General characteristics of algae
Algae are multicellular, predominantly aquatic, eukaryotic photosynthetic organisms that do not have tissues or whose body is not differentiated into vegetative organs (i.e., belonging to the subkingdom of lower plants).
❖ Systematic divisions of algae (differ in the structure of the thallus, the set of photosynthetic pigments and reserve nutrients, characteristics of reproduction and development cycles, habitat, etc.): ■ Golden; ■ Green (examples: spirogyra, ulotrix); ■ Red (examples: porphyry, phyllophora); ■ Brown (examples: lessonia, fucus); ■ Characeae (examples: hara, nitella); ■ Diatoms (example: Lycmophora), etc. The number of algae species is more than 40 thousand.
❖ Algae habitat: fresh and salt water bodies, wet soil, tree bark, hot springs, glaciers, etc.
❖ Ecological groups of algae: planktonic, benthic (in detail), terrestrial, soil, etc.
Planktonic forms are represented by green, golden and yellow-green algae, which have special adaptations to facilitate transport by water: reducing the density of organisms (gas vacuoles, lipid inclusions, gelatinous consistency) and increasing their surface (branched outgrowths, flattened or elongated body shape, etc.) .
Benthic forms live at the bottom of reservoirs or envelop objects in the water; They are attached to the substrate by rhizoids, basal discs and suckers. In the seas and oceans they are represented mainly by brown and red algae, and in fresh water bodies - by all departments of algae, except brown algae. Benthic algae contain large chloroplasts with a high chlorophyll content.
Terrestrial or aerial algae (usually Green or Yellow-green algae) form deposits and films of various colors on the bark of trees, wet stones and rocks, fences, roofs of houses, on the surface of snow and ice, etc. When there is a lack of moisture, terrestrial algae become saturated with organic and inorganic substances.
Soil algae (mainly Yellow-green, Golden and Diatoms) live in the thickness of the soil layer at a depth of 1-2 m.
Similarities and differences
Despite all the diversity and heterogeneity of the group, algae also have common features - the presence of chlorophyll and a photoautotrophic type of nutrition. In turn, multicellular algae are united by the following characteristics:
- lack of a pronounced conduction system;
- tendency to live in humid conditions and aquatic environments;
- lack of a covering shell;
- lack of a clear division of the body into separate organs.
Some varieties of algae stand out from the general mass by their ability to heterotrophy (a method of feeding the body when the food is ready-made organic objects) or osmotrophy (the property of the cell membrane to allow nutrients to pass through).
There are also algal organisms that can capture nutrients through the cellular mouth (in particular, euglena, dinophyte algae).
Features of the structure of algae
The body of the algae is not divided into vegetative organs and is represented by a strong, elastic thallus (thallus) . The structure of the thallus is filamentous (examples: ulotrix, spirogyra), lamellar (example: kelp), branched or bushy (example: chara). Dimensions - from 0.1 mm to several tens of meters (for some brown and red algae). The thallus of branched and bushy algae is dissected and has a linear-segmented structure; in it one can distinguish the main axis, “leaves” and rhizoids.
Some algae have special air bubbles that hold the thallus near the surface of the water, where there is the possibility of maximum light capture for photosynthesis.
The thallus of many algae secretes mucus, which fills their internal cavities and is partially discharged out, helping to better retain water and prevent dehydration.
The cells of the algae thallus are not differentiated and have a permeable cell wall, the inner layer of which consists of cellulose, and the outer layer of pectin substances and (in many species) a number of additional components: lime, lignin, cutin (retaining ultraviolet rays and protecting cells from excessive loss of water during low tide), etc. The shell performs protective and supporting functions, while providing the opportunity for growth. With a lack of moisture, the shells thicken significantly.
The cytoplasm of the cell in most algae forms a thin layer between the large central vacuole and the cell wall. The cytoplasm contains organelles: chromatophores , endoplasmic reticulum, mitochondria, Golgi apparatus, ribosomes, one or more nuclei.
