Protozoa is a Phylum of microorganisms which are characterised by having unicellular and non-fungal body. Being single cell based Animals they are the most simple form of all living organisms. They are not a natural group but simply a collection of organisms. Protozoans are often called the 'Animal-Like Protists' because they subsist entirely on other organisms for food. Protozoa includes Animals like Ciliates, Flagellates, Sarcodines and Apicomplexans.

• Protozoa Classification

• Protozoa Characteristics

• Protozoa Locomotion

• Protozoa Distribution

• Protozoa Evolution

• Protozoa Reproduction

There are more than 65,000 described Species, of which over half are fossil. Protozoa play an important role as zooplankton, the free-floating aquatic organisms of the oceans. Here, they are found at the bases of many food chains, and they participate in many food webs. Protozoa also act as pathogens and parasites, causing a variety of diseases amongst all Animals including humans. Malaria is caused by a number of Plasmodium Species, most importantly 'Plasmodium falciparum'.

Protozoa Classification

Traditional Classification

Flagellates - Example Giardia lamblia.
Amoeboids - Example Entamoeba histolytica.
Sporozoans - Example Plasmodium knowlesi.
Ciliates - Example Balantidium coli.

Modern Classification

i)  Phylum Sarcomastigophora

a) Subphylum Mastigophora

Class - Phytomastigophorea.
Class - Zoomastigophorea.

b) Subphylum Opalinata

c)  Subphylum Sarcodina

Superclass - Rhizopoda.
Superclass - Actinopoda.

ii)  Phylum Labyrinthomorpha

iii)  Phylum Apicomplexa

iv)  Phylum Microspora

v)  Phylum Myxozoa

vi)  Phylum Ciliophora

d)  Subphylum Postciliodesmatophora

Class - Karyorelictea.
Class - Spirotrichea.

e)  Subphylum Rhabdophora

Class - Prostomatea.
Class - Litostomatea.

f)  Subphylum Cyrtophora

Class - Phyllopharyngea.
Class - Nassophorea.
Class - Oligohymenophorea.
Class - Colpodea.

Characteristics of Protozoa

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• Unicellular - They are mostly Unicellular but some can be Multicellular or Colonial organisms also.

• Microscopic Organisms - Most Protozoa Species are so small that they cannot be seen with naked eye, they are only visible through a Microscope.

• Presence of Double-Membrane Mitochondria - Almost all Protozoa contain Double-Membrane Mitochondria; the inner membrane forms finger-like extensions (or cristae) into the mitochondrial interior, and the outer membrane forms the boundary of the organelle. Cellular respiration occurs at Mitochondria. Protozoa which live in the intestinal tract of their hosts or those which live in anaerobic environment lack Mitochondria.

• Aerobic Respiration - Most Protozoal Species are Aerobic. Their respiration is based on the oxidation of the six-carbon glucose molecule to single-carbon carbon dioxide molecules and water via the Embden-Meyerhof pathway, tricarboxylic acid cycle  (Krebs cycle), and cytochrome systems. There are some Anaerobic Species which live in the human intestine and animal rumen.

• Digestive System - Protozoa may absorb food via their cell membranes, some, e.g. Amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food. All Protozoa digest their food in stomach-likes compartments called 'Vacuoles'.

• Locomotion - Protozoa exhibit diverse modes of locomotion.

Basic types of Locomotion

1)  Flagellar - Flagellar propulsion is employed by the flagellates and during some stages in the life cycles of certain Sarcodines. The flagellum is a whip-like structure found not only in Protozoans but in higher organisms as well (such as in sperm, the Male reproductive cells of higher animals).

2)  Ciliary - Cilia are a type of flagellum arranged in closely aligned longitudinal rows called 'Kineties'. It is used by Ciliates.

3)  Amoeboid Movement - Amoeboid Movement is achieved by Pseudopodia and involves the flow of cytoplasm as extensions of the organism. The basic locomotory organelle is the Pseudopodium.

• Paraphyletic - They constitute their own kingdom under the Integrated Taxonomic Information System 2009 classification.

• Extremely Diverse - Protozoa are extremely diverse, not only in their structure but also in their feeding habits, reproduction and locomotion. The Flagellates exhibit the greatest diversity of nutrition among the Protozoa. Many contain pigments and obtain food like plants through sunlight. Other feed on Algae, Bacteria, and other Protozoa.

