Genomic and physiological properties of Anoxybacterium hadale gen. nov. sp. nov. reveal the important role of dissolved organic sulfur in microbial metabolism in hadal ecosystems

Hadal zones account for the deepest 45% of the oceanic depth range and play an important role in ocean biogeochemical cycles. As the least-explored aquatic habitat on earth, hadal ecosystems contain a vast diversity of so far uncultured microorganisms that cannot be grown on conventional laboratory culture media. Therefore, it has been difficult to gain a true understanding of the detailed metabolic characteristics and ecological functions of those difficult-to-culture microorganisms in hadal environments. In this study, a novel anaerobic bacterial strain, MT110^T, was isolated from a hadal sediment-water interface sample of the Mariana Trench at 10,890 m. The level of 16S rRNA gene sequence similarity and percentage of conserved proteins between strain MT110^T and the closest relatives, <i>Anaerovorax odorimutans</i> DSM 5092^T (94.9 and 46.6%) and <i>Aminipila butyrica</i> DSM 103574^T (94.4 and 46.7%), indicated that strain MT110^T exhibits sufficient molecular differences for genus-level delineation. Phylogenetic analyses based on both 16S rRNA gene and genome sequences showed that strain MT110^T formed an independent monophyletic branch within the family Anaerovoracaceae. The combined evidence showed that strain MT110^T represents a novel species of a novel genus, proposed as <i>Anoxybacterium hadale</i> gen. nov. sp. nov. (type strain MT110^T = KCTC 15922^T = MCCC 1K04061^T), which represents a previously uncultured lineage of the class Clostridia. Physiologically, no tested organic matter could be used as sole carbon source by strain MT110^T. Genomic analysis showed that MT110^T had the potential capacity of utilizing various carbon sources, but the pathways of sulfur reduction were largely incomplete. Our experiments further revealed that cysteine is one of the essential nutrients for the survival of strain MT110^T, and cannot be replaced by sulfite, leucine, or taurine. This result suggests that organic sulfur compounds might play an important role in metabolism and growth of the family Anaerovoracaceae and could be one of the key factors affecting the cultivation of the uncultured microbes. Our study brings a new perspective to the role of dissolved organic sulfur in hadal ecosystems and also provides valuable information for optimizing the conditions of isolating related microbial taxa from the hadal environment.