A bioindicator is an organism that is used to assess (indicate) the state of an ecosystem. The term refers to an organism or a group of related organisms (species, genus, family) whose abundance and physiological state or behavior reflects the state or a change in the environment in which it is located. Bioindicators are useful especially in assessing the impact of humans on the environment in terms of pollution and the reduction of the living space of organisms.
Biomonitoring is a method that uses organisms to obtain information about the quality of the environment. It is based on the general characteristic of living organisms in which they respond to changes in the environment. It enables the monitoring of spatial and temporal trends of a pollutant that is deposited or transported locally, regionally or continentally. Biomonitoring is used to determine pollution at the national and local levels.
The use of plants as indicators of environmental quality has been known for as long as agriculture itself. This was proved by studying the ancient peoples using plants as indicators of soil fertility. In agriculture, individual plants show the state of nutrients in the soil (too much or too little of a substance in the soil) or certain physical properties of the land.
The total power of sources of anthropogenic impact is much greater than the power of natural sources. Lead, in the course of human exposure, enters the biosphere much more often than with natural processes. The most damage to the environment can be caused by various types of pollution with heavy metals. The air in industrial centers is contaminated with heavy metals such as copper, lead, nickel, mercury, vanadium, zinc and chromium. Such air pollution appears in various parts of plants and soil, and generally affects the environment. According to research, various soils are filters against the flow of man-made substances, and can be used as a chemical barrier to cleanse the biosphere.
In general, organisms in the natural environment that are investigated, are passive indicators, while organisms that are grown for a certain period of time and under controlled conditions, are active indicators.
Higher plants, especially from the phylum Spermatophyta, as well as lower plants; mosses, lichens, algae, are often used as bioindicators. Which group or species are chosen depends on the purpose of the control and on the characteristics of the ecosystem being investigated.
For indicator plants it is important whether the major source of pollution comes via air or soil. In spreading pollution through both media it is important that:
1. the plants are geographically distributed over a wide area;
2. there are genetically equalised plants;
3. the plants are not demanding in terms of habitat;
4. they can be grown under controlled conditions in the laboratory;
5. have a long vegetative period (in all seasons);
6. have as few seasonal growth forms as possible.
Environmental pollution
Humans pollute the environment with their activity. Industry, transport, agriculture, forestry, energy production, etc. always include and produce several substances, directly or as by-products, which are harmful to the environment. These substances are extremely widespread today, being released into the air, soil and water effecting and changing the natural conditions of the ecosystems. These substances include air pollutants such as sulfur dioxide, fluorides, nitrogen oxides, heavy metals, particulate matter (including soot), as well as secondary pollutants such as ozone, Peroxyacyl nitrates, formaldehyde (all of which a part of smog), etc. All herbicides, insecticides, industrial fertilisers and a whole spectrum of various organic and inorganic substances released in industrial processes, are pollutants. The presence of all these substances act as a disturbance in the whole ecosystems as well as in lives of individual organisms and their populations.
A toxic substance (pollutant) can enter the plant organism directly from the air or indirectly from the soil. From the air, it enters the plant most easily through the leaf gaps, partly through the cuticle and through all cells of the epidermis of leaves, stems, flowers and fruits. The plant can be damaged by the substance itself, or by compounds formed from it.
The response of the plant depends on the type of pollutants, on the dose it receives, its physiological state (age, nutrition) and factors of the environment (temperature, light, humidity, etc.). In addition to bioindication based on response or active bioindication, there is also bioindication based on accumulation. Some plants can accumulate substances without being damaged, or there is no detectable damage at a relatively high content of pollutants. Some examples: accumulators of sulfur compounds (various Brassicaceae, Fabaceae, special cultivars of Lolium sp.), heavy metals (lichen mosses, Brassicaceae, some conifers), nitrates (spinach, Rumex sp.), of various pesticides, etc.