Chromatophores are algal organelles containing photosynthetic pigments, ribosomes, DNA, lipid granules and pyrenoids . Unlike the chloroplasts of higher plants, chromatophores are more diverse in shape (can be cup-shaped, ribbon-shaped, lamellar, star-shaped, disc-shaped, etc.), size, number, structure, location and set of photosynthetic pigments.
In shallow-water ( green ) algae, the photosynthetic pigments are mainly chlorophylls a and b, which absorb red and yellow light. In brown algae, which live at medium depths where green and blue light penetrates, the photosynthetic pigments are chlorophylls a and c, as well as arotin and fucoxanthin , which have a brown color. In red algae, which live at depths of up to 270 m, the photosynthetic pigments are chlorophyll d (characteristic only for this group of plants) and reddish-colored phycobilins - phycoerythrin, phycocyanin and allophycocyanin, which absorb blue and violet rays well.
Pyrenoids are special inclusions that are part of the chromatophore matrix and are a zone of synthesis and accumulation of reserve nutrients.
Algal reserve substances: starch, glycogen, oils, polysaccharides, etc.
Types of organization
Algae are very different in appearance. There are several main types of body organization:
- Amoeboid. A feature of this type is the absence of a hard cell membrane, which makes significant deformation of the body possible. Such organisms are capable of unique movements due to changes in shape. Their speed is quite low.
- Monadic. As a rule, these are unicellular algae characterized by a constant body shape. Their peculiarity is the presence of flagella.
- Coccoid type. These are single-celled organisms that do not have organelles that facilitate movement and are not able to change the shape of their body. The cell wall is usually thickened and may have projections and pores. Such algae tend to form colonies. Some species are capable of locomotion by secreting mucus.
- Palmelloid. The body of such organisms is quite large and has a constant shape. A feature of such organisms is the ability to attach to any substrate. As a rule, this is a formation of several coccoid cells united by a common mucous mass, although the cells themselves are not connected to each other in any way.
- Threaded. Cells are united to form chains or threads, which can be simple or branching. Their filaments float freely in the water column or can be attached to the substrate. They can also unite into colonies that will grow diffusely (all the cells that make up the filament divide), intercalary (the growth zone is located in the middle), apical (by multiplying cells located at the ends of the filaments) or basal (the growth zone is located at the base of the body of an algal organism) in ways.
- Variegated. The structure of such an algae colony contains two systems of threads. One spreads along the substrate, attaches to it and has a horizontal position. The second is a system of vertical threads extending from the creeping one. Systems of threads in this case have different functions: horizontal ones perform the function of support, vertical ones - assimilation.
- Lamellar. The body of the organism in this case has the shape of a plate consisting of one or several cell layers. Sometimes such an organism can acquire a hollow tubular structure inside.
- Siphonal. This type is also called noncellular, since cellular partitions are absent in such organisms. The body of the algae in these cases resembles one large cell containing several nuclei.
- Charophytic. Such organisms consist of a large shoot with a multicellular structure and branches extending from it.
- Sarcinoid. The colony in this case arises as a result of the division of one parent cell and is a group of algal cells enclosed in the shell of the mother cell. The coating is very elastic and tends to stretch.
- Pseudoparenchymatous. The body of the organism is formed as a result of the adhesion and fusion of branching threads.
Algae propagation
Algae reproduce asexually and sexually.
❖ Organs of reproduction of algae (unicellular): ■ sporangia (organs of asexual reproduction); ■ gametangia (organs of sexual reproduction).
❖ Methods of asexual reproduction of algae: vegetative (thallus fragments) or single-celled zoospores.
❖ Forms of the sexual process in algae: ■ isogamy - the fusion of motile gametes of the same structure and size, ■ heterogamy - the fusion of mobile gametes of different sizes (the larger one is considered female), ■ oogamy - the fusion of a large immobile egg with a sperm, ■ conjugation - the fusion of the contents of two unspecialized cells.
The sexual process ends with the formation of a diploid zygote, from which a new individual is formed or motile flagellated zoospores , which serve for the dispersal of the algae.
