Like other green sulfur bacteria C. tepidum requires light and specific compounds to perform
anoxygenic photosynthesis.[3]C. tepidum differs from other green sulfur bacteria in that it cannot easily use H2 or Fe2+ as electron donors, relying on elemental sulfur, sulfide, and thiosulfate instead.[5][6][7] To fulfill their metabolic requirements, they reside primarily in anaerobic sulfur rich environments such as anaerobic levels of
stratified lakes and lagoons, anaerobic levels of layered organic
bacterial mats, and in hot springs where there is abundant sulfur.[7]C. tepidum and other green sulfur bacteria also play a large role within the
carbon and
sulfur cycles.[7] Within the sulfur cycle, they contribute to the oxidative branch by oxidizing reduced sulfur compounds.[8] Within anaerobic sediment layers C. tepidum is able to couple carbon and sulfur cycling in a metabolically favorable way.[8]
Photosynthetic mechanism
As it was mentioned before, C. tepidum performs anoxygenic photosynthesis. Within each cell there are 200–250
chlorosomes[3] that are attached to the
cytoplasmic side of
reaction centers inserted within the inner
cell membrane.[3] The
ellipsoidal shaped complexes act as light harvesting antenna to capture energy.[3] Within each chlorosome are 215,000 ± 80,000
bacteriochlorophyll C[4] that act as pigment molecules and absorb unique wavelengths of light relative to their color.[4]C. tepidum contains genes that play an important role in the methylation of the C-8 and C-12 carbons of bacteriochlorophyll C. This methylation allows for BChl C levels to fluctuate in response to a change in the availability of light, resulting in a high efficiency of light harvesting and allowing C. tepidum to survive in areas of very low light intensity.[9][10][11] Light energy is harvested by the chlorosomes and used in conjunction with H2, reduced sulfur compounds, or ferrous iron to preform redox reactions and provide energy to fix CO2 via the
reverse tricarboxcylic acid cycle.[3]
Genome structure
C. tepidum contains a genome that contains 2.15 Mbp, within there are a total of 2,337 genes (of these genes, there are 2,245
protein coding genes and 56
tRNA and
rRNA coding genes).[12] It's synthesis of
chlorophylla and
bacteriochlorophyllsa and c make it a
model organism used to elucidate the biosynthesis of
bacteriochlorophyllsc.[13] Present in the genome of C. tepidum are a multitude of genes that protect the bacterium against the presence of oxygen. The fact that such a large part of the genome is used to encode for protections against oxygen points to the possibility that C. tepidum spent a long period of its evolutionary history in proximity to oxygen, and therefore needed pathways that ensured that living in the presence of oxygen would not substantially harm the bacterium.[14][15] Several of its
carotenoidmetabolic pathways (including a novel
lycopenecyclase) have similar counterparts in
cyanobacteria.[16][17]
^
N.-U. Frigaard, et al. (2006). B. Grimm, et al. (eds.). Chlorophylls and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications. Vol. 25. Springer. 201–221.