Biotechnology of extremely thermophilic archaea

Abstract

Although the extremely thermophilic archaea (Topt ≥ 70°C) may be the most primitive extant forms of life, they have been studied to a limited extent relative to mesophilic microorganisms. Many of these organisms have unique biochemical and physiological characteristics with important biotechnological implications. These include methanogens that generate methane, fermentative anaerobes that produce hydrogen gas with high efficiency, and acidophiles that can mobilize base, precious and strategic metals from mineral ores. Extremely thermophilic archaea have also been a valuable source of thermoactive, thermostable biocatalysts, but their use as cellular systems has been limited because of the general lack of facile genetics tools. This situation has changed recently, however, thereby providing an important avenue for understanding their metabolic and physiological details and also opening up opportunities for metabolic engineering efforts. Along these lines, extremely thermophilic archaea have recently been engineered to produce a variety of alcohols and industrial chemicals, in some cases incorporating CO2 into the final product. There are barriers and challenges to these organisms reaching their full potential as industrial microorganisms but, if these can be overcome, a new dimension for biotechnology will be forthcoming that strategically exploits biology at high temperatures.

Figure 1.

In vitro synthetic pathway for hydrogen production, adapted from Myung et al. (2014), Kim et al. (2017). Enzymes from extremely thermophilic archaea: isoamylase from Sulfolobus tokodaii, 4-α-glucanotransferase from Thermococcus litoralis, NADPH:rubredoxin oxidoreducase and [NiFe]-hydrogenase from Pyrococcus furiosus.*Enzyme from extremely thermophilic archaeon.

Figure 2.

CO₂ fixation cycles including those found exclusively in extremely thermophilic archaea. The DC/4-HB (blue) and 3-HP/4-HB (green) are found exclusively in extremely thermophilic archaea. These cycles share many intermediates with the reverse TCA (rTCA) (yellow) cycle as well as the 3-HP bicycle (gray) from green non-sulfur bacteria. TCA, tricarboxylic acid; 4-HB, hydroxybutyrate; 3-HP, 3-hydroxypropionate; DC, dicarboxylate

Figure 3.

Sulfur and iron oxidation pathways in extremely thermophilic archaeal species applicable to biomining. Biotic oxidation of ferrous iron (Fe²⁺) drives the supply of ferric iron (Fe³⁺) to abiotically dissolve the ore sulfides to elemental sulfur and polysulfides, rendering sulfur species available for biotic oxidation. Biotic and abiotic reactions are listed in Table 2.

Figure 4.

Metabolic engineering temperature shift strategy demonstrated in P. furiosus. (a) Heterologous pathway from M. sedula inserted in P. furiosus; (b) temperature shift strategy with growth phase near T_opt of host organism and production phase at T_opt of pathway; (c) transcriptomes revealed that central metabolism in recombinant strain was minimally affected by temperature shift compared to parent strain. Mal-CoA = malonyl CoA reductase; Mal SA = malonyl semialdehyde reductase.