The protozoa are acellular, microscopic organisms found everywhere, in water; soil, on the body of plants and animals. Although they are minute and apparently insignificant, but they are of considerable economic value to the mankind.
The activities of these organisms can be divided the following two heads:
1. Beneficial Protozoa.
2. Harmful Protozoa.
ADVERTISEMENTS:
1. Beneficial Protozoa:
The protozoa are useful in the following ways:
i. Food:
Protozoa provide food for insect larvae, crustaceans and worms, which are taken by large animals like fishes, lobsters, clams, and crabs, which are eaten by man. Thus they form sources of food supply to man both directly and indirectly.
ADVERTISEMENTS:
ii. Symbiotic Protozoa:
Certain protozoa like Trichonympha and Colonymphya etc. live in the gut of termite, which help in the digestion of cellulose. The digested cellulose is utilized by the host.
iii. Insect Control:
Several protozoa control harmful insects by persisting their bodies.
ADVERTISEMENTS:
iv. Helpful in Sanitation:
A large number of protozoa living in polluted water feed upon waste organic matters and thus purify it. Many protozoa feed upon bacteria and play important role in the sanitary betterment and keeping water safe for drinking.
v. Industry:
The skeletal deposits of marine protozoa (Foraminifera and Radiolaria) from occanic ooze at the sea-bottom. About 30% of oceanic bed is covered with the Globigerina ooze. These skeletal deposits are put to many uses. Some are employed as filtering agents, others are made into chalk and still others are used for abrasives.
vi. Building Material:
The skeletal deposits in due course of time change into the limestone rock. Limestone is provided by the Globigerina ooze, such as that of cliffs of Dover, which have played an important role in the defence of England. Comerina (Formally known as Nummulites, are the largest protozoa, which form limestone. Thus, limestone-beds are used as building material.
vii. Oil Exploration:
Petroleum is organic origin. The skeletal deposit of Forminifera and Radiolaria are often found in association with oil deposits. In this mean; they help in the exact location of oil.
viii. Scientific Study:
ADVERTISEMENTS:
Many protozoa are used in biological and medical researches. A Holotricha. Tetrachymena geleii is used in nutritional research. The effects of various foods and poison have been investigated on this protozoan.
2. Harmful Protozoa:
The protozoa are harmful in following ways:
i. Pollution of Water:
Drinking water in natural condition is made unpalatable by the reproduction of some free-living protozoa in it. For example, Uroglenopsis brings flashy odour like that of cod- liver oil. Peridinium emits smell resembling that of clam-shells.
ii. Destruction of Animals of Food Value:
dinoflagellates like Noctiluca and Gonyaulax, when become abundant, are responsible for turning the ocean red. The water becomes foul and cause toxic reaction to molluscs like clam oysters and mussels and they become unfit for eating by human beings.
iii. Destruction of Wooden Articles:
Some flagellater like Trichonympha and Colonympha live in the gut of termites and help in the process of cellulose digestion. In the absence of these flagellates the termites will die or change their diet. Thus such protozoa indirectly help in distraction of wooden articles and books.
iv. Reduction in Fertility of Soil:
It has been observed that about 200- 300 species of protozoa are present in soil. These protozoa feed on nitrogen-fixing bacteria thus reduce the fertility of soil.
v. Parasitic Protozoa:
Parasitism is an association of two organisms in which one organism lives at the expense of the other giving nothing in return. The first animal is called parasite and other the host. Parasitism is a specialized mode of life and various level of it are found in protozoa. There are two type of parasites on the basis of their survival Facultative parasites are those that can live for various lengths of time without the host. The obligatory parasites are those which can-not survive when separated from their host.
Depending upon the nature and degree of intimacy and the placement of the parasites on their host these are two types of parasites:
a. Ectoparasites:
When the parasite lives on the external surface of the host, it is called ectoparasite. A few species like Hydramoeba hydroxena (found on epidermis of Hydra), Costa necatrix found in the epidermal cells of fishes, and Ichthyophthirius multifilis, inhibiting in the skin of fishes. They feed on the liquefied host cells causing various degrees of cellular necrosis. These parasites are transferred directly from host to host.
b. Endoparasites:
When the parasite lives inside the body of the host such parasite is called endoparasite.
Following types of endoparasites are there depending upon their location:
i. Coelozoic:
When the parasite lives in the alimentary canal or cavities of host body. For example Entamoeba histolytica, Giardia lamblia, Balantidium coli etc.
ii. Histozok:
Which occupies space between the host cells. For example young trophozoic of Monocystislives in sperm morula of earthworm. Trypanosoma gambiens lives in the blood plasma of vertebrates.
iii. Cytozoic:
These live inside the host’s cell. For examples Plasmodium and Babesia in the blood cells of hosts. Sarcocystis lives in the striped muscles of host.
Effects of Parasitism on the Parasite:
Parasitic protozoa are usually simplified in their structure. They do not possess any elaborate organelles, except the ciliate parasite. The endoparasites show maximum structural simplicity. They lack contractile vacuoles, locomotory and ingestory organelles.
However, coelozoic forms like Trichomonas, Giardia and Balantidum possess locomotory and ingestory organelles. Surplus food is stored as glycogen, volutin granules and chromatid bodies. Many parasites are aerobic as well as anaerobic depending on the availability of oxygen. In some protozoa like Gregarina develops the organelles of attachment. Occurrence of complicated life-cycle often involving cyst- formation and rapid multiplication by merogony and schizogony.
Effects on Hosts:
The parasites are of two types non-pathogenic and pathogenic. The non-pathogenic parasites have no noticeable ill-effects on the host. For example Entamoeba coli, Trichomonas hominis, Endolimax nana etc. The pathogenic parasites are those which cause ill-effects or diseases to the hosts.
Transmission of Parasite to other Host and its Infection:
The infection of new host takes place in two steps first they exist of parasite from the first host, and second the transmission in the second host.
Following ways are there in which the parasitic protozoa reach new hosts:
i. Active Migration:
This occurs in epizoic parasites of aquatic hosts e.g., Ichthyophthirius which actively comes out of the host i.e. fish and infects the other host.
ii. Direct Transfer:
This type of infection takes place on coming in contact of the other host. E.g. Trichomonas vaginalis etc.
iii. Accidental Infection:
In this method the new host in infected accidentally swallowing of the parasite in the encysted condition along with food and water e.g., Conccidia, Entamoeba histolytica etc.
iv. Infection by Intermediate Host:
This takes place in blood-parasite like Plasmodium and Trypanosoma in which a vector transmits the parasite through blood sucking (like Anopheles female mosquito, fly). The developmental stages may or may not be found in the vector.
v. Congential Transfer:
The germinative infective, taking place at or before birth, may occur in several ways as placental transfer takes place in case of Plasmodium in man, ovarian transfer as takes place in Nosema in silkworm etc.