Dimethylsulfoniopropionate (DMSP) is an important marine osmolyte. Aphotic environments are only recently being considered as potential contributors to global DMSP production. Here, our Mariana Trench study reveals a typical seawater DMSP/dimethylsulfide (DMS) profile, with highest concentrations in the euphotic zone and decreased but consistent levels below. The genetic potential for bacterial DMSP synthesis via the dsyB gene and its transcription is greater in the deep ocean, and is highest in the sediment.s DMSP catabolic potential is present throughout the trench waters, but is less prominent below 8000 m, perhaps indicating a preference to store DMSP in the deep for stress protection. Deep ocean bacterial isolates show enhanced DMSP production under increased hydrostatic pressure. Furthermore, bacterial dsyB mutants are less tolerant of deep ocean pressures than wild-type strains. Thus, we propose a physiological function for DMSP in hydrostatic pressure protection, and that bacteria are key DMSP producers in deep seawater and sediment.

The Mariana Trench is the deepest location on earth and harbors unique microbial communities as evidenced by 16S rRNA gene amplicon and metagenomic sequencing. Obtaining culturable microorganisms from the Mariana Trench will contribute to a further understanding of hadal biogeochemical processes and act as a unique microbial reservoir with potential applications. Here, 825 bacterial strains, identified by 16S rRNA gene sequencing, were isolated from 12 water depths (0-10,400 m) of the Mariana Trench with 2216E and R2A media at 4 °C or 28 °C on four cruises during 2015-2017. These bacteria belong to four phyla, nine classes, 27 orders, 45 families and 108 genera. Alphaproteobacteria, Gammaproteobacteria, Actinobacteria_c, Bacilli and Flavobacteriia were the most abundant classes, accounting for 37.9%, 33.0%, 11.8%, 8.6% and 8.0% of the total bacterial isolates, respectively. 2216E and R2A media were found to have a better selectivity to Bacilli and Flavobacteriia, respectively. Fifty strains were potential novel bacterial species with a 16S rRNA gene similarity < 98.65%, and a higher percentage of novel strains were obtained from R2A than 2216E medium. Additionally, 301 (150 species) out of 354 strains (178 species) selected from each depth could degrade at least one of the ten kinds of macromolecules tested. These results indicate that there is a high diversity of culturable bacteria in the Mariana Trench and they can produce a variety of extracellular enzymes. Our study provides a valuable resource of microorganisms for investigating their biogeochemical roles in the Mariana Trench and for industrial applications.

A Gram-staining-negative, strictly aerobic, long-rod shaped with no flagellum and yellow-pigmented bacterium designated strain ZXX205T, was isolated from the hadal seawater at the depth of 7500 m in the Mariana Trench, Pacific Ocean. Phylogenetic analysis based on 16S rRNA gene sequences placed strain ZXX205T within the genus Winogradskyella and strain ZXX205T was most closely related to Winogradskyella flava KCTC 52348T and Winogradskyella echinorum KCTC 22026T with 96.9 % and 96.6 % sequence similarity, respectively. The sequence similarities to all other type strains were 96.3 % or less, and to the type strain Winogradskyella thalassocola LMG 22492T was 94.1 %. Growth occurred in the presence of 0–9.0 % (w/v) NaCl (optimum 3.0 %), at 4–45 °C (optimum 28 °C) and pH 6.0–9.0 (optimum pH 7.5). The sole respiratory quinone was menaquinone 6 (MK-6). The dominant cellular fatty acids (>10 %) of strain ZXX205T were iso-C15 : 0, iso-C15 : 1 G, iso-C16 : 0 3-OH and iso-C16 : 0. The polar lipids profile contained predominantly phosphatidylethanolamine, four glycolipids, four unidentified aminolipids and three unidentified lipids. The genomic DNA G+C content was 35.5 %. The DNA–DNA relatedness (DDH) values between strain ZXX205T and the most closely related species Winogradskyella flava and Winogradskyella echinorum were 21.1 and 20.4 %, respectively. Based on polyphasic taxonomic analysis, strain ZXX205T is considered to represent a novel species in the genus Winogradskyella of the family Flavobacteriaceae , for which the name Winogradskyella ouciana is proposed. The type strain is ZXX205T (=MCCC 1K03851T=JCM 33665T).