31 Bioindicators

Dr Sunil Mittal and Dr Hardeep Kaur

1. Introduction

The term ‘Bioindicators’ refers to any organisms, species, communities or biological processes, which help to qualitatively monitor the status of the environment in which they live by means of their function as well as their population. The occurrence of any environmental change or problem within an ecosystem can be predicted very easily by any changes in the population status, behavior, and physiology of such organisms. The identification of any species as a bioindicator is reflected by any fluctuation in the abundance of the species in response to any environmental change in a particular habitat. Bioindicators are useful in providing information about the health of an ecosystem since the members of these species are highly sensitive to the changes in their surroundings (figure 1). Therefore, the sampling and studying about the population dynamics of such organisms makes it possible to monitor any ecological change. This in turn, helps in identifying the positive and negative impacts of human activities in the ecosystem. A bioindicator has specific requirements regarding some physical or chemical variables, such that variations in the presence or absence, morphology, physiology, population or behavior of the given species suggest that the given physical or chemical variables are outside their preferred limits. Generally, bioindicators are designated as species which respond to anthropogenic impacts on the environment. An all-inclusive definition of a bioindicator states: “a species or a group of species that voluntarily imitates the abiotic or biotic state of an environment, represents the effect of environmental change on a habitation, community or ecosystem or is indicative of the diversity of a subset of taxa or the whole diversity within an area”. Bioindicators help in the assessment of the quality of the environment and how it changes over a given period. These can be plants, animals, algae, lichens, zooplanktons, insects, amphipods, molluscs, echinoderms, and other micro-organisms. Even human nail & hair can also be used as bioindicators. Bioindicators find usage in monitoring the quality of air, soil and water in any region and in the assessment of the overall biodiversity. These can also help in the determination of the progress of mitigative measures implemented for environmental conservation and thus play a significant role in nature conservation too. The information provided by bioindicators is adequate and reliable and might be difficult to acquire or compute in such a quick manner using other means.

The usefulness of bioindicators is most observed in given three situations:

when it is impossible to measure the indicated environmental factor when the measurement of the indicated factor is very difficult

when the measurement of the environmental factor is possible and easy but the interpretation is quite difficult

Figure 1: Response of different bioindicators to environmental stress

Source: https://www.nature.com/scitable/knowledge/library/bioindicators-using-organisms-to-measure-environmental-impacts-16821310

2. Characteristics of Bioindicators

The identification of any organism, species or process as an ideal bioindicator requires possession of certain characteristics such as:

• it should be widely distributed
• it should be highly abundant in a certain environment it should be site specific and highly sensitive
• it should have restricted mobility
• it should accumulate and concentrate the toxin to measurable levels
• it’s position in the tropical system should be clear with fine feeding strategy and a constant rate of metabolism
• it should possess a low genetic and ecological variability
• it’s response should be representative of the responses of the other taxa or the ecosystem as a whole
• it should be cost effective and robust while handling
• it’s sampling, sorting as well as storage should be simple
• it should have an environmental significance and economic importance

Figure 2: Characteristics of bioindicators in the context of their use in biomonitoring

3. Classification of Bioindicators

Bioindicators can be classified into different categories (as illustrated in figure 3), on the basis of:

• aim of bioindication
• application of bioindicators
• on the basis of International Union of Biological Sciences (IUBS)

Figure 3: Schematic diagram for classification of bioindicators

3.1. Based on aim of bioindication

When classified on the basis of aim of bioindication, bioindicators are of four types:

i. Early warning indicators: The bioindicators which can show the first signs of disturbance in the environment are called early warning indicators. These reveal signs before most other species are affected since they have very quick and sensitive responses to any environmental change. For e.g ants.

ii. Compliance indicators: The bioindicators which help in confirming the accomplishment of maintenance or restoration goals are known as compliance indicators. For example, measurement of fish characteristics is a bioindicator for the sustainability of the population or community as a whole.

iii. Diagnostic indicators: The indicators which help in the examination of the detected environmental changes or disturbances are known as diagnostic indicators. These indicators are generally measured on the individual or sub-organismal level.

iv. Accumulation indicators: The bioindicators which help in studying the effects on different biological organization levels are known as accumulation indicators. For e.g. mosses, lichens, etc.