Water bioindicators
Aquatic macrophytes can accumulate significant amounts of heavy metals in their tissues, making them suitable bioidicators of heavy metal pollution. The accumulation depends on many biotic and abiotic factors; temperature, pH and ions dissolved in water. The uptake of pollutants can take place through the roots or through the surface of the leaves (examples: Typha angustifolia and Potamogeton pectinatus).
Diatoms are the most important group for monitoring among algae, as they are present along the entire watercourse with a great diversity of species and clearly expressed ecological characteristics. Diatoms can be present in clean as well as heavily polluted waters and are very sensitive to changes in salinity and increases of phosphate concentration.
Species tolerant to increased pollution: Synedra ulna, Gomphonema parvulum, Navicula rhynchocephala, Cymbella ventricosa, Oscilatoria sp., Tribonema sp.
The sea
In addition to direct destruction, a serious threat is eutrophication, mainly nitrogen and phosphorus input. Eutrophication is a pollution process that occurs when a body of water becomes oversaturated with nutrients, the result is the rapid growth of algae and aquatic plants that overgrow the areas. Plants die and decay. The decay of organic material uses up oxygen in the water. Nitrogen fertilisers that run off the fields, nutrients from animal excrement and human sewage are the main cause of eutrophication. Increasing nutrient input also reduces the growth and reproduction of corals and allows the growth of macroalgae, which leads to changes and deterioration in the coral reef community.
Species of tropical seaweeds that take up nitrogen and accumulate it in their leaves have proven to be favorable bioindicators of increasing nutrients (mainly inorganic nitrogen). The ratio of 15N/14N in seagrass leaves can be used as an indicator of the increase of nitrogen compounds, when they are present in lower concentrations, not yet causing noticeable changes in the flora.
Air pollution
Physiological, chemical and biological properties of lichens and mosses enable their use as biomonitors of air pollution.
Lichens are particularly sensitive to SO2, nitrogen oxides, HF, O3, peroxyacetylnitrate and heavy metals, which can comprise up to 0,1-5 % of the lichen’s dry weight. They indicate long-term pollution. The sensitivity of lichens is primarily conditioned by the proportion of the thallus that is in contact with air or with the base, thus the size of the absorption surface and the related supply of water and minerals. Most commonly used as biomonitors are the Parmelia sulcata, Hypogymnia physodes and Xanthoria parietina. They are often used as indicators of heavy metal accumulation. It was found that lichens represent diverse accumulation properties to pollutants. Their advantage is that they grow in a geographically diverse area – they are found in various ecosystems, they grow all year round and do not have seasonal growth patterns. On the other hand, lichens are very difficult to grow under controlled conditions in the laboratory, controlled reproduction is not possible, so all the lichens used for experiments come from the natural environment. This can lead to problems if there is a shortage of lichens in heavily polluted areas and have to be sourced from distant areas where the climatic conditions are different.
Mosses are generally less sensitive to heavy metals and other toxic substances compared to higher plants and accumulate them much more. Some species are even tolerant to toxic elements and can grow in highly polluted areas with high concentrations of heavy metals and fluorine. Mosses are used as active bioindicators, they almost completely meet the requirements for laboratory use as growth and reproduction are possible under controlled conditions.
Ozone affects photosynthesis, respiration, metabolism and many other processes in plants. Damage to plants due to exposure to ozone is chlorosis (loss of chlorophyll) – visible in the form of pointy faded spots. This occurs when the breakdown of chlorophyll is greater than its synthesis. Chloroses later turn into necrosis, areas of dead tissue. Damage first appears on the upper (older) leaves, as these are the most exposed to ozone and on the upper surface of the leaves. The effects of ozone also include disturbances in adaptation to extreme weather conditions and other environmental stresses (increased susceptibility to disease development, greater vulnerability to pest attacks). Changes also occur in the course of physiological processes, such as photosynthesis, respiration, transpiration, which in turn affects the growth and productivity of plants. Externally, this is reflected in visible damage and loss of leaves and, as a result with cultivated plants, a smaller crop. The mentioned physiological damage can also occur without outwardly visible signs.
Monitoring the occurrence of ozone, the most commonly used plant is tobacco Nicotiana tabacum and also Trifolium repens where there is a positive correlation between the degree of leaf damage and ozone concentration.
