Explain how bryophytes act as indicators of pollution.
Answers
Bryophytes are green land plants which lack a
vascular system and are simple both morphologically and anatomically. The growth potential in
bryophytes is not as highly polarized as vascular
plants. Bryophytes grow in a variety of habitats
especially in moist places on soil, rocks, trunks and
branches of trees and fallen log. They obtain
nutrients directly from substances dissolved in
ambient moisture. Some substances are probably
absorbed directly from the substrate by diffusion
through the cells of the gametophyte. Bryophytes
are used as reliable indicators of air pollution (Le
Blanc & Rao, 1975). They are exploited as bryometers instrument for measuring phytotoxic air
pollution. They either independently or together
with lichens can be valuable organisms in developing an index of atmospheric purity (IAP) which is
based on the number, frequency-coverage and
resistance factor of species. This index can provide
a fair picture of the long-range effects of pollution
in a given area (Rao, 1982). There are two
categories of bryophytes in response to pollution:
• which are very sensitive to pollution and show
visible symptoms of injury even in the presence
of minute quantities of pollutants. This serve as
good indicators of the degree of pollution and
also of the nature of pollutant.
• which have the capacity to absorb and retain
pollutants in quantities much higher than those
absorbed by other plant groups growing in the
same habitat. These plants trap and prevent
recycling of such pollutants in the ecosystem
for different periods of time. Analysis of such
plants gives a fair idea about the degree of
metal pollution.
Pollutants
Pollutants may be gaseous such as carbon
monoxide (CO), fluorides, hydrocarbons (HC),
hydrogen sulphide (H2S), nitrogen oxides (NO),
Ozone (O3), sulphur dioxide (SO2), aldehydes, lead
and automobile exhaust fumes. NOX and NH3 are
primary gaseous pollutants which are strongly
phototoxic. SO2 and NO2 are readily converted to
strong acids by oxidation and solution in atmosphere as water droplets to form acid rain. The
smoke and SO2 is produced by the combustion of
traditional fuel of automobiles, together with various metals released into atmosphere by smelting
and other heavy metal industries. Nitrogen oxides
have also greatly increased in the cities with the rise
of use of automobiles. Particulate pollutants are
dust, particles of metallic oxides, coal, soot and fly
ash, cement, liquid particles, heavy metal and
Govindapyari et al. 35-41
radioactive materials. Ozone (O3) is a secondary
pollutant formed by the action of sunlight on
nitrogen dioxide and on certain hydrocarbons. It is
more phytotoxic than the primary pollutants. NH3 a
pollutant is also being released by intensive animal
rearing. Other pollutants are agricultural pesticides
and fertilizers and various forms of aquatic pollution. The air pollutant either in a gaseous state
mixed with air or in a liquid state affected by dew,
rain, or snow, will be noxious to bryophytes attached to the bark.
Effect of pollutants on bryophytes
There is an impoverishment of bryophytic communities in and around cities and industrial areas
(Gilbert, 1968).Urban areas comprise a series of
habitats with a variety of substrates and moisture
regimes and are subject to varying levels of pollution. Bryophytes occupying certain substrates
appear to more sensitive to air pollution than others.
Bryophytes have been disappearing from urban
industrial environments because of their sensitivity
to polluted air. Species diversity in a polluted area
varies not only with the distance from the source of
pollution but also with the type of substrate.
Air pollution inhibits gametangial formation and
sexual reproduction in bryophytes. They also reduce photosynthesis by degrading chlorophyll and
growth of plants and eventually cause their death.
Metals and metalloid are accumulated by bryophytes from the substratum, wind-blown or in wet
deposition. When the metal enters the cell, it
inhibits the photosynthetic activity. Enzymes and
membrane are poisoned when a heavy metal gains
access to the cell interiors. Mercury is particularly
toxic low concentration greatly inhibited photosynthesis, temporarily increased respiration, reduced
chlorophyll levels and caused loss of intracellular
K + from Rhytidiadelphus squarrosus (Brown &
Whitehead, 1986). It is evidenced that when the
pollution level goes down, the percentage frequency
of species goes up, which subsequently increases
the fertility percentage. This situation varies from
species to species depending on the prevailing
climatic conditions in the area. Bryophytes die
within a short period of time depending on the level
of pollution, when transferred along with their
substrates from unpolluted to polluted areas in a
city or around a factory.