3.2. Based on different applications of bioindicators

When classified on the basis of applications, bioindicators are of four types:

i. Environmental bioindicators: these refer to the species which respond to any disturbance in the environment and thus reveal the signs of any environmental change or disturbance. They help in the diagnosis of the environmental status for making any environmental policy. For e.g. animals, macro-invertebrates, sentinels etc.

ii. Ecological bioindicators: The species which are sensitive to fragmentation of the habitat, pollution or other disturbances or stresses in the environment are called ecological indicators. For e.g. lichens, plant indicators etc.

iii. Biodiversity bioindicator: The species which are the indicators for measurable parameters of biodiversity such as genetic parameters, landscape parameters, etc. and represent the species richness of a community are known as biodiversity bioindicators. For e.g. animals, plants and microbial indicators.

iv. Pollution bioindicators: The bioindicators which are helpful in the detection of presence of pollutants in a region are known as pollution bioindicators. For example, animal and plant indicators.

3.3. Classification of bioindicators on the basis of International Union of Biological Sciences (IUBS)

The International Union of Biological Sciences (IUBS) has divided bioindicators into following six groups after recognizing the importance of bioindicators in monitoring environmental changes:

• Microbiology Botany
• Zoology
• Cell Biology and Genetics
• Comparative Physiology
• Hydrobiology

3.3.1 Microbiology

Microbes quickly detect different types of the environmental pollution such as soil and water pollution. Some microbes help in the decomposition of some pollutants while some of them are sensitive to some toxins or harmful substances. The reduction in the population of the sensitive species or increases in the population status of the tolerant species indicate changes taking place in the environment. These changes in the species diversity may be the result of the presence of some toxic substances. For example, presence of sulphur can be easily detected in the microbial muds from continental and intercontinental water bodies. Some microbes are also helpful in the assessment of variations induced in the marine environment by various anthropogenic activities such as E. coli, Pseudomonas, Clostridium, Streptococcus etc. The presence of pesticides in soil is indicated by Cyanobacteria. For example, Nostoc microscopicums indicates the pollution of soil by pesticides such as Dithane, Deltan, Phorate, etc.

Oil spillage or oil pollution can be easily monitored using yeast, actinomycetes, some filamentous fungi, and bacteria. Some microbial species such as Penicillium sps and Aureobasidium pullulans can utilize oil fractions from waste oil.

3.3.2. Botany

This group can further be sub-classified into the following two categories since different groups of plants indicate the changes in the environmental status:

• Lower plants
• Higher plants

A. Lower plants

The taxa involved in lower plants as bioindicators include a wide range of lichens, algae, fungi (leaf yeasts), etc. which help in the detection of both short term as well as long term effects on the basis of appropriate choice of organisms. The tolerance level or resistance towards a substance in an environment varies with species. For example, lichens are considered ideal bioindicators or monitoring agents due to their vulnerability and resilience to different environmental effects. Some lichens can bloom only in the fresh or unpolluted air while some of their species are resistant even to much polluted air. The lichen Lechanora is good biological indicators of a wide spectrum of the environment. The lichen thalli indicate the presence of sulphur dioxide and fluorine in the atmosphere. Lichens are also used for survey of long life. The presence of some radionuclides such as cesium-137, strontium-30 released during nuclear explosions, in the environment can also be monitored using lichens. The lichen thalli are able to absorb fluorine and heavy metals including lead even when they are dead.

Lichens generally thrive on rocks and trees surfaces or soil and do not have roots and cuticle. They attain all their nutrients directly from the atmosphere rather than from the soil. They are such ideal monitoring agents because of their extreme sensitivity to the toxins present in the air as well as due to their high surface area to volume ratio which further leads to the accumulation of toxins or contaminants from the air (figure 4). The lichens are also considered because of being cost effective and inexpensive agents in the evaluation of air pollution when compared to most of the physical and chemical monitors.

Figure 4: Lichens as bioindicators

Source: https://www.nps.gov/articles/lichens-as-bioindicators.htm

Various algae are too used in environmental monitoring such as Viva and Enteromorpha for monitoring the water quality of estuaries, Cladophora and Stigeolonium for heavy metal contamination of water, Chlorella for monitoring the presence of toxic substances in water bodies, Skeletonema costatum, Amphidiwn carterae, Pavlova lutheri, etc. for the indication of oil pollution, etc.

B. Higher plants

A wide range of higher plants serve as bioindicators. Air pollution is monitored using some sensitive species whereas some tolerant (indicator) species are used for the determination of soil conditions. Plants also serve as indicators of presence of various heavy metals such as Al, Ca, Co, Cu, Zn, Cd, Pb, Hg, Mo, Mn, Ag, etc. and have also been used to detect and monitor gaseous pollutants such as sulphur dioxide(SO2), ozone(O3), nitrogen oxides, etc (figure 5).

