Alkane degradation mechanism of Mixta calida HXX308 isolated from sediment of the Mariana Trench

The Challenger Deep of the Mariana Trench, which is the deepest site in the ocean, contains rich deposits of n-alkanes in its sediments. However, the alkane metabolic processes of the bacteria in this extreme environment were not well understood. In this study, we isolated a strain Mixta calida HXX308 (Proteobacteria) from sediment samples of the Challenger Deep (10,816 meters below sea level). HXX308 grows under pressures ranging from 0 to 40 MPa, with optimal growth at lower pressures. Additionally, it degrades approximately 20% of eicosane at both atmospheric pressure (0.1 MPa) and 20 MPa. Metabolic profiling indicated that HXX308 possesses a complete aerobic alkane metabolism pathway, along with nitrate reduction and sulfate reduction pathways, which support its adaptation to the trench's anoxic environment. Comparative genomic studies showed that most strains in the genus Mixta contain the alkane-degrading gene LadB. Characterization of the LadB gene in HXX308 confirmed its role in the degradation of medium-to long-chain alkanes (C18-36). HXX308 is the first Mixta strain isolated from marine environment. Although this strain originated from the trench, its hydrocarbon metabolic characteristics are similar to those of cultures of terrestrial origin, suggesting that the alkanes in these sediments are likely from the terrestrial environment. Our study enhances the understanding of alkane-degrading in the phylum Proteobacteria and provides insights into the environmental adaptation of M. calida HXX308 in the Mariana Trench.

Albibacterium profundi sp. nov., isolated from sediment of the Challenger Deep of Mariana Trench, and reclassification of Pedobacter indicus as Albibacterium indicum comb. nov

A rod-shaped, white-pigmented, non-motile, Gram-stain-negative bacterium, designated RHL897T, was isolated from sediments collected at the Mariana Trench Challenger Deep (10,816 m). Strain RHL897T was strictly aerobic and grew at 4-37 °C, pH 6.0-10.0 and in the presence of 0-11.0 % (w/v) NaCl. Its genomic DNA G+C content was 41.2%. Metabolic analysis revealed mechanisms for salt tolerance, abundant metal ion transport proteins and stronger resistance to heavy metals such as arsenic and mercury compared to the closest reference strains, likely linked to adaptation to the hadal sediment environment. The predominant menaquinone was MK-7, and the major polar lipids were phosphatidylethanolamine, an unidentified aminophospholipid and an unidentified glycolipid. The main fatty acids were iso-C15 : 0, summed feature 3 (C16 : 1  ω7c and/or C16 : 1  ω6c) and iso-C17 : 0 3OH. Strain RHL897T exhibited the highest 16S rRNA gene sequence similarity to the type strain of Pedobacter indicus (97.9%) and Albibacterium bauzanense (96.1%). Phylogenetic trees constructed based on 16S rRNA gene sequences and a 549 core gene set indicated that strain RHL897T was closely related to P. indicus and A. bauzanense, with all three species clustering within a distinct clade. Combined with the analyses of average nucleotide identity, average amino acid identity and digital DNA-DNA hybridization, strain RHL897T represented a novel species of the genus Albibacterium, for which the name Albibacterium profundi sp. nov. is proposed. The type strain is RHL897T (=MCCC 1K09221T=KCTC 102276T). Furthermore, the revised phylogeny with the inclusion of RHL897T suggested that P. indicus should be reclassified under the genus Albibacterium and renamed Albibacterium indicum.

Oceanobacter antarcticus sp. nov., isolated from surface seawater of the Weddell Sea in Antarctica

The poles of the Earth harbour many novel micro-organisms that have not yet been isolated and identified. Here, a Gram-stain-negative, motile, aerobic and rod-shaped bacterial strain, designated as wDCs-4T, was isolated from surface seawater collected from the Weddell Sea in Antarctica. It grows at 4-40 °C (optimum 10-15 °C), pH 4-8 (optimum 7) and in the presence of 0-4% NaCl (w/v, optimum 0.5-1%). The complete 16S rRNA gene of strain wDCs-4T had maximum sequence identity with Oceanobacter mangrovi SM2-42T (97.2%), followed by Thalassolituus oleivorans MIL-1T (96.5%) and Oceanobacter kriegii 197T (96.2%). Phylogenetic analyses based on the 16S rRNA gene and genome sequences showed that strain wDCs-4T was closely clustered with the members of the genus Oceanobacter and formed an independent clade, which could be considered a monophyletic taxon. The average nucleotide identity values between strain wDCs-4T and the members of the genera Oceanobacter and Thalassolituus were 77.7-78.1 and 77.4-80.5%, respectively. The corresponding digital DNA‒DNA hybridization values are 19.5-20.1 and 20.5-22.4%, respectively. The major fatty acids (>5%) of strain wDCs-4T comprised summed feature 5 (C18:0 ante/C18:2  ω6,9c or C18:2  ω6,9c/C18:0 ante) and C16:0. The predominant respiratory was Q-8. The polar lipid profile consisted of phosphatidylethanolamine, phosphatidylglycerol, aminolipids and unknown polar lipids. The draft genome size was 4.58 Mbp, with a DNA G+C content of 53.4 mol%. On the basis of polyphasic analyses, strain wDCs-4T represented a novel species in the genus Oceanobacter, for which the name Oceanobacter antarcticus sp. nov. was proposed. The type strain was wDCs-4T (=MCCC 1A20726T=KCTC 8314T).

