ABSTRACT
Life in Movile Cave (Romania) relies entirely on primary carbon fixation by bacteria oxidizing sulfide, methane and ammonia with oxygen, nitrate, sulfate, and ferric iron. There, large spherical-ovoid bacteria (12-16 μm diameter), rich in intracellular sulfur globules, dominate the stable microbial community in the surface water of a hypoxic Air Bell. These were identified as Thiovulum sp. (Campylobacterota). We obtained a closed genome of this Thiovulum and compared it to that of Thiovulum ES. The genes for oxidizing sulfide to sulfate are absent, therefore, Thiovulum likely avoids constant accumulation of elemental sulfur either by oxidizing sulfide to sulfite which is then excreted, or via dissimilatory nitrate reduction to ammonia using the formate-dependent nitrite reductase or hydroxylamine oxidoreductase. Thus, Thiovulum, found also in other caves, is likely important to both S and N cycles in subterranean aquatic ecosystems. Additionally, using electron microscopy, we suggest that in absence of motor-like structures along the membrane, the peritrichous flagella-like structures are type IV pili, for which genes were found in both Thiovulum genomes. These pili may play a role in veil formation, connecting adjacent cells. The force exerted by coordinated movement of such pili may partly explain the exceptionally fast swimming of these bacteria.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
ORIGINALITY SIGNIFICANCE STATEMENT
We identify Thiovulum sp. as a dominant bacterium in subsurface floating microbial agglom-erations in a hypoxic cave system. We show for the first time that Thiovulum can be found in high abundance in planktonic environments dissociated from solid surfaces. We provide the complete genome of this organism and suggest that it is capable of dissimilatory nitrate reduction to ammonia using sulfur as the electron donor, thus contributing to both the cave’s nitrogen and sulfur cycles. Based on sequence similarity, this clade of Thiovulum spp. may be common and important in other sulfidic caves. Last, we propose that the short peritrichous flagella-like structures of Thiovulum are type IV pili rather than actual flagella.