Soil pollution – heavy metals
Heavy metals are either pollutants or are naturally present in the environment in high concentrations – plants as bioindicators can be used in three ways:
1. monitoring the presence/absence of certain ecotypes, plant species or plant communities;
2. study of physiological state of plants such as staining/discoloration (chlorosis);
3. measurng the elemental concentration in certain parts of the plant tissue.
For bioindicators of soil contamination with heavy metals, the following are used: Taraxum officinale, Populus nigra, Plantago lanceolata, Capsella bursa-pastoris, Nerium oleander and some species from the genus Amaranthus.
Mangroves also act as a sink and buffer by immobilizing heavy metals before they reach the aquatic ecosystem. They are highly tolerant and various species are known to accumulate heavy metals.
Extremely high heavy metal content in the soil affects the vegetation and the survival of only some plant species. Certain plant species grow even on soils extremely loaded with heavy metals. Leguminous grasses and amaranth accumulate more heavy metals, thereby contribute to a decrease in the content of heavy metals in the soil and thus restore soil fertility. It is important to know the methods and parameters that can be used to assess the toxicity of heavy metals for plants, as well as the tolerance of plants to individual heavy metals. These methods use data on the growth of the root system, relative growth of plants, production of plant biomass, heavy metals concentration in soil and plants, growing conditions and physiological properties of plants. Chemical analyses of plant tissue are most often used to indicate the nutritional status of plants, which consequently reveals the soil’s supply of nutrients and the concentratons of certain elements.
The presence of heavy metals in the environment reduces the abundance of plants, reduces the species composition and causes morphological changes or even their collapse. Heavy metals have a negative effect on metabolic links in the course of their action on enzymes. In the case of replacement of the parent metal in the enzymes, the catalytic performance decreases. Chloroplasts and mitochondria are disrupted by the presence of heavy metals, namely at the subcellular level of ion transport and functions of cell membranes.
Ecosystem state indicators and mycobioindication
The forest ecosystems through time adapt to changes in the environment, but it cannot keep up with the rapid changes brought about by pollution. Bioindication is used at several levels: symptomatic declines, analysis of sulfur content, other bioindicative and physiological analyses and measurements.
Cultivated plants, on the other hand, are more plastic, capable of greater adaptation and, as such, more resistant to pollutants than forest plants. In addition, many of them are annual or biennial plants, or are grown only for one or a few seasons and the cumulative effect of the effect of pollutants is not evident.
Pollutants (by air and soil) also affect ectomycorrhizal fungi. Mycorrhizae are an important factor in reducing stress, but for most mycorrhizal fungi, pollution from industrial emissions causes a significant decline in the proportion of mycorrhizae in trees. In addition to the reduction of mycorrhizae, qualitative changes also occur and it has been observed that more primitive types of ectomycorrhizae predominate in heavily polluted areas. Thus ectomycorrhizal fungi can be early indicators of pollution. Sensitive fungi that first disappear from contaminated areas: all species from the genus Phelldon and Hydnellum and some species from the genus Suillus, Trisholoma and s. The most tolerant types of fungi are Amanita rubescens, Loccaria laccata and Rusula ochroleuca, etc.
Conclusion
In addition to their irreplaceability in the ecosystems, plants are also important as indicators of the state of the environment. They are and are used as bioindicators, thereby determining the level of pollution and the state and quality of the environment itself.
We can monitor whether the level of pollution is rising or stagnating, the presence and quantities of the pollutants and the reason for the pollution in the area being researched.
Sources:
Parzych A., Astel A., Zduńczyk A., Surowiec T. Evaluation of urban environment pollution based on the accumulation of macro- and trace elements in epiphytic lichens. 2015
Holt E. A., Miller, S. W. 2011. Bioindicators: Using organisms to measure environmental impacts.
Khanieva I. M., Bekuzarova S. A., Abdulkhalikov R.Z., Boziev A. L., Shogenov Yu. M. 2020. Bioindicators and environmental protection.
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