The phylogeny and metabolic potentials of an n-alkane-degrading Venatorbacter bacterium isolated from deep-sea sediment of the Mariana Trench

Recently, several reports showed that n-alkanes were abundant in the hadal zone, suggesting that n-alkanes could be an important source of nutrients for microorganisms in hadal ecosystems. To date, most of the published studies on the microbial capacity to degrade hydrocarbons were conducted only at atmospheric temperature and pressure (0.1 MPa), and little is known about whether and which microbes could utilize n-alkanes at in situ environmental conditions in the hadal zone, including low temperature and high hydrostatic pressure (especially >30 MPa). In this study, a piezotolerant bacterium, strain C2-1, was isolated from a Mariana Trench sediment at depth of 5,800 m. Strain C2-1 was able to grow at in situ temperature (4°C) and pressure (58 MPa) with n-alkanes as the sole carbon source. Phylogenetically, strain C2-1 and related strains (TMPB967, ST750PaO-4, IMCC1826, and TTBP476) should be classified into the genus Venatorbacter. Metagenomic analysis using ~5,000 publicly available datasets showed that Venatorbacter has a wide environmental distribution in seawater (38), marine sediments (3), hydrothermal vent plumes (2), Antarctic ice (1), groundwater (13), and marine sponge ecosystems (1). Most Venatorbacter species are non-obligate n-alkane degraders that could utilize, at a minimal, C16-C18 n-alkanes, as well as other different types of carbon substrates, including carbohydrates, amino acids, peptides, and phospholipids. The type II secretion system, extracellular proteases, phospholipase, and endonuclease of Venatorbacter species were robustly expressed in the metatranscriptomes of deep-sea hydrothermal vents, suggesting their important contribution to secondary productivity by degrading extracellular macromolecules. The identification of denitrifying genes suggested a genus-specific ecological potential that allowed Venatorbacter species to be active in anoxic environments, e.g., the oxygen-minimal zone (OMZ) and the deeply buried marine sediments. Our results show that Venatorbacter species are responsible for the degradation of hydrocarbon and extracellular macromolecules, suggesting that they may play an important role in the biogeochemistry process in the Trench ecosystems.

Genome-based taxonomic rearrangement of Oceanobacter-related bacteria including the description of Thalassolituus hydrocarbonoclasticus sp. nov. and Thalassolituus pacificus sp. nov. and emended description of the genus Thalassolituus

Oceanobacter-related bacteria (ORB) are a group of oligotrophic marine bacteria play an underappreciated role in carbon cycling. They have been frequently described as one of the dominant bacterial groups with a wide distribution in coastal and deep seawater of global oceans. To clarify their taxonomic affiliation in relation to alkane utilization, phylogenomic and comparative genomics analyses were performed based on currently available genomes from GenBank and four newly isolated strains, in addition to phenotypic and chemotaxonomic characteristics. Consistently, phylogenomic analysis robustly separated them into two groups, which are accordingly hydrocarbon-degrading (HD, Thalassolituus and Oleibacter) and non-HD (NHD, Oceanobacter). In addition, the two groups can also be readily distinguished by several polyphasic taxonomic characteristics. Furthermore, both AAI and POCP genomic indices within the HD group support the conclusion that the members of the genus Oleibacter should be transferred into the genus Thalassolituus. Moreover, HD and NHD bacteria differed significantly in terms of genome size, G + C content and genes involved in alkane utilization. All HD bacteria contain the key gene alkB encoding an alkane monooxygenase, which can be used as a marker gene to distinguish the members of closely related genera Oceanobacter and Thalassolituus. Pangenome analysis revealed that the larger accessory genome may endow Thalassolituus with the flexibility to cope with the dynamics of marine environments and thrive therein, although they possess smaller pan, core- and unique-genomes than Oceanobacter. Within the HD group, twelve species were clearly distinguished from each other by both dDDH and ANI genomic indices, including two novel species represented by the newly isolated strains alknpb1M-1 ^T and 59MF3M-4 ^T , for which the names Thalassolituus hydrocarbonoclasticus sp. nov. and Thalassolituus pacificus sp. nov. are proposed. Collectively, these findings build a phylogenetic framework for the ORB and contribute to understanding of their role in marine carbon cycling.