Thermotoga maritima
Outline of a Thermotoga maritima section showing the "toga"
Scientific classification Edit this classification
Domain: Bacteria
Phylum: Thermotogota
Class: Thermotogae
Order: Thermotogales
Family: Thermotogaceae
Genus: Thermotoga
Species:
T. maritima
Binomial name
Thermotoga maritima
Huber et al., 1986

Thermotoga maritima is a hyperthermophilic, anaerobic organism that is a member of the order Thermotogales. T. maritima is well known for its ability to produce hydrogen (clean energy) and it is the only fermentative bacterium that has been shown to produce Hydrogen more than the Thauer limit (>4 mol H2 /mol glucose).[1] It employs [FeFe]-hydrogenases to produce hydrogen gas (H2) by fermenting many different types of carbohydrates.[2]

History

First discovered in the sediment of a marine geothermal area near Vulcano, Italy, Thermotoga maritima resides in hot springs as well as hydrothermal vents.[3] The ideal environment for the organism is a water temperature of 80 °C (176 °F), though it is capable of growing in waters of 55–90 °C (131–194 °F).[4] Thermotoga maritima is the only bacterium known to grow at this high a temperature; the only other organisms known to live in environments this extreme are members of the domain Archaea. The hyperthermophilic abilities of T. maritima, along with its deep lineage, suggests that it is potentially a very ancient organism.[5]

Physical attributes

Thermotoga maritima is a non-sporulating, rod shaped, gram-negative bacterium.[1][6] When viewed under a microscope, it can be seen to be encased in a sheath-like envelope which resembles a toga, hence the "toga" in its name.[6]

Metabolism

As an anaerobic fermentative chemoorganotrophic organism, T. maritima catabolizes sugars and polymers and produces carbon dioxide (CO2) and hydrogen (H2) gas as by-products of fermentation.[6] T. maritima is also capable of metabolizing cellulose as well as xylan, yielding H2 that could potentially be utilized as an alternative energy source to fossil fuels.[7] Additionally, this species of bacteria is able to reduce Fe(III) to produce energy using anaerobic respiration. Various flavoproteins and iron-sulphur proteins have been identified as potential electron carriers for use during cellular respiration.[7] However, when growing with sulfur as the final electron acceptor, no ATP is produced. Instead, this process eliminates inhibitory H2 produced from fermentative growth.[7] Collectively, these attributes indicate that T. maritima has become resourceful and capable of metabolizing a host of substances in order to carry out its life processes.

Clean energy (biohydrogen) from T. maritima

Energy is a growing need of the world and it is expected to grow in the next 20 years.  Among various energy sources, hydrogen serves as the best energy carrier due to its higher energy content per unit weight. T. maritima is one of fermentative bacteria that produces hydrogen to levels that approach the thermodynamic limit (4 mol H2/ mol glucose). However, similar to other fermentative bacteria, the hydrogen yield in this bacterium does not go beyond 4 mol H2 / glucose (Thaeur limit) because of its inherent nature to use more energy for its own cell division to grow rapidly than producing H2. Because of these reasons fermentative bacteria have not been thought to produce higher amounts of hydrogen at a commercial scale. Overcoming this limit by improving the conversion of sugar to H2 could lead to a superior H2 producing biological system that may supersede fossil fuel-based H2 production.

Metabolic engineering in this bacterium led to development of strains of T. maritima that surpassed the Thauer limit of hydrogen production.[1] One of the strains, also known as Tma200, produced 5.77 mol H2/ mol glucose which is the highest yield so far reported in a fermentative bacterium. In this strain, energy redistribution, and metabolic rerouting through the pentose phosphate pathway (PPP) generated excess reductants while uncoupling growth from hydrogen synthesis. Uncoupling of growth from product formation has been viewed as a viable strategy to maximize the product yield which has been achieved in the higher hydrogen producing bacterium. Similar strategies can be adopted for other hydrogen producing bacterium to maximize product yields.

Hydrogenase activity

Hydrogenases are metalloenzymes that catalyze the reversible hydrogen conversion reaction: H2 ⇄ 2 H++ 2 e−. A Group C [FeFe]-hydrogenase from Thermotoga maritima (TmHydS) has showed modest hydrogen conversion activity and reduced sensitivity to the enzyme's inhibitor, CO, in comparison to Group A prototypical and bifurcating [FeFe]-hydrogenases.[8] The TmHydS has a hydrogenase domain with distinct amino acid modifications in the active site pocket, including the presence of a Per-Arnt-Sim (PAS) domain.

