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published on 10 May 2008 in air

Bioindicators

Monitoring the environment
Ecologic research is more and more oriented towards an assessment of the quality of the environment and its resources. The techniques and instruments that are required for carrying out such a demanding task are many and increasingly sophisticated. Often the realization of an environmental assessment plan must take into account the complex nature of the factors that influence the environment (direct and indirect anthropical factors and natural factors), the large number of criteria that condition the choice of the type of intervention (types of polluting agents, characteristics of the environment in which operations are carried out, the aims, times, etc.) and in particular it must overcome the main obstacle that  prevents constant surveillance operations of the natural/environmental heritage : the economic expense.
An example is clearer than many words: discovering that a particular area houses a dangerous pollutant, as for example dioxin, does not indicate the possible presence, in the same area, of other pollutants that may be equally dangerous. In fact it isn’t possible to search for each and every pollutant that may theoretically be found in the environment because a similar strategy would probably subtract resources from those areas in which the environmental risks have been documented and verified.
For this reason scholars turn more and more frequently towards the contribution that is made available by a scientific sector that is rapidly growing, that of environmental biomonitoring: the study of the quality of the environment with the help of biological indicators.
Environmental biomonitoring not only proves how useful natural sciences can be to point out the damages caused on the environment by human activities, it also provides proof of how the environment can be kept under control at lower costs respect to those foreseen by other methods. In this case, furthermore, the results of a basic naturalistic science are a useful source of information for other fields of inquiry, specially in the biomedical field.

The identity card
Apart from the term itself, that may seem difficult to explain in simple words, a bioindicator is no more than a living “organism” (or a set of organisms) characterized by particular traits that are very useful for the work of researchers. In practice, a bioindicator is an animal, vegetable or microbial system whose morphological variations or changes in behaviour can be used to formulate conclusions regarding the environmental conditions they are to be found in.
So, the first fundamental characteristic of a bioindicator, is that it must be able to respond to the chemical changes in the environment through alterations in its spontaneous condition. The second fundamental characteristic is that these alterations, in order to really provide information, must be easy to measure by the researchers, possibly at costs that are not too high.
It is evident that  bioindicators may also include organisms that vary greatly in their biological organization and ecological function.  Autotrophic species and also the heterotrophic species can be good bioindicators, or the taxa of a taxonomic range that is superior to that of the species (for example a family), and in some cases even the entire biocenosis of an ecosystem. Due to this enormous potential variability of the bioindicators, it may be added that the biological responses that may be useful for the studies on bioindication, are equally variable.
This consideration however needs an important specification: if it is true that almost any organism may “indicate” a process (that is taking place, or which has taken place) of deterioration of the environment, it is also true that in actual fact some organisms carry out this function much better than others, for two fundamental reasons:

  • because by their nature they “express” the chemical variations in the environment more faithfully and rapidly;
  • because their biological parameters are easier (and cheaper) to determine than those of others.

In general, therefore, the inalienable requisite that defines a good environmental  bioindicator is as follows: it must be widely distributed and easy to identify,  to sample and analyze.

Sensibility
More simply, any alterations in the habitat enable the identification of two types of reactions in living creatures: reactions involving sensibility, and reactions involving tolerance. In some proportion, it is on these two quite general classes of biological reactions that bioindication procedures are based.
Therefore at this stage a compulsory question arises: with what criteria are bioindicators of the first, rather than of the second type used?
The question is important, because when the time comes to decide to inquire on the biological impact of pollutants in order to programme environmental maintenance interventions, it is necessary to act at the lowest cost, and with good probabilities of success. In other words,  both the costs and advantages that the environmental monitoring strategy involves must be estimated correctly. However, for this reason it is indispensable that at least a general picture of the phenomena that are to be analyzed is available from the very beginning. In practice, preliminary information that can enable the researchers to use the techniques and instruments that are most suited to the particular type of pollution that they are required to control, is necessary.
For particular toxic agents that currently represent a typical component of environmental pollution, biomonitoring becomes more efficient if organisms of the first type are exploited, i.e. those which do not show much tolerance to chemical changes in the environment.
Lichens for example are organisms that are particularly suited to indicate the diffusion of these toxic agents, specially in the case of atmospheric pollutants such as sulphur dioxide: in this case bioindication is based on the assessment of their presence and abundance in the examined area. In applied ecological research these bioindicators have given proof of their efficacy,  so much so that they have been used to define pure air indexes in geographical districts affected by atmospheric emissions. What those in the sector define as  “lichen desert” – a phenomenon that can be noted in urban areas where atmospheric pollution is very high –  describes a degenerative situation  caused by the chemical components in the air, provoked, in fact, by an excessive concentration of sulphur dioxide. As it is lichen-toxic, in fact, sulphur dioxide in these cases is responsible for the total absence of lichen communities in the environment, which points out a high level of pollution, which is severe also because of its consequences on human health (sulphur dioxide is also toxic for humans).
However there are other cases in which it is possible to use the sensitive types of bioindicators, which show alterations in their parts and functions, such as leaf necrosis in the tobacco plant in the presence of photochemical pollution (ozone) or ecological characteristics, such as their distribution in space and/or time in the habitat, which guarantee other very efficient solutions to monitor the environment. Scientific literature today accounts for an extraordinary spectrum of bioindicators not only for atmospheric pollution but also for water and soil pollution, without forgetting that excellent bioindicators can also be parts of our own organism, and our metabolism.

Tolerance and bioaccumulation
As mentioned above, however, there is also an alternative strategy to monitor the chemical interference in the environment, by biological methods. This strategy is based on the use of the other class of bioindicators: the bioindicators that indicate tolerance. This second group of organisms, unlike the first group, not only has a significant resistance to the substances polluting the environment, but also manages to stock them in its tissues. The phenomenon is known to ecologists as “bioaccumulation”, and enables researchers to measure the concentrations of polluting agents, and it is also possible to assess the time of exposure, integrating basic information and data regarding the absorption rate of the materials by the single organisms.
Typical pollutants that can be monitored through bioaccumulation are, for example, heavy metals, that are easily intercepted by various types of organisms, among which, today, the most important and reliable are mosses. In fact, since these plants do not have any protective external cuticles and are provided with many sites with a negative load in the cell walls, they act as powerful electrostatic exchangers that can capture positive particles even at very low concentrations, thus being transformed into excellent heavy metal accumulators. For these and other peculiarities, some mosses are currently designated  among the more recommended biological systems for environmental assessment of low-concentration polluting agents. Through the same mechanism, furthermore, mosses also offer the possibility of  monitoring aromatic polycyclic hydrocarbons (APH) and the so-called persistent organic contaminants (such as many solvents and pesticides) that are discharged solely by humans and do not involve the inconvenience of having to discriminate  their possible spontaneous presence in the environment (an inconvenience that can occur with heavy metals instead).

Written by Carlo Modenesi

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
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