In this article, I briefly describe the purple and green phototropic bacteria.
Phototropic bacteria
These are a group of bacteria that derive their energy for growth from sunlight, and their source of carbon comes from carbon dioxide or organic carbon. The phototropic bacteria can be categorized into two groups, i.e., anoxygenic phototrophic bacteria and oxygenic phototrophic bacteria. Anoxygenic phototrophic bacteria are gram-negative bacteria that can use light as an energy source. They are anaerobic bacteria as they do not evolve oxygen during photosynthesis. Aerobic anoxygenic phototrophic bacteria, a different group, are obligate aerobes and capture energy from light by anoxygenic photosynthesis.
Anaerobic anoxygenic phototrophic bacteria
These are Gram-negative bacteria that contain bacteriochlorophyll and can use light as an energy source. These organisms do not evolve O2 during photosynthesis. The anoxygenic bacteria grow phototrophically only under anaerobic conditions and are incapable of forming O2. Anaerobic anoxygenic phototrophic bacteria occur in anaerobic freshwater or marine environments.
They may occur beneath the surface of shallow aquatic environments rich in organic matter, such as stagnant ponds, ditches, and salt marsh pools, or sometimes they have a much deeper habitat, such as at the bottom of a lake. The various water-insoluble carotenoid pigments present in these bacteria absorb light energy and transmit it to the bacteriochlorophyll. The bacteriochlorophyll and the carotenoid pigments of anoxygenic bacteria can absorb light energy in the blue to blue-green range. This is very helpful when anoxygenic bacteria occur in the depths of a lake because blue light can penetrate water a greater distance than red light.
The carotenoids determine the color of anoxygenic phototrophic bacteria. Based on pigmentation, they can be divided into two major groups, i.e., green bacteria and purple bacteria.
The green phototropic bacteria
These are Gram-negative bacteria that contain bacteriochlorophyll and can use light as an energy source. The green phototrophic bacteria (figure 1) contain bacteriochlorophyll type c or d and minor amounts of bacteriochlorophyll a. The bacterial cultures are green or brown.
The green-sulfur bacteria
The family Chlorobiaceae contains the green-sulfur bacteria that are obligatory anaerobic photoautotrophic bacteria. The organisms are unable to grow in the dark, even under microaerophilic conditions. The cells can be visualized as ovoid, bean-shaped, or rod-shaped in appearance. The bacteria multiply only by binary fission.
They do Photosynthesis by using bacteriochlorophyll (BChl) c, d, or e in addition to BChl a and chlorophyll a. The green-sulfur bacteria live as photolithotrophs using H2S as the electron donor for CO2 fixation. The granules of elemental sulfur are deposited only outside the cells, and the sulfur can eventually be oxidized to SO42-. The bacterium Chlorobium tepidum has emerged as a model organism for the group, and though only ten genomes have been sequenced, these are quite comprehensive of the family’s biodiversity. Their small dependence on organic molecule transporters and transcription factors also indicates that these organisms are adapted to a narrow range of energy-limited conditions.
The green non-sulfur bacteria
The family Chloroflexaceae contains the green non-sulfur bacteria. These bacteria form flexible filaments, also called green flexi bacteria. They possess gliding mobility. The organisms are mainly photo-organotrophic, i.e., organic substances serve both as carbon sources and as electron donors for the reduction of carbon dioxide. They can also grow as photolithotrophs with H2S as the electron donor. They can grow aerobically in the dark as chemoheterotrophs.
The main bacterial genus, Chloroflexus, is thermophilic as it occurs in hot springs where it forms green or orange mats. In hot springs, Chloroflexus will form a mat with H2S, without cyanobacteria, and maybe green in color. In these conditions these are photoautotrophs, with sulfide as the electron donor. These strains are obligately anaerobic and photoautotrophic, still, they grow best as photoheterotrophs.
The bacterium Chloroflexus auranticus is found in alkaline hot springs (pH 5.5-10), where it forms an orange mat below a layer of cyanobacteria. The bacterial genus Chloroflexus under these conditions is probably living photoheterotrophically, depending upon the cyanobacteria for fixed carbon. These strains are facultatively aerobic. The Chloroflexus cells occur as filaments or trichomes and exhibit gliding motility.
