All you need is a small puddle of water to be able to find some protista, which are very small microorganisms; commonly they may be found in the thin film of water covering particles of the ground. There are a number of parasite species, some of which cause severe diseases in man, such as malaria; others live in symbiosis with other animals, for example in their intestines, and their action facilitates the digestive processes.
Protista are microscopic, mainly unicellular organisms, i.e. consisting of one cell only. Unlike Monera, which have no distinguished nucleus, Protista have a nucleus and this is why they are called eukaryotes. Their genetic material (DNA) is in the nucleus, wrapped in a membrane that separates it from the cytoplasm. Protista are the kingdom with the highest degree of variability, which includes micro-organisms with very different shapes, structures and living conditions.
All Protista can reproduce asexually, i.e. they can duplicate without exchanging any genetic material. This is the most frequently used method to increase the number of individuals. But, in case of need, they can recombine their genetic inheritance, i.e. reproduce through “sexed” methods. All Protista have an aerobic metabolism, i.e. they need oxygen to live. They consist of two large groups: autotrophic and heterotrophic protista.
They can perform photosynthesis and mainly consist of unicellular algae. They can be divided into a number of systematic groups according to the shape of their cells and the type of photosynthetic pigments they use. Chrysophyta or golden algae: they live in both sea and freshwater; the most common ones are diatoms, which are equipped with a typical siliceous shell (SiO2), consisting of two parts joined with each other like a box and a lid. The shell is provided with many small holes through which the cell communicates with the external environment…
These Protista are also known as Protozoa, which means “first animals”; in fact, the term does not mean that all protozoa are the ancestors of the animals, but refers to the fact that they ingest food, which is typical of animals. Some of these Protista are predators and feed on bacteria and other Protista, i.e. they feed on dead organic matter, while others are parasites that exploit the resources of several organisms. They can be divided into four main groups depending on how they move: amoeboid protozoa, flagellate protozoa, sporozoa and ciliates.
A long food chain
Even if protista are rather small in size (usually between few microns to a few millimetres), they are very important for natural organisation, given the important role that photosynthetic protozoa play in the food chain of the aquatic organisms. They mainly feed on bacteria and thus play an essential role in the food chain, since they are the main producers on which all other organisms depend. The number of these organisms belonging to marine plankton can be huge; they have been found even as at 1,000 and 5,000 metres deep.
Together at all costs: the symbiosis
There are cases of symbiosis between animals and protozoa, in particular unicellular algae. The animals in which this phenomenon is most common are Radiolaria and Foraminifera (among protozoa), Coelenterata, Ctenophora, Platyhelminthes and molluscs (among Metazoa). In omnivore or herbivore species, such as, for instance, Radiolaria and Foraminifera, the algae adapt inside the cell by developing systems to resist the attack of some digestive enzymes of the host, especially cellulase.
Another case of symbiosis is the one between the mollusc Tridacna, which lives on the reefs of the Asian Pacific region, and the zooxanthellae. The latter live on the edge of the coating of the mollusc, which contains “hyaline bodies”, i.e. transparent bodies, that make it easier for light to penetrate; the zooxanthellae crowd around these bodies to perform the photosynthesis. The zooxanthellae are symbionts, also of Coelenterata, and reproduce through eggs. In the Mediterranean Sea, they live only in some sea anemones, but in the Tropical seas the symbionts live in very many species of Coelenterata, especially in the Madreporaria of the reefs.
The most ancient fossil traces of the first eukaryote cells date back to approximately 1 and a half billion years ago. Prokaryotes had appeared two billion years earlier, and up to then they had been the only inhabitants: in the intervening time, the bacteria had had the opportunity to differentiate into two different lines of evolution. One of these originated the first eukaryotes, i.e. Protista, following the spread of bacteria that could perform the photosynthesis (cyanobacteria), enriching the atmosphere with oxygen.
Protista live anywhere, provided there is even a little amount of water. There are marine and freshwater species that live both floating on the water or on the seabed. A small water pond is enough for these micro-organisms to live; they can usually be found even in the thin film of water that covers the soil particles. Finally, there are many parasitic species, some of which transmit serious diseases to man. Here are some of the most important diseases that affect man:
- the sleeping sickness caused by trypanosomes, such as, for instance, the Tripanosoma gambiense, transmitted to man by some species of the tsetse fly;
- toxoplasmosis, a disease affecting nearly 50% of the human population; harmless in itself, it becomes dangerous when it affects a pregnant woman since it is transmitted to the child, causing deformities;
- malaria, transmitted to man by the anopheles, parasited by the Plasmodium. This disease still causes approximately 3 million victims a year, of whom one million are African children.
Cold beer that burns: Ciguatera
Intoxication from Ciguatera is caused by the fish that feed on plants or small fish, which in their turn have accumulated the toxin from Dinoflagellata, such as Gambierdiscus toxicus. The larger the fish, the higher the amount of accumulated toxin. The symptoms, that usually begin 15 to 30 minutes after eating contaminated fish, include abdominal pain, nausea, vomiting, diarrhoea, numbness of the tongue and pharynx, toothache, walking problems, blurred vision, rashes, itchiness, tears, weakness, muscle spasms, lack of motor co-ordination, sleeplessness and occasionally irregular breathing.
A fire-red lake
Lake Tovel, in the Trentino region, is known all over the world for the exceptional phenomenon of the “reddening of its waters”. During the hottest hours of some summer days, large portions of the lake surface took a deep red colour, offering an incredible sight. The spectacular colour was due to the strong concentration of a unicellular alga of the group Dinoflagellata, the Glenodinium sanguineum (also known as Woloszynskya coronata). This alga, which is just 1/50 mm long, under specific stress conditions, stores special pigments, called carotenoids, which make it look completely red.
Man paints the sea red
Under favourable conditions, Dinoflagellata can rapidly proliferate and form large clusters. Red tides are the most conspicuous proofs of the massive presence of Dinoflagellata. In 1986, an increase in the Dinoflagellata population of the type Gymnodium brevi caused a significant red tide along the coasts of the Gulf of Texas. It extended 500 km along the coasts and caused the death of over 22 million fish in 2 months. Mollusc fishing was banned along 3/4 of the coast of the Gulf of Texas, south of Galveston, which caused a loss of oysters for a total of 1,4 million US dollars.
Micro-organisms purify water
Before being discharged into rivers and lakes, waste waters must be purified. One of the systems used for this purpose is activated-sludge purification. This process utilises the water self-purification principle, in which the micro-organisms (bacteria, protozoa and metazoa) use the organic substances contained in the water, transform them and remove them from the water. The aerobic process is the most commonly used one, i.e. oxygen is supplied to trigger the biological processes in which the organic substances are oxidised.
In the last decades is being undertaken a “biotechnological solution” which is capable of removing polluting agents from water: phytodepuration is based on the self-purification capacity of acquatic ecosystems through physical, chemical and biological processes carried out by vegetal organisms and bacteria. Plants involved are macro and microphytes which are specifically selected according to some characteristics as their capacity to adapt to the environment which needs to be decontaminated and their rapid growth with formation of biomass; in any case, the species employed for phytodepuration are water plants or hygrophilous plants which grow in moist environments.