GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

18. PHOTOTROPHIC BACTERIA AND PHOTOSYNTHESIS

18.1. CHARACTERISTICS OF PURPLE AND GREEN BACTERIA

According to the ninth edition of Bergey's Manual of Systematic Bacteriology, the Class Anoxyphotobacteria comprises two orders (Table 18.1). The first order, Rhodospirillales (purple Bacteria), unites the families Rhodospirillaceae (purple non-sulfur bacteria) and Chromatiaceae (purple sulfur bacteria). The second order, Chlorobiales (green bacteria), unites the families Chlorobiaceae (green sulfur bacteria) and Chloroflexaceae. They form a morphologically diverse group including cocci, rods, curved forms (spirilla, vibrios), as well as both motile and non-motile species.

Table 18.1.

Anaerobic phototrophic bacteria

Bchl - bacteriochlorophyll, PM - Cell/33.html">Plasma Membrane

Following the 16S rRNA analysis of prokaryotes conducted in the mid-1980s, green phototrophic bacteria were assigned to Group 9 (Chloroflexus) and Group 10 (Chlorobium), whereas purple bacteria were placed in Group 11, which was segregated into the separate class Proteobacteria in 1988.

In the book "The Prokaryotes" (1999), green phototrophic bacteria of the family Chlorobiaceae are placed in Section 5, and Representatives of the family Chloroflexaceae in Section 7. Purple phototrophic bacteria are included in Part C: Proteobacteria (see Section 7).

Purple bacteria. A common feature of members of the order Rhodospirillales is that their photosynthetic apparatus (light-harvesting systems and reaction centers) is located on internal membranes (thylakoids) formed by invaginations of The Plasma Membrane. Thylakoid structures can be vesicular, tubular, or lamellar. The characteristic chlorophyll for these bacteria is chlorophyll a. CO₂ fixation occurs via The Calvin Cycle. Purple bacteria are capable of using Organic compounds as hydrogen Donors and/or carbon sources.

Based on their ability to use elemental sulfur as an electron donor, purple bacteria are divided into sulfur and non-sulfur species. A typical representative of purple sulfur bacteria is Chromatium vinosum, and of non-sulfur bacteria, Rhodospirillum rubrum. Sulfur bacteria are easily recognized by their intracellular sulfur inclusions (appearing as highly light-refractive globules).

Green bacteria. These bacteria are characterized by the presence of chlorosomes—Organelles attached to the cytoplasmic membrane that contain the characteristic light-harvesting pigment, bacteriochlorophyll (c, d, or e). In addition, they contain small amounts of bacteriochlorophyll a, which is directly associated with photosynthetic Reaction Centers and localized in the cytoplasmic membrane. Green bacteria are incapable of fixing carbon dioxide via the Calvin cycle (they lack the enzyme ribulose bisphosphate carboxylase). CO₂ assimilation occurs through the reductive Tricarboxylic Acid Cycle.

A typical species of green sulfur bacteria is Chlorobium limicola, and of non-sulfur bacteria, Chloroflexus aurantiacus.

Metabolic features of Purple and green bacteria. These bacteria exhibit a versatile METABOLISM. For instance, many purple non-sulfur bacteria are capable of both anaerobic growth in the light and aerobic growth in the dark (utilizing organic substrates). Other groups are strict anaerobes and obligate phototrophs. Many species use molecular hydrogen as a hydrogen donor (representatives of the genera Rhodobacter and Chromatium, as well as rhodospirilla and Chlorobium, grow in the light using H₂ and CO₂). Certain species of these bacteria can utilize hydrogen sulfide or elemental sulfur as a hydrogen (electron) donor. Both CO₂ and organic substrates are assimilated. The vast majority of these phototrophic bacteria are capable of MOLECULAR Nitrogen Fixation. Poly-β-hydroxybutyrate, Polysaccharides, and polyphosphates accumulate as storage compounds.

Pigments of the photosynthetic apparatus. Due to their Photosynthetic Pigments, sufficiently dense Suspensions of phototrophic bacteria display green, blue-green, purple-violet, red, brown, and pink colors. The color depends on The Nature and quantitative ratio of the pigments. Individual pigments can be identified by the absorption spectra of intact Cells. Chlorophylls, for example, are responsible for absorption maxima in the blue (< 450 nm), red, and infrared (650–1100 nm) Regions of the spectrum. Absorption in the 400–550 nm region is caused by carotenoids, whereas in cyanobacteria, absorption in the 550–650 nm region is due to phycobiliproteins.

Carotenoids serve two main Functions: they participate in Photosynthesis as light-harvesting pigments, absorbing light energy and transferring it to chlorophyll; and they protect chlorophyll against photooxidation. Blue-green mutant forms of purple bacteria that lack carotenoids can grow only under dim light and are killed by high light intensity.

Distribution of phototrophic bacteria. Phototrophic purple and green bacteria inhabit the anaerobic zones of various Water bodies, shallow ponds, slow-flowing waters, lakes, and marine bays. Purple sulfur bacteria frequently form coatings of various shades of red (ranging from pale pink to deep red) On the surface of mud or decaying organic matter. Sometimes they swarm above the mud surface, forming a layer about 10 cm thick ("water bloom"). Intensive reproduction of purple sulfur bacteria is also observed in shallow ponds whose surface is covered with a dense layer of duckweed or water lily leaves.

This peculiar biological filter absorbs those spectral components of light that can be utilized by green Algae and cyanobacteria, while transmitting light that is absorbed by bacteriochlorophylls and dark-red carotenoids. Consequently, anaerobic phototrophic bacteria thrive beneath such a green cover.

Seasonal mass development of purple sulfur bacteria is also observed in the anaerobic zones of lakes below the thermocline (at a depth of 10–30 m). Here, the bacteria find the substances they need—hydrogen sulfide, CO₂, and organic compounds. Infrared solar radiation, which is absorbed by bacteriochlorophylls, does not penetrate to such depths. Under these conditions, the energy maximum falls within the 450–500 nm spectral region—precisely the region where carotenoids absorb. The high carotenoid content in the cells of purple bacteria enables their photosynthetic metabolism at great depths. Accordingly, at such depths among green sulfur bacteria, carotenoid-rich brown forms also predominate.