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). It employs [FeFe]-hydrogenases to produce hydrogen gas (H2) by fermenting many different types of carbohydrates.
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. 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). Thermotoga maritima and Geothermobacterium ferrireducens are the only bacteria 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. 16S rRNA sequencing of Thermotoga maritima determined that the bacteria belong to the eubacteria phyla. However, as of a study published in 1986, the bacteria were not determined to have close relationships to any other group within this phyla. The majority of the lipids in the bacteria are unique in structure compared to other eubacteria. The peptidoglycan of T. maritima has unique structures and components as well. The peptidoglycan structure contains equal amounts of l- and d-lysine. A study published in 2009 revealed that d-lysine plays a crucial role in peptidoglycan synthesis, functioning as a substitute for l-lysine and forming novel cross-links within the structure.
Physical attributes Thermotoga maritima is a non-sporulating, rod shaped, gram-negative bacterium. 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.
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. 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. 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. However, when growing with sulfur as the final electron acceptor, no ATP is produced. Instead, this process eliminates inhibitory H2 produced from fermentative growth. 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 biohydrogen 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. 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. In addition to hydrogen production, Thermotoga maritima DSM3109 expresses a novel thermostable transketolase (TKtmar) that has an optimum temperature above 90°C and tolerates organic solvents. This enzyme can be used as a biocatalyst for industrial applications that use sugar transformations as it can from a stereospecific carbon–carbon bond used in the synthesis of prochiral ketones which are important precursors in biofuel production.
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 shown modest hydrogen conversion activity and reduced sensitivity to the enzyme's inhibitor, CO, in comparison to Group A prototypical and bifurcating [FeFe]-hydrogenases. 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.
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