Thiovulum sp. strain imperiosus: the largest free-living Epsilonproteobacteraeota strain lives in a marine mangrove environment.

A large (47.75 ± 3.56 µm in diameter) bacterial strain forming white veils is described from a marine mangrove ecosystem. High sulfide concentrations (up to 8 mM of HS) were measured on sunken organic matter (wood/bone debris) under laboratory conditions. This sulfur-oxidizing bacterium colonized the organic matter, forming a white veil. According to conventional scanning electron microscope (SEM) observations, bacterial cells are ovoid and slightly motile by numerous small flagella present on the cell surface. Large intracytoplasmic internal sulfur granules were observed, suggesting a sulfidic-based metabolism. Observations were confirmed by elemental sulfur distribution detected by energy-dispersive X-ray spectroscopy (EDXS) analysis using an environmental scanning electron microscope (ESEM) on non-dehydrated samples. Phylogenetic analysis of the partial sequence of 16S rDNA obtained from purified fractions of this Epsilonproteobacteraeota strain indicates that this bacterium belongs to the cluster and could be one of the largest ever described. We propose to name this species Thiovulum sp. strain imperiosus.