A group of energetic plants, most of which contain chlorophyll in their cells and are able to carry out the process of photosynthesis, releasing oxygen into the external environment. They live in fresh and saline marine reservoirs. Some algae are bioindicators of water pollution.

Algae, a large and heterogeneous group of primitive, plant-like organisms. With few exceptions, they contain the green pigment chlorophyll, which is necessary for nutrition through photosynthesis, the synthesis of glucose from carbon dioxide and water. Colourless algae are very rare, but in many cases the green chlorophyll is masked by pigments of a different colour. In fact, among the thousands of species in this group, you can find forms coloured in every shade of the solar spectrum. Although algae are sometimes described as the most primitive of organisms, this is a view that can only be accepted with considerable reservations. While many of them lack the complex tissues and organs of seed plants, ferns, and even mosses and liverworts, all the processes necessary for the growth, nutrition, and reproduction of their cells are very similar, if not identical, to those found in plants. Algae are therefore physiologically very complex. Algae are the most numerous, the most important to the planet and the most widespread photosynthetic organisms. There are many of them everywhere - in fresh water, on land and in the oceans, which is more than can be said for liverworts, mosses, ferns or seed plants. With the naked eye, algae can often be seen as small or large patches of green or other coloured foam ('slime') on the surface of the water. On soil or tree trunks they usually look like green or blue-green slime. In the sea, layers of large algae (macrophytes) resemble red, brown and yellow shiny leaves of various shapes.

Algae vary widely in size, from microscopic forms a few thousandths of a centimetre in diameter or length, to sea giants over 60 metres long. Many algae are unicellular or consist of several cells forming loose aggregates. Some are tightly organised colonies of cells, but there are also true multicellular organisms. The cells may be connected at the ends, forming chains and filaments - both branched and unbranched. The whole structure as a whole sometimes looks like a small disc, a tube, a club or even a tree, and sometimes resembles a ribbon, a star, a boat, a ball, a leaf or a strand of hair. The surface of the cells can be smooth or covered with a complex pattern of spines, papillae, pits and ridges. In most algae, the general structure of the cells is similar to that of green plant cells, such as corn or tomatoes. A rigid cell wall, composed mainly of cellulose and pectin substances, surrounds the protoplast, in which the nucleus and cytoplasm are separated by special organoids - plastids. The most important of these are chloroplasts, which contain chlorophyll. The cell also has fluid-filled cavities - vacuoles - which contain dissolved nutrients, mineral salts and gases. However, this cell structure is not characteristic of all algae. In diatoms, one of the main components of the cell wall is silica, which forms a kind of glass shell. The green colour of the chloroplasts is often masked by other substances, usually pigments. A small number of algae do not have a rigid cell wall at all.

Many aquatic vegetative cells and algae colonies, as well as some types of their reproductive cells, move quite rapidly. They are equipped with one or more xiphoid appendages - flagella - that propel them through the water column. Some algae, lacking a cell wall, are able to pull parts of their body forward and pull the rest up with them, allowing them to "crawl" on hard surfaces. This movement is called amoeboid because the well-known amoebas move in the same way. The straight-line or zigzag movement of diatoms - which have a solid cell wall - is probably caused by water currents generated by the various trickling motions of their cytoplasm. Sliding, crawling, waving movements of algae more or less firmly attached to the substrate are usually accompanied by the formation and liquefaction of slime.

Almost all unicellular algae can reproduce by simple cell division. The cell divides into two, both daughter cells - and this process can, in principle, continue indefinitely. Since the cell only dies by "accident", we can speak of a kind of immortality. A special case is cell division in diatoms. Their shells consist of two halves (valves) that fit together like two parts of a soap dish. Each child cell receives a parent leaf and completes the second leaf itself. So one leaf of a diatom may be new, and the second inherited from a distant ancestor. The protoplast of some vegetative cells can separate, forming mobile or immobile spores. From these, after a long or short period of rest, a mature alga develops. This is one of the forms of asexual reproduction. In sexual reproduction, male and female gametes are formed in algae. The male gamete fuses with the female, i.e. fertilisation occurs, and a zygote is formed. After a period of dormancy, which can last from a few weeks to several years depending on the type of algae, the zygote begins to grow and eventually gives rise to an adult. The gametes vary greatly in size, shape and motility. In some algae, the male and female gametes are structurally similar, while in others they are distinctly different, representing sperm and eggs. Thus, sexual reproduction in algae has many forms and levels of complexity.

It is difficult to find a place on the planet where algae do not exist. They are usually thought of as aquatic organisms, and indeed the vast majority of algae live in puddles and ponds, rivers and lakes, seas and oceans, where they can become very abundant at certain times of the year. Algae attach themselves to rocks, stones, pieces of wood, aquatic plants or float freely as part of the plankton. Sometimes this suspension of billions of microscopic forms reaches the consistency of pea soup and fills vast expanses of lakes and oceans. This phenomenon is called an "algal bloom". The depth at which algae can be found depends on the transparency of the water, i.e. its ability to transmit the light necessary for photosynthesis. Most algae are concentrated in a surface layer a few decimetres thick, but some green and red algae are found at much greater depths. Some species can grow in the ocean at depths of 60-90 metres. Some algae, even frozen in ice, can remain viable in a state of suspended animation for many months.