Impact of acetate on CO2 fixation pathways in thermophilic and hydrogenotrophic bacteria

The Wood-Ljungdahl (WL) pathway and reductive tricarboxylic acid (rTCA) cycle are the dominant chemolithotrophic CO2 fixation pathways in bacteria inhabiting aphotic geothermal and deep-sea hydrothermal ecosystems. However, the activity of these bacterial metabolic systems in ecosystems with available organic carbons remains unclear. Here, we examined the impact of extracellular acetate on the CO2-fixation pathways of three thermophilic hydrogen-oxidizing and non-acetogenic bacteria using 13C tracer-based metabolomics. Under chemolithoautotrophic conditions, Thermodesulfatator indicus and Hydrogenobacter thermophilus fixed CO2 through the WL pathway and rTCA cycle, respectively, whereas Thermovibrio ammonificans, which has been suggested to operate both of these pathways, exhibited significant CO2 fixation through only the rTCA cycle. Under chemolithomixotrophic conditions with acetate, H. thermophilus and T. ammonificans assimilated both CO2 and acetate via the rTCA cycle. In contrast, acetate suppressed CO2 fixation through the WL pathway in T. indicus and was used as the primary carbon source under chemolithomixotrophic conditions. These results suggest that the contribution of the WL pathway for CO2 fixation might be overestimated in ecosystems where acetate is available. Moreover, the present findings indicate that simultaneous CO2 fixation through both the WL pathway and rTCA cycle in a cell, which has been proposed as a possible metabolic strategy for CO2-fixation in ancestral life, is not advantageous in extant microorganisms.