I KINGDOM MONERA

We will examine two groups of Kingdom Monera: Eubacteria (true bacteria) and Cyanobacteria (blue-green algae). Some texts consider these groups as subkingdoms and others consider them as divisions of Kingdom Monera.

Eubacteria

Bacteria (singular, bacterium), which belong to group Eubacteria are small, relatively simple, single-celled organisms. They have existed on earth longer and are more widely distributed than any other organisms. They are found in almost every imaginable habitat: in air, soil, water, in extreme temperatures, and in harsh chemical environments. Some bacteria can be photosynthetic, using H₂S rather than H₂O, as a source of electrons, but most are heterotrophic, absorbing nutrients from the surrounding environment.

Bacteria are called "prokaryotes", from the Greek for "prenucleus". Their one distinguishing characteristic is that they do not contain membrane bound "organelles". Particularly their genetic material is not bound by a nuclear envelope. Bacteria do not have chromosomes, as described in last term. Instead, their genetic material is a single circular loop of DNA (Figure 1). They reproduce by the process of "binary fission", where the cell duplicates its components and divides into two cells (Figure 2). In other words, the cell pinches into two without the complex movement of chromosomes seen in mitosis. Newly produced cells usually become independent, but they may remain attached in linear chains or grapelike clusters. In favourable environments, individual bacterial cells rapidly proliferate, forming colonies consisting of millions of cells.

Asexual reproduction (by binary fission) is the only method of reproduction among bacteria, however this only increases bacterial numbers not their genetic variation. Genetic variation in bacteria does occur and is accomplished by 4 methods. Three methods are by obtaining genes from the environment or other bacteria. The fourth method is simply by mutation. This is not a directed method but, by pure chance, a mutation could be beneficial for the bacteria and with the rapid generation time of bacteria (measured in minutes), a beneficial mutation can quickly predominate. The first method by which bacterial genetic variation is accomplished is by transformation. Transformation is the process by which a bacterium "picks up" a gene or genes from the environment. Conjugation is the process by which genes (on plasmids) are transferred from one bacterium to another by conjugation bridges. This transfer can be between bacteria of the same genus and species, between different species in a genus or between two bacteria of different genera. Lastly, transduction is the process by which genes are transferred from one bacterium to another by viruses.

 

Figure 1: Typical prokaryotic cell (non-photosynthetic bacterial cell).

Figure 2: Binary fission in bacteria.

 

Microscopic examination of bacterial cells reveals that most bacteria can be classified according to three basic shapes: bacilli (rods), cocci (spheres), and spirilla (spirals, or corkscrews). Examine slide #89, which is a composite slide that shows each of these three major bacterial types. You need to scan the whole slide in order to find all three shapes. It is best to use the lowest magnification of the microscope to initially find these cells, and then switch to the highest power to observe them in more detail. Also, note Figure 3, which shows each of these different bacterial types.

Figure 3: Bacterial types.

 

Most bacteria in this group are heterotrophic, which means that they derive their energy from organic molecules made by other organisms. Many heterotrophic bacteria are important in the ecosystem as decomposers because they feed on dead organic matter and release nutrients locked in dead tissues. Specifically, they secrete enzymes that cause the breakdown of organic matter in dead organisms and their wastes, a practice that has earned them an alternative name "saprobe" (sapros from the Greek, meaning "rotten" or "putrid"). This activity is ecologically crucial and is undoubtedly the most important of all bacterial activities. Decomposition releases such key ions as nitrates, phosphates, and sulphates, which then become available to other organisms. Other heterotrophs are parasites, often referred to as pathogens. They cause many of the diseases of plants and animals, including those of humans.

Cyanobacteria

Cyanobacteria are commonly known as blue-green algae. They are autotrophic, which means that they derive their energy from photosynthesis or the oxidation of inorganic molecules. In addition to chlorophyll a, they contain phycocyanin (blue) and phycoerythrin (red). Because of various proportions of these pigments, only about half of cyanobacteria are actually blue-green in color; many range in color from brown to olive green. They live in aquatic environments including oceans, ponds, lakes, tidal flats, and moist soil. Cyanobacteria exist mostly as colonies and filaments and sometimes as single cells. The cyanobacteria can move. For example, filamentous forms such as Oscillatoria sp. rotate in a screw like manner, while the gelatinous forms glide along in a mucus-like slime they produce. Blue-green algae produce gelatinous capsules, which are often lighter than water and therefore help keep the algae up near the surface of the water where there is the most sunlight. As far as is known, reproduction in the cyanobacteria is by fission only.

The cells of the cyanobacteria are prokaryotic but reveal a considerable level of complexity (see Figure 4). Their chlorophyll is integrated into thylakoids, extensions of the cell membrane. Actually, the entire photosynthetic cell is comparable to a eukaryotic chloroplast. Photosynthesis in the cyanobacteria is nearly identical, biochemically, to that of the algae and the green plants. Like the algae and plants, their photosynthetic pigments include chlorophyll a and the accessory pigment beta-carotene, although they lack chlorophyll b. The glucose produced by the cyanobacteria in the process of photosynthesis is stored in their own form of starch, which is similar to animal glycogen. These characteristics make cyanobacteria predecessors of the eukaryotic chloroplasts.

Figure 4: Typical cell of cyanobacteria.

A number of cyanobacteria produce specialized, nitrogen-fixing cells called heterocysts. Their role is to incorporate atmospheric nitrogen into a form useful for producing amino acids and other nitrogen containing molecules. Some cyanobacteria also produce spores (akinetes) that are resistant to drying. These spores allow cyanobacteria to survive unfavorable environmental conditions. Examine the prepared slide of Anbaena sp. (slide # 76) and note the vegetative (photosynthetic) cells and the heterocysts (Figure 5). Anabaena sp. is a very common filamentous blue-green alga found in stagnant ponds late in the summer.

 

Figure 5: Anabaena sp

Examine the prepared slide of Gloeocapsa sp. (slide # 78), which is another very common colonial blue - green alga, which produces a thick gelatinous sheath (Figure 6).

 

What do you suppose is the function of the sheath?

             

Figure 6: Gloeocapsa sp.