The purple phototrophic bacteria
The purple bacteria (figure 2) are proteobacteria that are phototrophic. They contain the chlorophyll pigments bacteriochlorophyll a or b, together with various carotenoids, which give them colors ranging between purple, red, brown, and orange. The reaction centers on the cell membrane, where photosynthesis takes place, and is folded into the cell to form sacs, tubes, or sheets, thus increasing the available surface area. The purple bacteria do not produce oxygen, as water is not the reducing agent involved in photosynthesis. In purple sulfur bacteria, the reducing agent is either sulfide or elemental sulfur.
The purple non-sulfur bacteria
This group of bacteria belongs to the family Rhodospirillaceae. The purple non-sulfur photosynthetic bacteria constitute a non-taxonomic group of versatile organisms. In this group, most of the bacteria can grow as photoheterotrophs, photoautotrophs, or chemoheterotrophs switching from one mode to another depending on the conditions available. The conditions pertain to the degree of anaerobiosis, availability of carbon source (CO2 for autotrophic growth, organic compounds for heterotrophic growth), and availability of light (needed for phototrophic growth).
Under aerobic conditions, the cultures appear orange-brown to purple-red. Under anaerobic conditions, some cultures appear the same as under aerobic conditions, whereas some appear greenish-yellow in color. The organisms grow in anaerobic aquatic environments, such as mud and stagnant water, although they can survive in air. The purple non-sulfur bacteria exhibit a diversity of shapes, i.e., helical, rod-shaped, ovoid, or spherical cells.
The purple non-sulfur bacteria are photoorganotrophs. Some species can grow autotrophically by using hydrogen sulfide as the electron donor. Photosynthesis occurs only under anaerobic conditions and in the presence of light. These bacteria attain chemotrophic growth by respiration, although there are some exceptional strains and species that can obtain energy by fermentation or anaerobic respiration.
The purple-sulfur bacteria
This group of bacteria belongs to the family Chromatiaceae. These are a group of anaerobic proteobacteria capable of photosynthesis and are often found in hot springs or stagnant water. Most species are anaerobic and cannot grow in the dark. Unlike plants, algae, and cyanobacteria, they do not use water as their reducing agent and thus do not produce oxygen. Instead, they use hydrogen sulfide as the reducing agent, which is oxidized to produce granules of elemental sulfur. This, in turn, may be oxidized to form sulfuric acid.
The cultures appear orange-brown to purple-violet. The purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates. They can also be found in “sulfur springs” where geochemically or biologically produced hydrogen sulfide can trigger the formation of blooms of purple sulfur bacteria. These bacteria cannot thrive in oxygenated environments, so anoxic conditions are necessary for photosynthesis. These bacteria can be ovoid to rod-shaped, coccoid or helical in appearance. All the genera of this bacteria are capable of photolithotrophic growth, using H2S or elemental sulfur as the electron donor for CO2 fixation.
Conclusion
The phototropic bacteria can be categorized into two groups, i.e., anoxygenic phototrophic bacteria and oxygenic phototrophic bacteria. Anoxygenic phototrophic bacteria are gram-negative, anaerobic bacteria that can use light as an energy source.
The green phototrophic bacteria are Gram-negative bacteria that contain bacteriochlorophyll and can use light as an energy source. The green-sulfur bacteria are obligatory anaerobic photoautotrophs that are unable to grow in the dark even under microaerophilic conditions.
The green non-sulfur bacteria form flexible filaments and are also called green flexi bacteria. They possess gliding mobility. The main bacterial genus, Chloroflexus, is thermophilic as it occurs in hot springs where it forms green or orange mats.
The Purple bacteria are proteobacteria that are phototrophic. They contain the chlorophyll pigments bacteriochlorophyll a or b, together with various carotenoids, which give them colors ranging between purple, red, brown, and orange. The purple non-sulfur bacteria exhibit a diversity of shapes, i.e., helical, nonprosthecate rod-shaped, ovoid, or spherical cells.
The organisms grow in anaerobic aquatic environments, such as mud and stagnant water, although they can survive in air. The purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic habitats where hydrogen sulfide accumulates. These bacteria cannot thrive in oxygenated environments, so anoxic conditions are necessary for photosynthesis. These bacteria can be ovoid to rod-shaped, coccoid, or helical in appearance.
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I, Swagatika Sahu (author of this website), have done my master’s in Biotechnology. I have around twelve years of experience in writing and believe that writing is a great way to share knowledge. I hope the articles on the website will help users in enhancing their intellect in Biotechnology.