Higher order taxa
Domain: Bacteria Phylum: Proteobacteria Class: Alphaproteobacteria Order: Rhodospirillales family: Rhodospirillaceae (Molisch 1907)
Description and significance
Pararhodospirillum photometricum is a Gram-negative, spiral, rod-shaped bacterium. R. photobacterium got its name by having a spiral, rod shape. It got its name photobacterium from the words phôs-otos, meaning light, and metricus, relating to measuring; metrica. Photometricum was intended to mean light measuring. They measure between 1.1-1.5µm wide. One complete turn of a spiral is 2.5-4µm wide and 4-7µm long. Cells that are 14-30µm long are common as well. R. photometricum is motile and has a brown color. Cultures are brown-orange to brown-red or dark brown. Photosynthetic pigments are bacetriochlorophyll a and carotenoids of the spirilloxanthin series (Brenner et al.).
Their internal photosynthetic membranes consist of several lamellar stacks forming a sharp angel with the cytoplasmic membrane. Because it mainly lives in fresh waters, there is no requirement of salt. This bacteria does not go through aerobic growth. It is important to note that R. rubrum does go through aerobic growth, and unlike R. photometricum, it is able to go through the oxidation of sulfide. R. photometricum and R. rubrum are currently the only ones in the genus Rhodospirillum because other species were placed into different genus regarding many factors such as genetic distance among spiral-shaped PNSB (Brenner et al.).
Genome and genetics
R. photometricum belongs to the Phototrophic Alphaproteobacteria group as R. photometricum and R. rubrum are one of the only ones in their genus that have large type cytochrome c2. This is different as opposed to the genus Phaeospriillum who have small type cytochromes c2. Based on their 16S rDNA sequence analysis, different phylogenetic relationships are observed while differentiating various different phototrophic Alphaproteobacteria. Manipulation of the 16S rDNA aids in tracing the Rhodospirillum genus and where they are on the phylogenetic tree with other Alphaproteobacteria.
Three species of the genus Aquaspirillum belong to the Alphaproteobacteria based on rRNA-DNA hybridization and 16S rRNA gene sequencing. Two of these species, A. itersonii and A. peregrinum were closely related to the species R. rubrum and R. photometricum.
Nutrition and metabolism
This species mainly lives in Fresh water environments at an optimal temperature of 25-30°C and an optimal pH level of 6.5-7.5. These cells grow photoheterotrophically under anoxic conditions in the light with various organic compounds such as carbon and electron sources (Brenner et al.).
The carbon sources that they use range from a whole variety. Nitorgen sources such as ammonia,alanine, glutamate, and asparagine are used. Nicotinic acid is required as a growth factor.
Little research and studies have been made of R. photometricum. This species is very sensitive to oxygen and does not grow under oxic conditions in the dark. What is speculated is that it is unable to induce a second electron transport chain in the presence of oxygen and therefore depends on microoxic conditions in which the internal membrane system and the light-driven electron transport chain are fully expressed. The cells are fully pigmented under these growth conditions (Chandler et al.).
Ecology / Pathology
Ecology: R. photometricum mainly lives in stagnant, fresh water environments at an optimal temperature of 25-30°C and an optimal pH level of 6.5-7.5. These habitats have to be exposed to light as the cells go through photosynthesis, harboring major quinone components, Q-8 and RQ-8 (Brenner et al.).
Pathology: There has not been any further research into the pathogenicity of the bacteria unto humans, however exposure to the bacteria does not cause any lethal outcomes. Since it lives as an autotroph, producing its energy from photosynthesis and the breaking down of sugars, isolating them from the light would kill them. Also, isolating them form Nicotinic acid would greatly affect their growth as it is a required growth factor.
Research has been performed on its light harvesting capabilities as well in order to see its potential in harvesting light using pigment-protein complexes, called chromatophores. They constructed a model of a chromatophore from R. photometricum based on atomic force microscopy data. They were able to characterize the intercomplex excitation transfer network and dive into the relationship between the close-packing and light-harvesting efficiency of R. photometricum's photosynthetic capabilites (Chandler et al.).
Recent research that has been studied upon R. photometricum includes a change in the classification of the genus. Research claimed that Rhodospirillum photometricum made it evident that the genus still remained heterogeneous in phenotypic and genetic properties, thus a change in the classifcication was incited (Lakshmi et al.). Major differences were observed in the carotenoid composition within the genus. Many different species contained lycopene and rhodopin, and in addition, the structure of the intracytoplasmic membrane system is not a prime characteristic to differentiate between different genera, therefore, a different genus with identical internal membrane systems was chosen to be more ideal. R. photometricum changed from being in the Rhodospirillum genus to the Pararhodospirillum genus.
Brenner, D, Krieg, N, and Staley, J. Bergey's Manual of Systematic Bacteriology. Second Edition. East Lansing, MI. Bergey's Manual Trust. 2005. 120-123.
Chandler DE, Strumpfer J, Sener M, Scheuring S, Schulten K. Light harvesting by lamellar chromatophores in Rhodospirillum photometricum. Biophys J. 2014. 106:2503-10.
Lakshmi KV, Divyasree B, Ramprasad EV, Sasikala Ch, and Ramana ChV. Reclassification of Rhodospirillum photometricum Molisch 1907, Rhodospirillum sulfurexigens Anil Kumar et al. 2008 and Rhodospirillum oryzae Lakshmi et al. 2013 in a new genus, Pararhodospirillum gen. nov., as Pararhodospirillum photometricum comb. nov., Pararhodospirillum sulfurexigens comb. nov. and Pararhodospirillum oryzae comb. nov., respectively, and emended description of the genus Rhodospirillum. Int J Syst Evol Microbiol. 2014. 64:1154-9.
Molisch, H. Die Purpurbakterien nach neueren Untersuchungen. Gustav Fischer Verlag, Jena (1907). pp. 1-95.