Figure 5: Ozone damage to plants

Source: http://theinconvenientskeptic.com/2010/12/ethanol-ozone-and-the-epa/

Some of the well-known examples of higher plants as bioindicators as well as pollutant scavengers include:

• Anthroxanthum spp is tolerant to zinc
• Agrost is tolerant to copper
• Festuca is tolerant to lead
• Otlmpatiem is tolerant to calcium

The plants serve as ideal monitoring agents due to various phenotypic, metabolic and anatomical changes in the plant system and the type of changes help in the determination of the nature of compounds to which the plants are exposed. The damage caused on exposure to ozone gas can be reflected by the weathered flakes of tobacco or chlorotic flakes of pine needle. The damage caused by peroxyacetyl nitrate can be indicated by the collapse, glazing and bronzing of leaf cells in plants.

Some of the physiological and anatomical changes in plants include the effects on the nature of stomata, pigmentation, chlorosis and bleaching. For example, the inhibition of photosynthesis and enolase due to fluorine damage, bleaching of perianth and stamen injury by mercury poisoning, etc.

3.3.3. Zoology

Valuable data about the accumulation of chemicals in animals and their role as efficient bio monitors can be achieved by the study of both individual species as well as whole communities. Accumulation in animals occurs in different degrees in different organs which allow more effective choices for choosing suitable indicator species. The animals are designated as bioindicators even when there is a relative absence of accumulation since marked changes have been seen to be induced in the development and reproduction of particular species on being subjected to environmental pollution. The bioindication in animals also takes place by the accumulation of chemicals within food chains which further leads to biomagnification.

Some of the examples of animals as biological indicators include:

• Heavy metal and pesticide pollution in water can be done monitored by fish, daphnia, silver carp, etc.
• Freshwaters can be indicated by the presence of zooplanktons such as rotifers and cladocerans
• Radioactive pollution in the soil can be indicated using earthworms
• Frogs: the health of frogs is an indicator of the health of the biosphere as a whole because most frogs generally require suitable habitat in both the terrestrial and aquatic environments. The frogs are generally very vulnerable to environmental changes and disturbances since their skin absorbs toxic chemicals very easily and thus they make ideal indicators of environmental pollution

3.3.4. Human System as Bioindicators

Human cells, tissues, organs, organ systems, etc. also act as effective indicators of environmental stress. Human blood and urine can indicate the presence of toxic compounds on exposure to such substances. For instance, human hair, can trap metallic vapors and dust over a long period of time due to the affinity of these substances with the hair protein.

3.3.5. Cellular and sub-cellular components

The degree of tolerance, vulnerability and compliance in living organisms to various environmental factors and states are controlled by their characteristic genetic composition and physiological states. The process of biomonitoring can be effectively carried out by the examination of cellular and sub-cellular components of the living organisms, including chromosomes. Any chromosomal or genetic damage precisely indicates the extent of certain mutagenic agents in the environment. In resistant organisms, which are tolerant to toxic substances, a degradation of the pollutant may take place, and thus they make excellent indicators of environmental stress.

3.3.6. Comparative Physiology

The sense organs and other organ systems in various animals show behavioral responses on exposure to any kind of environmental stress or disturbance. Exposure of an organism to a pollutant may lead to coughing, interference with vital ionic or gaseous changes in skin pores, etc. A chemical may affect the functioning of various systems such as endocrine, nervous, muscular, cardiovascular and excretory systems. Such changes may be examined at morphological, biochemical or physiological levels in an organism and can thus indicate the presence of toxic substances.

3.3.7. Hydrobiology

Macro benthos as well as other biota living in the aquatic systems can be used to indicate the quality of various water systems. Biotic index in any water body is one such parameter for the purpose of biomonitoring.

4. Relevance of Bioindicators

The biological relevance of a bioindicator species is determined by certain characteristics such as little natural variability, stimuli to environmental stress, specificity, sensitivity, and the acquisition of biologically valid as well as measurable changes. An early warning indicator of potential adverse effects of any disturbance is the most useful indicator. However, a very sensitive response may lead to a false indication of very minor or biologically invalid changes and thus may not be considered relevant. An ideal biological indicator is the one reflecting irreversible damage as well as measurable changes.

5.  Importance of Bioindicators

5.1.  Ecological Importance

Bioindicators help in the determination of environmental changes, monitoring the level of pollution and its effects, and the health of an ecosystem as a whole. Bioindicators of different types provide numerous applications, for instance, the indication of increasing eutrophication and organic loading on Coral reefs, especially sponges. Bleached and broken corals and coral mortality are the indicators of past stress on coral reefs.

