In Situ Gene Expression Responsible for Sulfide Oxidation and CO2 Fixation of an Uncultured Large Sausage-Shaped Aquificae Bacterium in a Sulfidic Hot Spring

Abstract

We investigated the in situ gene expression profile of sulfur-turf microbial mats dominated by an uncultured large sausage-shaped Aquificae bacterium, a key metabolic player in sulfur-turfs in sulfidic hot springs. A reverse transcription-PCR analysis revealed that the genes responsible for sulfide, sulfite, and thiosulfate oxidation and carbon fixation via the reductive TCA cycle were continuously expressed in sulfur-turf mats taken at different sampling points, seasons, and years. These results suggest that the uncultured large sausage-shaped bacterium has the ability to grow chemolithoautotrophically and plays key roles as a primary producer in the sulfidic hot spring ecosystem in situ.

Fig. 1

Photos of the sulfur-turf (white microbial mat) thriving in the hot spring stream and RT-PCR detection of transcripts of genes related to reduced sulfur compound oxidation, respiration, and carbon fixation. The white arrow shows the discharge point of the spring. Hot spring water flows downward from the top to the bottom in this figure. dhsU1 and dhsU2, sulfide dehydrogenase; sqrX and sqrF, sulfide-quinone reductase; sorA, sulfite dehydrogenase; soxX, soxY, soxZ, soxA and soxB, sox complex; rhd1, rhd2 and rhd3, thiosulfate sulfurtransferase; ccoN, cbb-3 type cytochrome c oxidase; aclA, ATP citrate lyase; fumA, fumarate hydratase; forA, 2-oxoglutarate ferredoxin oxidoreductase; rpoZ, RNA polymerase; M, DNA marker.

Fig. 2

Hypothesized model of the active autotrophic sulfur-oxidation pathway in the uncultured LSSB in the in situ sulfur-turf ecosystem. The bold black arrow indicates the reaction that has been biochemically demonstrated in the uncultured LSSB and verified at the transcriptional level in this study. The bold gray arrows show the putative reactions verified at the transcriptional level in this study, but lacking direct biochemical evidence in the uncultured LSSB. The thin black arrows represent the flow of the substrates and products postulated. DhsU, sulfide dehydrogenase; Sqr, sulfide-quinone reductase; Sox, sox complex; Rhd, thiosulfate sulfurtransferase; CcoN, cbb-3 type cytochrome c oxidase; AclA, ATP citrate lyase; FumA, fumarate hydratase; ForA, 2-oxoglutarate ferredoxin oxidoreductase.