❖ Peculiarities of algae reproduction: ■ in some types of algae, each individual is capable of forming (depending on the time of year or environmental conditions) both spores and gametes; ■ in certain types of algae, the functions of asexual and sexual reproduction are performed by different individuals - sporophytes (they form spores) and gametophytes (they form gametes); ■ in the development cycle of many types of algae (red, brown, some green) there is a strict alternation of generations - sporophyte and gametophyte ; ■ gametes of algae, as a rule, have taxis, which determine the direction of their movement depending on the intensity of light, temperature, etc.; ■ flagellated spores perform amoeboid movement; ■ in seaweed, the release of spores or gametes coincides with the tide; there is no rest period in the development of the zygote (i.e., the zygote begins to develop immediately after fertilization, so as not to be carried away to the sea).
Life cycle
The life cycle of algae includes two alternating phases - haploid and diploid. The haploid stage is represented by the gametophyte, and the diploid stage is represented by the sporophyte. The haploid phase includes gametes: sperm, sperm, eggs.
When two gametes meet - sperm (n) and egg (n), a zygote (2n) is formed. A sporophyte (2n) matures from it. That. a diploid set of chromosomes is formed. On the sporophyte in zoosporangia, zoospores (n) are produced during meiosis. They form female and male gametophytes (n) by mitotic division. From gametophytes, sperm (n) and eggs (n) develop through mitosis, and when they fuse, a zygote (2n) is formed. The cycle ends.
Rice. 1. Life cycle of algae
The meaning of algae
❖ The importance of algae: ■ they produce organic substances through photosynthesis; ■ saturate water with oxygen and absorb carbon dioxide from it; ■ are food for aquatic animals; ■ are the ancestors of plants that colonized the land; ■ participated in the formation of mountain limestone and chalk rocks, some types of coal and oil shale; ■ green algae clean water bodies polluted with organic waste; ■ used by humans as organic fertilizers and feed additives in the diet of animals; ■ used in the biochemical, food and perfume industries for the production of proteins, vitamins, alcohols, organic acids, acetone, iodine, bromine, agar-agar (necessary for the production of marmalade, pastille, soufflé, etc.), varnishes, dyes, glue ; ■ many species are used for human food (kelp, some green and red algae); ■ some types are used in the treatment of rickets, goiter, gastrointestinal and other diseases; ■ sludge from dead algae (sapropel) is used in mud therapy; ■ can cause “blooming” of water.
Algae are the oldest microorganisms on Earth
Algae are one of the most ancient microorganisms on the planet, the habitat of which is large and diverse. They are the basis of the food chain for many animals and mammals. Their vital activity provides the necessary volumes of oxygen, and their thickets serve as a habitat for various species of invertebrates. The state of the biocenosis of water bodies must be in ideal balance, which cannot happen without algae. In combination with various organic compounds, they participate in the self-purification of water, are used in the agricultural sector and are actively used for the manufacture of necessary human household items.
Green algae
❖ Spirogyra
■ Habitat: fresh standing and slowly flowing water bodies, where it forms bright green mud; widespread in Belarus.
■ Body shape: thin thread-like; the cells are arranged in one row.
■ Features of the cell structure - cylindrical in shape with a well-defined cell wall; covered with a pectin shell and a mucous sheath. The chromatophore is ribbon-shaped, spirally twisted. The vacuole occupies most of the cell. The nucleus is located in the center and is connected by cords to the wall cytoplasm; contains a haploid set of chromosomes.
■ Reproduction: asexually carried out by breaking the thread into short sections; there is no sporulation. The sexual process is conjugation. In this case, two threads of algae are usually located parallel to each other and grow together with the help of copulation processes or bridges. Then the cell membranes at the points of contact of the threads dissolve, forming a through channel through which the contents of one of the cells move into the cell of the other thread and merge with its protoplast, forming a zygote with a dense membrane. The zygote divides by meiosis; 4 nuclei are formed, three of them die; from the remaining cell, after a period of rest, an adult develops.
❖ Ulotrix
■ Habitat: fresh, less often sea and brackish water bodies, soil;
■ The life cycle is complex, with alternating sexual and asexual generations: the multicellular gametophyte (sexual generation that forms gametes) is replaced by a unicellular sporophyte (asexual generation capable of forming spores).
