Survey of Genomes - Chlorobium tepidum TLS

Look out your window at that beautiful tree or shrub nearby. Now imagine it living in a hot springs at over 50C doing photosynthesis without oxygen as a byproduct but rather by excreting elemental sulfur. Kerry Vickers from the 2019 Hiram College Genetics course tells us about a microbial anaerobic thermophilic phototroph, Chlorobium tepidum strain TLS, that does just that.

Welcome to Genomics Revolution. This is Kerington Vickers from the 2019 Hiram College Genetics course hosting this episode on the genome of Chlorobium tepidum TLS. I will call this C. tepidum from here on out. This strain of thermophilic green sulfur bacteria was isolated from acidic high sulfide hot springs on North Island, New Zealand (3). 

This organism is part of the Chlorobiaceae family which are also called green sulfur bacteria. Green sulfur bacteria use sulfide ions as electron donors for photosynthesis. They are known to be found in depths of up to 145 m in the Black Sea, with low light availability. The genome of C. tepidum has been completely sequenced. The single circular chromosome in this organism is approximately 2.15 Mb in size. There are a total of 2,337 genes (of these genes, there are 2,245 protein coding genes and 56 tRNA and rRNA genes). This organism can be found growing in dense mats over hot springs as well as other warm muds and bodies of water that contains sufficient hydrogen sulfide.
So far we know that C. tepidum were found in hot springs in New Zealand, the whole genome is sequenced and contains 2,227 genes and we learned about green sulfur bacteria... what else is next? I bet you are wondering the growing conditions are for C. Tepidum.

It is the only known thermophilic member of its family which means it thrives at relatively high temperatures. It has been recorded to grow optimally at 48 degrees Celsius (this is 118.4 degrees Fahrenheit). If that was us we would not be able to survive in this type of environment for long periods of time. They grow best in pH levels between 6.0 and 4.5. They described the bacteria as gram negative and rod-shaped. It is able to harvest light through special photsynthetic systems known as chlorosomes.

These unique features make studies of Chlorobia important for understanding the evolution and mechanisms of photosynthesis and energy metabolism. So what makes this organism survive in these environments? Why should we care about this organism?

C. tepidum is a valuable model for the green sulfur bacteria because it is easily cultivated and naturally transformable. According to Wahlund et al., C. tepidum while under photautotraphic conditions can be cultured in about two hours which makes this faster than any other anoxygenic phototroph. This makes it the ideal tool for research in basic studies of photosynthesis and autotrophy in green bacteria.

Unlike Chromatium and Thiothrix who are two other sulfur-producing bacteria, C. tepidum deposits the elemental sulfur outside the cell. When comparing genomes, many of the genes are highly conserved among the photosythetic species. Their function is not clear but they are suspected to play an important role in photosynthesis or photobiology. C. tepidum has been shown to have strong similarities to many Archael species between their metabolic processes.
C. tepidum contains duplications of genes involved in biosyntheic pathways for photosynthesis and the metabolism of sulfur and nitrogen. Unlike regular photosynthesis that synthesizes foods from carbon dioxide and water and generates oxygen as a byproduct, while Chlorobia performs anoxygenic photosynthesis. This is helpful for humans unlike how Chlorobium tepidum does it because we cannot use their byproducts of elemental sulfur.

It is important to know that currently C. tepidum does not cause any diseases. Mutants have been found and studied such as bchK in C. tepidum and it lacked BChl c., this means the mutant grew slower. BChlc is important because all photoautotrophic organisms rely on chlorophyll (Chl) or bacteriochlorophyll (BChl)- based photosynthesis. BChl c in C. Tepidum is a mixture of four homologous that carry different modifications at the C8 and C12 positions and had been demonstrated that these side chains are derived from methylation reactions involving SAM. There is so much to learn from this genome and I hope this sparks your interest in it to further your knowledge, I know it has for me. Thanks for taking the time to listen to this guest speaker podcast.

(1) Eisen et al., 2002.  Proceedings of the National Academy of Sciences USA 99:9509-9514.  The complete genome sequence of Chlorobium tepidum TLS, a photosynthetic, anaerobic green-sulfur bacterium.
(2) Frigaard et al., 2003.  Photosynthesis Research 78: 93-117.  Chlorobium tepidum: insights into the structure, physiology, and metabolism of a green sulfur bacterium derived from the complete genome sequence.
(3) Frigaard et al., 2002.  Journal of Bacteriology 184:3368-3376.  Chlorobium tepidum mutant lacking bacteriochlorophyll c made by inactivation of the bchK gene, encoding bacteriochlorophyll c synthase.
(4) Wahlund et al., 1995.  Journal of Bacteriology. 173:2583-2588.  Genetic transfer by conjugation in the thermophilic sulfur bacterium Chlorobium tepidum.
(5) Wahlund et al., 1991.  Archives of Microbiology 156:81-90.  A thermophilic green sulfur bacterium from New Zealand hot springs, Chlorobium tepidum sp. Nov.