The terms bioindicator, biomonitor, bioaccumulator, and biomarker have all been used in varying ways to describe different approaches and techniques for studying biological responses to air pollution. In general, however, the field of biomonitoring can be seen as both a qualitative (bioindicator) and quantitative (biomonitor) approach to pollution control.
Biomonitors hold quantitative information on the health of an ecosystem. A biomonitor is also a bioindicator, except that it quantifies the impact or eventual outcome on an organism or ecosystem.
Bioindicators provide a pollutant measurement which can be compared with an instrument measurement. Bioindicators provide information on the quality of the environment and the actual condition of an organism or ecosystem, effectively a 'snap shot' of an ecosystem's health.
Bioindication can be active, for example in the use of deliberately-introduced plants, or passive, where organisms already present in the ecosystem are examined for their reactions.
Bioaccumulators are organisms that accumulate pollutants within their tissues. They may be less sensitive to, or indeed unaffected by, air pollution but are nevertheless good indicators of the exposure of the ecosystem to a pollutant.
Biomarkers are biochemical, cellular, physiological or behavioural variations in the tissue, body fluids or whole body of an organism that provide evidence of exposure to chemical pollutants, and may (or may not) also indicate a toxic effect.
Well designed biomonitoring schemes can help focus risk assessment and management efforts where they are needed and demonstrate the effectiveness of voluntary or regulatory control measures. Well designed schemes have a number of characteristics and use validated biomonitoring methodologies and indicator organisms that are appropriate to the purposes of the scheme. Osborn et al (2000) and Pakeman et al (2000) discuss the key features of monitoring schemes and methodologies and suggest types of organisms that might be used as indicators and different ways in which they can be deployed.
Essentially receptors must be:
- readily identifiable in the field;
- widely distributed within the geographical range of predicted emissions;
- easy to sample repeatedly throughout the year, to capture temporal variability;
- affordable, where the cost of sampling, including the cost of any laboratory analyses that may follow, should be such that sufficient replicates and statistical rigour can be obtained.
Although biomonitoring techniques are usually simple, non-continuous and relatively inexpensive on an individual basis, the natural variability of the environment may require large numbers of samples to be taken to meet the required statistical precision. Consequently, biomonitoring should not necessarily be seen as a low-cost approach to compliance assessment. The requirement for a statistically sound method places stringent constraints on the sampling design. Knowledge of the potential confounding factors that modify direct and indirect responses of biota to air pollutants is crucial.
Approaches of this kind will be very appropriate to those regulatory regimes aimed at preventing the quality of the environment from deteriorating or ensuring that its quality is enhanced. The Birds and the Habitats Directives are based on such regimes. Some approaches of this kind (relevant to soil organisms and the soil ecosystem) are discussed in Spurgeon et a.l 2002 which although written from a contaminated land perspective is based, in part, on work done on soils impacted by air pollution from a smelter (e.g. Long et al., 2008).
Research is needed to validate these new approaches and convert them into practical biomonitoring schemes that would be useful tools in the sustainable management of the environment.
Example Biomonitoring Methods
(taken from Bealey et al 2008)
Measuring the accumulation of chemical elements in plant tissue
Measuring the accumulation of pollutants in plant tissue is a common method that has been used to assess the effects of many pollutants. Total tissue nitrogen (N) has been measured for many years in all types of plant tissue to assess atmospheric nitrogen deposition (Bobbink et al., 1993; Pitcairn et al., 1995, 1998; Leith at al. 2005; Sutton et al, 2004). This method requires both field sampling and laboratory analyses, but is relatively inexpensive compared to other types of physical monitoring. Any pollutant gradient detected may serve as a useful pointer to the source, and will thus be of value for pollutant mapping and source apportionment.
