Thiocapsa cells are spherical to ovoid and reproduce by binary division. The cells do not move, and then quadruplets are formed due to continuous division in two vertical planes. In some adverse situations or natural environments, cells can become nearly regular aggregates surrounded by mucus. According to commonly used classification methods, Thiocapsa is a gram-negative bacteria. The bacteria contain bacteriochlorophyll a or b and carotenoids, which are located on the inner membrane of the cyst or cast. There are no air bubbles in the bacteria.
Figure 1. Colonies of Thiocapsa sp. Ya-2003 bacteria on the Van Niel medium. ()
Thiocapsa is an anaerobic photoautotroph; however, one of its type species, Thiocapsa rosepersicina, can undergo oxidative metabolism in the dark under micro-aerobic to aerobic conditions. In the presence of hydrogen sulfide, the bacteria can perform photosynthesis, and then they produce sulfur as an intermediate oxide and store spherical elemental sulfur in the cells. In this process, molecular hydrogen can act as an electron donor, and photosynthesis does not release molecular oxygen. Due to the photosynthetic pigments, the cell suspensions have different shades of orange-brown to lavender and red.
The cells of Thiocapsa roseopersicina are spherical, 1.2-3μm in diameter, usually 1.5μm. Individual cells are usually surrounded by a sticky capsule. Commonly seen are double spherical aggregates, quadruplets and irregular stacks, which are usually surrounded by a mucus layer. Bacterial cells do not move and have a cystic photosynthetic membrane system. Individual cells are colorless under the microscope. Cell suspensions and aggregates were pale red to rose red.
Thiocapsa roseopersicina is an anaerobic photoautotroph. All strains were able to grow under micro-aerobic to aerobic dark conditions in the presence of fructose, glycerol or organic acids. The optimum growth pH of the bacteria is: 7.0-7.5. The optimum growth temperature is 25°-30°C. In photosynthesis, photosynthetic electron donors are: sulfide, thiosulfate, sulfur and molecular hydrogen. Photosynthesizes acetate, fructose, fumarate, glycerol, malate, pyruvate and succinate. Most strains cannot utilize sulfite, lactate, propionate or peptone, primary alcohol, benzoate, butyrate, citrate, formate, alpha-ketoglutarate, tartrate and amino acids. In addition, most strains also have assimilatory sulfate reduction.
Thiocapsa pfennigii has a cell diameter of 1.2-1.5 μm in logarithmic growth phase and is 2.5 μm long before division. The quiescent cells are 0.8-1.0 μm in diameter. Stored spherical elemental sulfur is usually located in the center of the cell. There is an extensive endomembrane system within the cell that occupies most of the cytoplasm. The membrane system consists of ribbon-like branched tubular bundles that are connected to the cytoplasmic membrane. The single cell of the bacteria was colorless, and the color of the cell suspension was brown-orange.
Thiocapsa pfennigii is an obligate photoautotroph and is strictly anaerobic. The optimum growth pH range of the bacteria is: 6.5-7.5; optimum growth temperature about 25℃. Photosynthetic electron donors: sulfide and sulfur. In the presence of sulfide and hydrogen sulfate, bacteria can photoassimilate acetate and propionate, but cannot utilize thiosulfate and butyrate.
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