Genomic composition

The genome of T. maritima consists of a single circular 1.8 megabase chromosome encoding for 1877 proteins.[9] Within its genome it has several heat and cold shock proteins that are most likely involved in metabolic regulation and response to environmental temperature changes.[7] It shares 24% of its genome with members of the Archaea; the highest percentage overlap of any bacteria.[10] This similarity suggests horizontal gene transfer between Archaea and ancestors of T. maritima and could help to explain why T. maritima is capable of surviving in such extreme temperatures and conditions. The genome of T. maritima has been sequenced multiple times. Genome resequencing of T. maritima MSB8 genomovar DSM3109 [11][9] determined that the earlier sequenced genome was an evolved laboratory variant of T. maritima with an approximately 8-kb deletion. Moreover, a variety of duplicated genes and direct repeats in its genome suggest their role in intra-molecular homologous recombination leading to genes deletion. A strain with a 10-kb gene deletion has been developed using the experimental microbial evolution in T. maritima.[12]

Genetic system of Thermotoga maritima

Thermotoga maritima has a great potential in hydrogen synthesis because it can ferment a wide variety of sugars and has been reported to produce the highest amount of H2 (4 mol H2/ mol glucose).[4] Due to lack of a genetic system for the past 30 years majority of the studies have been either focused on heterologous gene expression in E. coli or predicting models since a gene knockout mutant of T. maritima remained unavailable.[13] Developing a genetic system for T. maritima has been a challenging task primarily because of a lack of a suitable heat-stable selectable marker. Recently, the most reliable genetic system based on pyrimidine biosynthesis has been established in T. maritima.[14] This newly developed genetic system relies upon a pyrE− mutant that was isolated after cultivating T. maritima on a pyrimidine biosynthesis inhibiting drug called 5-fluoroorotic acid (5-FOA). The pyrE− mutant is an auxotrophic mutant for uracil. The pyrE from a distantly related genus of T. maritima rescued the uracil auxotrophy of the pyrE− mutant of T. maritima and has been proven to be a suitable marker.

For the first time, the use of this marker allowed the development of an arabinose (araA) mutant of T. maritima. This mutant explored the role of the pentose phosphate pathway of T. maritima in hydrogen synthesis.[14] The genome of T. maritima possesses direct repeats that have developed into paralogs.[12] Due to lack of a genetic system the true function of these paralogs has remained unknown. Recently developed genetic system in T. maritima has been very useful to determine the function of the ATPase protein (MalK) of the maltose transporter that is present in a multi-copy (three copies) fashion. The gene disruptions of all three putative ATPase encoding subunit (malK) and phenotype have concluded that only one of the three copies serves as an ATPase function of the maltose transporter.[15] It is interesting to know that T. maritima has several paralogs of many genes and the true function of these genes is now dependent upon the use of the recently developed system. The newly developed genetic system in T. maritima has a great potential to make T. maritima as a host for hyperthermophilic bacterial gene expression studies. Protein expression in this model organism is promising to synthesize fully functional protein without any treatment.

Evolution

Thermotoga maritima contains homologues of several competence genes, suggesting that it has an inherent system of internalizing exogenous genetic material, possibly facilitating genetic exchange between this bacterium and free DNA.[7] Based on phylogenetic analysis of the small sub-unit of its ribosomal RNA, it has been recognized as having one of the deepest lineages of Bacteria. Furthermore, its lipids have a unique structure that differs from all other bacteria.[4]