■ Gametophyte body shape: filamentous, unbranched; in reservoirs it forms bright green bushes about 10 cm high.
■ Structural features of the gametophyte: the cells are cylindrical in shape, covered with a thick cellulose membrane, arranged in one row. The thallus threads are attached to the substrate by a colorless conical basal cell, which performs the functions of a rhizoid. All cells, except the basal one, are capable of dividing, causing the continuous growth of the thallus. The chromatophore has the form of a plate forming an open belt or ring (cylinder) along the cell wall.
■ Structure of the sporophyte: the sporophyte is formed by one flagellated cell, which is a zygote surrounded by a dense membrane and equipped with a mucous stalk, with which the sporophyte is attached to the substrate.
■ Reproduction: - asexual - by disintegrating the filament into short sections or by forming flagellated zoospores (4 zoospores are formed in one cell); the sexual process is isogamous; after fertilization, the zygote floats, then settles to the bottom, loses its flagella and forms a sporophyte. After a period of dormancy, the sporophyte nucleus divides and zoospores are formed, which then germinate.
Charovaya algae
Charal algae (or charophytes ) are a highly organized group of freshwater algae ranging in length from several tens of centimeters to several meters, similar in appearance to higher plants - horsetails.
Habitat: clean fresh water bodies with hard water, saturated with soluble calcium salts, at a depth of 1-5 meters or more; form thickets on muddy or sandy bottoms.
Structural features: the body is whorled, its main axis (“stem”) is divided by nodes into long (up to 1 m) internodes. Each internode is formed by one long multinucleate cell, often covered with a crust. From each node there are four or more side shoots (“leaves”). The organs of attachment of the thallus to the substrate are filamentous rhizoids. Chromatophores contain chlorophylls a and b and carotenoids. The reserve product is starch.
Reproduction: ■ asexual - by nodules or sections of the thallus; no disputes are formed; ■ the sexual process is oogamous.
The organs of sexual reproduction are multicellular; male - antheridia (in which several hundred biflagellate sperm develop) and female - oogonia (one egg is formed in each of them). Sperm penetrate the egg through a gap in the oogonia. After fertilization, an oospore (zygote) develops with a thick shell, which germinates after a dormant period.
Red and brown algae
Red algae
■ They live in seas and oceans (sometimes in fresh water bodies) at a depth of more than 200 meters. ■ The dimensions of the thallus range from fractions of a millimeter to several tens of centimeters (most species). ■ Shape of the thallus - filamentous, bushy, lamellar, crust- and coral-shaped, etc. ■ Color - from bright red to bluish-green and yellow. ■ The structure of the thallus is filamentous or lamellar. ■ Organs of attachment—rhizoids, suckers, soles. ■ Chromatophores are lamellar, containing chlorophylls a and d and the pigments phycoerythrin (red) and phycocyanin (blue). ■ They reproduce asexually (by spores) and sexually. In the development cycle of most red algae, there is an alternation of generations - gametophyte and sporophyte.
Brown algae. ■ These are the most highly developed (in terms of morphological and anatomical differentiation of the thallus) among all groups of lower plants;
they may be the ancestors of higher plants. ■ They live in seas and oceans (usually at depths of 40-100 m). ■ The dimensions of the thallus range from a few centimeters to 100 m or more. ■ The shape of the thallus is branched-bushy, lamellar or ribbon-like, divided into stem- and leaf-like “organs”. ■ Color - yellowish-brown. ■ The structure of the thallus is lamellar or ribbon. In most representatives, thalli have a false or true tissue structure (there are assimilation, storage, and conductive tissues). Some representatives have air capsules to maintain an upright position in the water. ■ Organs of attachment - rhizoids or basal disc - disc-shaped growth at the base of the thallus. ■ The thallus grows as a result of intercalary growth or due to division of apical cells. ■ The chromatophores are lamellar, contain chlorophylls o and c and the pigments carotene and fucoxanthin (have a brown color). ■ They reproduce asexually (by spores) and sexually. There is no rest period in the development of the zygote into a new individual. In the development cycle of most brown algae, there is an alternation of generations - gametophyte and sporophyte. Tags: plants
