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Optimising the use of lichens as biomonitors of atmospheric PAHs

Final Report Summary - POPLAIR (Optimising the use of lichens as biomonitors of atmospheric PAHs)

THE MAIN OBJECTIVE OF POPLAIR was to standardize the use of lichens as a method to monitor environmental polycyclic aromatic hydrocarbons (PAHs) and to provide a tool to complement existent air monitoring systems.

Lichens are symbioses of fungi and algae and/or cyanobacteria, which can be found growing almost everywhere in the planet, and which are outstanding accumulators of air pollutants. Unlike plants, lichens lack roots, and hence absorb nutrients and pollutants directly from the atmosphere. During decades, lichens have been successfully used to monitor a wide variety of air pollutants, and most recently to monitor PAHs.

PAHs are a large group of semi-volatile organic compounds, which have received increased attention in air pollution studies due to their carcinogenicity and mutagenicity. Measurements of PAHs in air are performed worldwide at air quality monitoring stations using active samplers. Active samplers capture the air forcing it to pass through a filter, which retains the particle-bound PAHs, and a polyurethane foam, which captures the gas phase PAHs. Air quality monitoring stations have the advantage of measuring PAHs in standard conditions; however, due to the high cost of installation, energy supply and maintenance, the number of stations is not enough to allow obtaining spatially resolute data. To overcome some of these limitations, the same Directive that regulates PAH concentrations in air (Directive 2004/107/EC from the European Union Law) recommends the use of other monitoring methods, notably living organisms, to complement data and to assess spatial deposition of PAHs.

Despite the increasing number of studies using lichens to monitor PAHs, there was still a gap of knowledge regarding how lichens intercept and accumulate gas phase and particle-bound PAHs, which factors contribute for the accumulation, and how PAHs impact lichens’ functioning.

WORK CARRIED OUT: Trying to accomplish the main objective, POPLAIR was divided into four tasks. In task 1 it was studied how lichens intercept and accumulate gas phase and particle-bound PAHs. Lichens were exposed to gas phase and particle-bound PAHs in laboratorial controlled conditions, as well as in outdoor environments. Making use of cutting-edge microscopy techniques, the fate of PAHs inside the lichens was tracked. In task 2 it was investigated if lichens are affected by PAH pollution. Different lichen species were exposed in vitro to increasing concentrations of PAHs and potential physiological impacts were measured. In task 3 it was studied to which extend the PAHs accumulated in lichens are representative of the atmospheric gas phase and particle-bound PAHs. For that, PAHs accumulated in lichens were compared with PAHs measured in air using passive and active air samplers. Finally, in task 4 it was studied how the use of lichens to monitor PAHs can be standardized so that it can be accepted by the stakeholders as a reliable monitoring technique.

MAIN RESULTS AND CONCLUSIONS: POPLAIR has revealed that accumulation of PAHs by lichens seems to be affected by a number of factors. One of the factors is related to the physico-chemical properties of each pollutant, together with environmental variables, which will dictate in which phase (gas or particle-bound) pollutants will be in contact with the lichen. Gas phase PAHs are readily uptaken by the lichen, apparently through a process where the alga plays a major role; whereas particle-bound PAHs seem to follow a similar accumulation process to the one reported for particle-bound metals.

Another factor affecting accumulation is related to the structure and composition of each lichen species. Whilst fruticose lichens tend to accumulate highest concentrations of low molecular weight PAHs; foliose lichens tend to accumulate highest concentrations of high molecular weight PAHs. The algae content of each lichen species has revealed to be a key factor on the uptake of gas phase PAHs, as species with higher algae content were observed to accumulate higher concentrations of these pollutants. Because of the accumulation of PAHs in lichens, some physiological effects for the algae, such as inhibition of the photosynthetic activity of the algae, degradation of chlorophyll, and production of stress proteins were observed. In POPLAIR it was found a level of PAHs that caused cell membrane disruption in lichens, being critical for lichens’ functioning. This critical level was assumed to be a reflex of the acute toxicity of the PAHs for the fungus partner of the lichen symbiosis (composing 90 % of the lichen thallus).

A third factor affecting accumulation of PAHs by lichens are environmental factors, notably climatic factors, such as humidity, temperature, sunlight, rain, wind. These may influence the uptake, as well as the loss of PAHs. Hydrated lichens promptly accumulate gas phase pollutants, probably due to the increased activity of the algae. Particle-bound PAHs are likely to be accumulated inside the lichen thallus in the intercellular spaces and/or retained at the lichen’s surface, where they can be washed-off by the rainfall or blow-off by the wind. Temperature may cause re-volatilization of PAHs from the lichen thallus and sunlight may have a role in the degradation of PAHs when they deposit on the lichen’s surface. Once accumulated inside the lichen thallus, PAHs may be protected from sunlight by the layer of pigments that some lichen species have on their surface. This layer of pigments has the role of protecting the algae from UV light, and thus, being PAHs sensitive to UV radiation, they are protected as well.

POTENTIAL IMPACT AND USE: From an air monitoring standpoint, POPLAIR has revealed that lichens work as a 2-in-1 tool in the sense that they accumulate POPs in both the gas phase and bound to particles in a continuous way during the time they are exposed (lifetime in the case of native lichens). This continuous uptake is the responsible for the high concentrations of POPs found in lichens in comparison with other monitors. The continuous uptake of POPs, together with loss processes, results in a dynamic equilibrium with the atmospheric levels, which allows calibrating PAH levels in lichens and in air. The use of lichens to monitor PAHs is especially useful to assess the spatial impact of PAHs emitted by known pollution sources, as well as to identify unexpected sources and to detect fugitive emissions. Lichens allow obtaining spatial and temporal resolute pollution data, which can be useful for health risk assessment.

SOCIO-ECONOMIC IMPACT: POPLAIR fits perfectly with the common European target of developing techniques for environmental biomonitoring of pollutants. The results obtained during the project enable to optimize the use of lichens to monitor PAHs and related pollutants in the industrial sector for environmental assessment studies. Additionally, the techniques explored by POPLAIR allow policy makers having a low-cost tool to manage and control pollution at a large spatial and temporal scale. Finally, during the project it was developed an educational card game about how lichen biodiversity is affected by environmental factors, which was tested in schools and proved to be effective to enhance students’ attention and retention.

TARGET GROUPS: Industrial sector, policy makers, environmental professionals (engineers, biologists, and ecologists), educational sector, and the civil society.

WEBSITE: Information of POPLAIR project (publications, activities carried out, etc.) can be found at