Changes in species composition or diversity provide a useful method for monitoring the status of an ecosystem and the impacts of emissions. Of these methods, Ellenberg indicator values are the best known (Ellenberg, H. (1979)). The Ellenberg N index consists of allocating a N-score to each plant species, so that the overall community has a total score based on a scale ranging from nutrient poor (1) to nutrient rich (10) (Sutton et al, 2004). Exploiter species can increase because of a lack of competition from eliminated species, or because of reduced competition with greater nutrient availability.
Visible injury impact
Symptoms of visible injury can be seen in plants and animals. Typical plant symptoms include leaf chlorosis (loss of chlorophyll), necrosis (tissue death) or abscission (leaf loss) (Taylor et al., 1986), while in mammals symptoms have included dental lesions in voles (Boulton et al., 1994).Visible impacts indicate the possibility of a potentially negative outcome on the plant species or ecosystem. However, visible symptoms may be only temporary, with the plant recovering at a later stage. Many other biotic and abiotic factors can mimic visible injury symptoms (chlorosis, necrosis or abscission of leaves), including drought, cold, heat, sun scald, nutrient deficiencies or excesses, other toxic chemicals (such as pesticides and herbicides), and pests and pathogens. It is therefore important to have some knowledge of the pollutants being emitted and the impacts expected.
Transplants - native and standardised
There is a long history of using standardised and native transplants as active bioindicators and bioaccumulators. For example, the sensitive BelW3 cultivar of the tobacco plant (Nicotiana tabacum), has been used for decades as a bioindicator for exposure to ground-level ozone (Ashmore et al., 1978)
Cytochrome P450 enzyme induction
Induction of cytochrome P450 enzymes is a biomarker of exposure, in that elevated levels in organisms (compared to levels in organisms from reference/clean areas) demonstrate exposure to organic compounds. It is caused by exposure to chemicals such as dioxins, PCBs (polychlorinated biphenyls), polycyclic aromatic hydrocarbons (PAHs) and numerous insecticides including organophosphates (OPs), carbamates and fungicides.
Increase in DNA adduct formation
DNA adducts form following exposure to organic pollutants, which bind to DNA resulting in genotoxicity to the cells. DNA adducts detected in organisms following exposure may lead to genetic changes in these organisms in future. While the pollutants in question may not be acutely toxic, they may result in delayed effects. Measuring DNA adducts can provide an early warning of possible long-term effects. This method is used to monitor PAH contamination in coastal areas of US and also to monitor aflatoxin exposure in humans (Gompertz et al., 1996).
Measuring histopathological lesions involves measuring changes in cellular morphology in the tissues of internal or external organs following exposure to chemicals, for example changes/enlargement of cells, proliferation of white blood cells, changes in epithelial cells and lung alveolar cells. Changes in morphology can be early warning signals (similar to DNA adducts) for diseases that may come to light after exposure has ceased. This method is used as part of a survey of UK coastal waters in the National Marine Monitoring Programme (NMMP) (Marine Pollution Monitoring Management Group, 1998).
Bait lamina strips
These strips measure the overall feeding activity of soil-dwelling organisms (including invertebrates and micro-organisms). They are a fast and inexpensive measure of soil function and can provide an overall indication of the health of an area, though they are not pollutant-specific. The rationale is that disturbed areas will have fewer soil-dwelling organisms and so the feeding rate will be low compared to undisturbed areas (which should have a high feeding activity rate).
The APIS lichen survey to assess nitrogen air quality is a new field guide for using epiphytic lichens to assess atmospheric nitrogen pollution effects on habitats. Accompanying the guide is a field manual which provides further information in support of the guide as well as recording sheets. A lichen indicator score (LIS) is first calculated using the presence or absence of N-sensitive and N-tolerant lichen species on either trunks or branches of oak or birch at any given location. Using the LIS score an indication of air quality at a given site can be determined. Survey results can be uploaded to the website to provide an interactive map of nitrogen air quality across the UK.
The OPAL air survey is studying lichens found on trees and also looking for tar spot fungus on sycamore leaves. Both can tell us a great deal about local air quality.