5.2. Assessing Human and Ecological Health

Ecological health includes the aspects of human health as well as the climatic conditions and water quality along with food stability for survival and aesthetic values. Ecological health involves proper maintenance of species diversity and populations as well as that of the functional characteristics like competitive interactions of the ecosystem. The sustainable availability of goods and services to all organisms in an ecosystem is an important aspect of a healthy ecosystem. Bioindicators are frequently used for the assessment of ecological health.

5.3. Monitoring water quality

Bioindicators help in efficient and cost-effective determination of water quality. For instance, the cellular metabolism of bioluminescent bacteria gets inhibited by the toxins present in water which leads to an effect on the intensity of light emitted by the bacteria. Thus, they are suitable for the analysis of water quality for the presence of environmental toxins. Water quality can also be monitored by the study of zooplankton community in aquatic systems and using macro-invertebrates such as bugs, since a decrease in their population status reflects degradation of water quality. Some tolerant bioindicator species include annelid worms and red midge larvae which are capable of breathing at the water surface and thus can thrive in polluted waters. Thus, different species respond in a specific manner to various environmental disturbances and can be used as efficient monitoring agents.

5.4. Monitoring Air Quality

Bioindicators can be used effectively for the assessment of air pollution which provides an early warning of sites which are vulnerable to the deposition of air pollutants. Some of the methods serving this purpose are epiphytic macro-lichen frequency, chemical analysis of mosses and community composition, etc.

Lichens and mosses can be used to monitor the quality of air by the measurement of various air pollutants such as fluoride and chloride content, ozone content, heavy metals, presence of radionuclides, etc. Therefore, lichens are considered as the high-quality indicators to monitor air pollution.

5.5. Monitoring Biodiversity

The current biodiversity crisis has led to an immediate need for urgent conservation policies and the identification of significant sites that promise the accomplishment of conservation goals. Bioindicators are potentially beneficial tools in achieving this. The disappearance of some noticeable animals such as birds, butterflies and mammals can play a major role in the monitoring of forest ecosystems.

The usually faced problems in the process of biodiversity conservation involves the absence of adequate resources such as sufficient data and these can be improved by conservation of some taxa (keystone species) which in turn provides the advantage of the protection of other taxa, species, or the complete biodiversity. Thus, indicator species can be effectively used as a conservation tool to recognize biodiversity hot spots.

6. Applications of Bioindicators

• Bioindicators have a wide range of applications. Some of these are as follows:
• Bioindicators can be effectively used as the indicators of water quality and thus, water pollution. For e.g. the presence of E. coli in water bodies is the indicator of water polluted by fecal matter.
• Bioindicators can be used for the assessment of air pollution. For e.g. the lichen thalli indicates the presence of fluorine in the atmosphere.
• Bioindicators determine an environmental disturbance or stress very easily and accurately.
• Bioindicators help in the assessment of the quality of the environment and thus play an important role in the maintenance of health of an ecosystem.
• Eutrophication in water bodies is indicated by the bioindicators. For e.g. presence of algal blooms in water body is the indicator of nitrates and phosphates.
• Bioindicators help in the measurement of the extent to which soil has been polluted. For e.g. Cyanobacteria are used as bioindicators of soil pesticides.
• Bioindicators can be effectively used as a conservation tool to recognize biodiversity hot spots.

7. Advantages of Bioindicators

Bioindicators are relevant, cost-effective and very reliable agents for environmental monitoring since the effect of pollutants on indicator species is clearly recognized. The techniques involved in bioindicator-based studies are relatively simpler, repetitive and feasible in a number of ecosystems. Indicator species are significant to monitor the environment because of their easy interpretation and less ambiguity than direct sampling and assessment of all plant and animal communities found in a certain ecosystem.

8. Disadvantages of Bioindicators

Environmental monitoring using bioindicators experiences a number of limitations such as the presence of sufficient number of individuals of the indicator species at the given locality, abundance and wide availability of species in the area of interest along with sufficient information about the physiological processes of uptake and accumulation of toxic substances or environmental contaminants in the indicators.

Conclusion

Bioindicators are biological tools which help in the determination of various disturbances taking place in the ambient environment. Several plants and organisms such as microorganisms, algae, lichens, mosses, invertebrates and vertebrates are used as bioindicators to analyze changes in the environment. Bioindicator based studies are usually simple and repeatable at different times by different individuals in different environmental conditions and are also suitable for the assessment of large areas. Bioindicator species indicate contaminant exposure and toxicity mechanisms in the environment provide early warning of future environmental damage and early indications of environmental recovery, thus help in ecosystem conservation.

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