• Mixotrophy - Mixotrophy is a property of certain microorganisms that can assimilate organic compounds as carbon sources but not as energy sources. Mixotrophy is a common phenomenon among free-living Ciliates and Sarcodines. Moreover, the degree of Mixotrophy varies from complete reliance on the symbiotic Alga or Algae to transitory retention of the plastids of phytoflagellate prey with only a partial dependence on photosynthesis to supplement the cell’s energy balance. Many of the Foraminiferans and Radiolarians possess symbiotic Algae. In some Foraminiferans and Radiolarians several different symbiotic Species of Algae may live within the Protozoan cytoplasm.

Size - Protozoa range in diameter from a few thousandths of a millimetre to several millimetres.

Protozoa: Geographical Range and Habitat

Protozoa are present in most soils and in aquatic habitats from the South to the North poles. Their habitat includes lakes, estuaries, sea and any moist habitat.

Diet - They prey upon unicellular or filamentous Algae, Bacteria, and Microfungi. Some Species are parasites of plants and animals.

History and Evolution of Protozoa

Protists dominated life on Earth 1.5 billion years ago. The presence of ancestors of modern Protozoa is evident from the calcareous shells of the 'Sarcodine Foraminiferans' and calcium-secreting 'Flagellate Coccolithophores' formed during the Cretaceous Period (145.5 to 65.5 million years ago) and the well-developed Foram limestones of the Paleozoic Era (542 to 251 million years ago), Early Cretaceous Epoch (145.5 to 99.6 million years ago), and Cenozoic Era (65.5 million years ago to the present). The present day Protozoa have undergone considerable evolutionary change. Complexity in Protists has evolved through specialisation of different parts of the cell - organelles and the cytoskeleton in particular. The main Protozoan groups like The Ciliates, Flagellates, Sarcodines, and Apicomplexans separated early in geologic history from Amoeboid and Flagellate ancestors and represented independent ancestral branches of the Protozoan evolutionary tree. The ancestral eukaryote organism is thought to have been an Amoeboid creature that relied on anaerobic metabolism. The evolution of Mitochondria (the centres of aerobic respiration in the cell) as organelles from Endosymbiotic Bacteria and the establishment of oxidative pathways allowed a more efficient cellular energy balance, which led the way to the evolution of an enormously diverse array of eukaryotic organisms. A considerable number of Protozoans became parasitic, a mode of life that evolved independently among the Protozoa many times. Ciliates and Sarcodines became symbionts in the intestinal tracts of both Vertebrates and Invertebrates as a result of surviving the digestive enzymes of the predator. Along their evolution path, they also laid the foundation for the rise of multicellular organisms.

Protozoa Reproduction

Protozoa reproduce asexually by the following means:

• Fission - One cell splits into two.

• Schizogony - Multiple fission. The nucleus divides many times before the cell divides. The single cell then separates into numerous daughter cells.

• Budding - Buds form and pinch off of the parent cell.

Some Protozoa also reproduce sexually by fusion of gametes. Free-living Protozoans normally only resort to sexual reproduction when environmental conditions become adverse, because this mode of reproduction enhances the fitness of the population and increases the chance of mutation. When food and other conditions are favourable, asexual reproduction is practised. Foraminiferan reproduce sexually as well as asexually. Sexual reproduction among the Ciliated Protozoans takes the form of Conjugation. During conjugation, two Ciliates line up side by side. The macronucleus, which plays no part in the process, disintegrates. A series of nuclear divisions of the micronuclei in each Ciliate then ensues, including a Meiosis, during which a number of Haploid Micronuclei are produced in both cells. All but one of these Haploid Micronuclei disintegrate. The remaining Haploid Micronucleus in each cell then divides through mitosis into two Haploid Nuclei (gamete nuclei). A bridge of cytoplasm forms between the two Ciliates, and one gametic nucleus from each cell passes into the other cell. The two gametic nuclei in each cell unite, thus restoring the diploid number of chromosomes. The micronucleus undergoes two mitotic divisions to produce four micronuclei; two of these will form the new micronuclei of the cell and two are destined to become the macronucleus. Following the process of Conjugation, normal binary fission proceeds. The number of macronuclei and micronuclei formed is dependent on the Species and remains the same as the original number.