L⁻¹ of pure MC-LR (MC2 and MC12), or water from the Karla reservoir amended (12 μg L⁻¹) or not with MC-LR. We measured MC levels in plants and rhizospheric soil and we determined effects on (i) plant growth and physiology (ii) the nitrifying microorganisms via q-PCR, (ii) the diversity of bacterial and fungal rhizospheric and epiphytic communities via amplicon sequencing. MC-LR and/or Karla water treatments resulted in the accumulation of MC in taproot at levels (480–700 ng g⁻¹) entailing possible health risks. MC did not affect plant growth or physiology and it did not impose a consistent inhibitory effect on soil nitrifiers. Karla water rather than MC-LR was the stronger determinant of the rhizospheric and epiphytic microbial communities, suggesting the presence of biotic or abiotic stressors, other than MC-LR, in the water of the Karla reservoir which affect microorganisms with a potential role (i.e. pathogens inhibition, methylotrophy) in the homeostasis of the plant-soil system. Overall, our findings suggest that MC-LR, when applied at environmentally relevant concentrations, is not expected to adversely affect the radish-microbiota system but might still pose risk for consumers’ health.

A national citizen survey quantified the abundance of epiphytic lichens that are known to be either sensitive or tolerant to nitrogen (N) deposition. Records were collected across the UK from over 10,000 individual trees of 22 deciduous species. Mean abundance of tolerant and sensitive lichens was related to mean N deposition rates and climatic variables at a 5 km scale, and the response of lichens was compared on the three most common trees (Quercus, Fraxinus and Acer) and by assigning all 22 tree species to three bark pH groups. The abundance of N-sensitive lichens on trunks decreased with increasing total N deposition, while that of N-tolerant lichens increased. The abundance of N-sensitive lichens on trunks was reduced close to a busy road, while the abundance of N-tolerant lichens increased. The abundance of N-tolerant lichen species on trunks was lower on Quercus and other low bark pH species, but the abundance of N-sensitive lichens was similar on different tree species. Lichen abundance relationships with total N deposition did not differ between tree species or bark pH groups. The response of N-sensitive lichens to reduced nitrogen was greater than to oxidised N, and the response of N-tolerant lichens was greater to oxidised N than to reduced N. There were differences in the response of N-sensitive and N-tolerant lichens to rainfall, humidity and temperature. Relationships with N deposition and climatic variables were similar for lichen presence on twigs as for lichen abundance on trunks, but N-sensitive lichens increased, rather than decreased, on twigs of Quercus/low bark pH species. The results demonstrate the unique power of citizen science to detect and quantify the air pollution impacts over a wide geographical range, and specifically to contribute to understanding of lichen responses to different chemical forms of N deposition, local pollution sources and bark chemistry.