Phototrophic Bacteria

Microbial Diversity

• Phylogenetic diversity refers to evolutionary relationships between organisms.
• Functional diversity refers to relationships in form and function between organisms.
• Physiological diversity refers to relationships in microbial metabolism and cellular biochemistry.
• Ecological diversity refers to relationships between organisms and their environment.
• Morphological diversity refers to relationships associated with outward appearance of organisms.

Proteobacteria

• Proteobacteria is the largest and most metabolically diverse group of all Bacteria.
• It constitutes the majority of known bacteria of medical, industrial, and agricultural significance.
• The phylum Proteobacteria can be divided into 5 classes, based on 16S rRNA sequences.
  • Alphaproteobacteria
  • Betaproteobacteria
  • Gammaproteobacteria
  • Deltaproteobacteria
  • Epsilonproteobacteria

Overview of Phototrophic Bacteria

• Photosynthesis is the conversion of light energy to chemical energy, which is carried out by phototrophs.
• Most phototrophs are also autotrophs.
• Photosynthesis requires light-sensitive pigments known as chlorophylls.
• Photoautotrophy requires ATP production and CO₂ reduction.

Phototrophic Proteobacteria

• Anoxygenic phototrophic proteobacteria consist of purple sulfur and purple non-sulfur bacteria.

Purple Bacteria

• It is a morphologically diverse group with an established classification based on morphological, phylogenetic, and physiological characteristics.
• Different genera fall within Alpha, Beta, and Gammaproteobacteria.
• These bacteria are Gram-negative.
• Purple bacteria contain bacteriochlorophylls a or b and carotenoid pigments, giving them different colors such as purple, red, and brown.
• They produce intra-cytoplasmic photosynthetic membrane systems in which the pigments are inserted, and the membranes have different shapes.
• The internal membranes allow purple bacteria to increase the amount of pigment to better utilize the available light.
• When cells are grown at high light intensities, fewer pigments are seen. At low light intensities, the membranes are packed with photopigments.

Purple Sulfur Bacteria

• Purple bacteria that utilize hydrogen sulfide (H₂S) as an electron donor for CO₂ reduction in photosynthesis are called purple sulfur bacteria.
• The sulfide is oxidized to chemical sulfur (S) which is stored as globules in the cell.
• The sulfur is later oxidized to sulfate (SO₄²⁻).
• These bacteria are strict anaerobes and usually photolithotrophs, and they utilize a Type II photosystem.
• Fixation of CO₂ happens by the Calvin cycle.
• Chromatium okeni is oval and rod shaped, with sulfur deposited internally.
• Thiiopredia rosea are spheres, non-motile, forming flat sheets of tetrad, with sulfur deposited internally.
• Thiospirillum jenense are flagellated spirals with sulfur deposited internally.
• Ectothiorhodspira mobilis deposits sulfur externally.
• Many purple sulfur bacteria can utilize other reduced sulfur compounds as photosynthetic electron donors.
• Thiosulfate (S₂O₃²⁻) is commonly used and is frequently used to grow lab cultures.
• All purple sulfur bacteria discovered are Gammaproteobacteria.
• Purple sulfur bacteria are generally found in illuminated anoxic zones of lakes and other aquatic environments where H₂S accumulates.
• They are found in sulfur springs where geochemically or biologically produced H₂S can trigger the formation of blooms.
• They are found in meromictic (permanently stratified) lakes, where there is denser (saline) water at the bottom and less dense (freshwater) at the top.
• The sulfide produced in the sediments diffuses upward into the anoxic bottom water, forming dense cell masses called blooms.
• These blooms are found in association with green phototrophic bacteria.
• The genera Ectothiorhodospira and Halorhodospira oxidize H₂S but produce S outside the cell.
• They are extreme halophiles (salt-loving) or alkaliphilic (grow best at pH above 9), and are considered extremophiles.
• Purple sulfur bacteria are found in saline lakes, soda lakes, and sewage lagoons.
• 200 strains of marine purple sulfur bacteria were isolated, and 2 strains showed antibiotic activity to Saccharomyces cerevisiae.
• These strains were Chromatium pupuratum.
• C. pupuratum strain NKPB 031704 produced pigmented compounds that exhibited broad-spectrum antibiotic properties.
• This was the first time that a photosynthetic bacteria was found to produce broad-spectrum antibiotics.
• It was also the first example of intracellular localization of antibiotic activity in a marine bacterium.

Purple Non-Sulfur Bacteria

• These bacteria were originally thought to be unable to use sulfide as an electron donor for the reduction of CO₂ to cell material.
• Although sulfide can be used by most species in this group, it is not at the levels used by the purple sulfur bacteria (1-3mM).
• Some purple nonsulfur bacteria can grow by fermentation or anaerobic respiratory metabolism.
• They can grow in the dark using aerobic respiration of organic or some inorganic compounds.
• The synthesis of the photosynthetic machinery is repressed by oxygen.
• The electron donor can be an organic or inorganic compound such as H₂.
• They are photoheterotrophs, using light as the energy source and an organic compound as the carbon source; this increases their competitiveness.
• Carbon sources include fatty acids, amino acids, sugars, alcohols, and aromatic compounds (benzoate, toluene).
• Virtually all purple non-sulfur bacteria can fix N₂.
• This helps them in outcompeting other bacteria in agricultural applications.
• Members of Alphaproteobacteria and Betaproteobacteria and are morphologically diverse.
• These use type II photosystem.
• Contain either bacteriochlorophyll a or b.
• Phaeospirillum fulvum is spiral shaped and polarly flagellated.
• Rhodoblastus acidophilus is rod shaped.

Rhodopseudomonas palustris

• Belongs to the genus Rhodopseudomonas, and its entire genome was sequenced by the Caroline Harwood team.
• Sequenced as model for studying anaerobic degradation of aromatic compound.
• 31% of its genome coded for energy metabolism, biosynthesis, C and N metabolism, and cellular processes.
• It has genes that oxidize hydrogen, thiosulfate, and carbon monoxide and uses them as energy sources.
• This bacteria has 3 nitrogenases, 5 benzene ring pathways, and 4 light-harvesting 2 systems.
• It is used in the degradation of benzene in the environment and TCP (2,4,6-Trichlorophenol), which is used in fungicides, herbicides, and insecticides.
• TCP is carcinogenic.
• It reduces chromium (VI) to chromium (III).
• Chromium (VI) has serious implications if consumed or inhaled, and it can cause lung cancer or cancer of the oral cavity or small intestine.
• It synthesizes cadmium sulfide (CdS) nanoparticles, using the enzyme cysteine desulfhydrase for this process.
• It also secretes a protein that stabilizes the nanoparticles produced.