References

  1. ^ a b c Singh R, White D, Demirel Y, Kelly R, Noll K, Blum P (September 2018). "Uncoupling Fermentative Synthesis of Molecular Hydrogen from Biomass Formation in Thermotoga maritima". Applied and Environmental Microbiology. 84 (17). Bibcode:2018ApEnM..84E.998S. doi:10.1128/aem.00998-18. PMC 6102995. PMID 29959252.
  2. ^ Merrill AH, Lingrell S, Wang E, Nikolova-Karakashian M, Vales TR, Vance DE (June 1995). "Sphingolipid biosynthesis de novo by rat hepatocytes in culture. Ceramide and sphingomyelin are associated with, but not required for, very low density lipoprotein secretion". The Journal of Biological Chemistry. 270 (23): 13834–13841. doi:10.1074/jbc.270.23.13834. PMID 7775441.
  3. ^ "Hyperthermophilic organism that shows extensive horizontal gene transfer from archaea". BioProject. National Center for Biotechnology Information. 2003. Retrieved January 14, 2012.
  4. ^ a b c Huber R, Langworthy TA, König H, Thomm M, Woese CR, Sleytr UB, Stetter KO (1986). "Thermotoga maritima sp. nov. represents a new genus of unique extremely thermophilic eubacteria growing up to 90°C". Archives of Microbiology. 144 (4): 324–333. doi:10.1007/BF00409880. S2CID 12709437.
  5. ^ Blamey JM, Adams MW (February 1994). "Characterization of an ancestral type of pyruvate ferredoxin oxidoreductase from the hyperthermophilic bacterium, Thermotoga maritima". Biochemistry. 33 (4): 1000–1007. doi:10.1021/bi00170a019. PMID 8305426.
  6. ^ a b c "Geothermal organisms". Montana State University. Retrieved January 14, 2012.
  7. ^ a b c d e Nelson KE, Clayton RA, Gill SR, Gwinn ML, Dodson RJ, Haft DH, et al. (May 1999). "Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima". Nature. 399 (6734): 323–329. Bibcode:1999Natur.399..323N. doi:10.1038/20601. PMID 10360571. S2CID 4420157.
  8. ^ Chongdar N, Birrell JA, Pawlak K, Sommer C, Reijerse EJ, Rüdiger O, et al. (January 2018). "Unique Spectroscopic Properties of the H-Cluster in a Putative Sensory [FeFe] Hydrogenase". Journal of the American Chemical Society. 140 (3): 1057–1068. doi:10.1021/jacs.7b11287.s001. PMID 29251926.
  9. ^ a b Latif H, Lerman JA, Portnoy VA, Tarasova Y, Nagarajan H, Schrimpe-Rutledge AC, et al. (April 2013). "The genome organization of Thermotoga maritima reflects its lifestyle". PLOS Genetics. 9 (4): e1003485. doi:10.1371/journal.pgen.1003485. PMC 3636130. PMID 23637642.
  10. ^ Nesbo CL, L'Haridon S, Stetter KO, Doolittle WF (March 2001). "Phylogenetic analyses of two "archaeal" genes in thermotoga maritima reveal multiple transfers between archaea and bacteria". Molecular Biology and Evolution. 18 (3): 362–375. doi:10.1093/oxfordjournals.molbev.a003812. PMID 11230537.
  11. ^ Boucher N, Noll KM (September 2011). "Ligands of thermophilic ABC transporters encoded in a newly sequenced genomic region of Thermotoga maritima MSB8 screened by differential scanning fluorimetry". Applied and Environmental Microbiology. 77 (18): 6395–6399. Bibcode:2011ApEnM..77.6395B. doi:10.1128/AEM.05418-11. PMC 3187129. PMID 21764944.
  12. ^ a b Singh R, Gradnigo J, White D, Lipzen A, Martin J, Schackwitz W, et al. (May 2015). "Complete Genome Sequence of an Evolved Thermotoga maritima Isolate". Genome Announcements. 3 (3): e00557–15. doi:10.1128/genomeA.00557-15. PMC 4447916. PMID 26021931.
  13. ^ Conners SB, Montero CI, Comfort DA, Shockley KR, Johnson MR, Chhabra SR, Kelly RM (November 2005). "An expression-driven approach to the prediction of carbohydrate transport and utilization regulons in the hyperthermophilic bacterium Thermotoga maritima". Journal of Bacteriology. 187 (21): 7267–7282. doi:10.1128/jb.187.21.7267-7282.2005. PMC 1272978. PMID 16237010.
  14. ^ a b White D, Singh R, Rudrappa D, Mateo J, Kramer L, Freese L, Blum P (February 2017). "Contribution of Pentose Catabolism to Molecular Hydrogen Formation by Targeted Disruption of Arabinose Isomerase (araA) in the Hyperthermophilic Bacterium Thermotoga maritima". Applied and Environmental Microbiology. 83 (4): e02631–16. Bibcode:2017ApEnM..83E2631W. doi:10.1128/aem.02631-16. PMC 5288831. PMID 27940539.
  15. ^ Singh R, White D, Blum P (September 2017). "Identification of the ATPase Subunit of the Primary Maltose Transporter in the Hyperthermophilic Anaerobe Thermotoga maritima". Applied and Environmental Microbiology. 83 (18): e00930–17. Bibcode:2017ApEnM..83E.930S. doi:10.1128/aem.00930-17. PMC 5583491. PMID 28687653.

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