Near-Complete Genome Sequence of Thalassospira sp. Strain KO164 Isolated from a Lignin-Enriched Marine Sediment Microcosm

Genomic stasis over millions of years in subseafloor sediment

One of the significant challenges in microbiology is to understand the extent and mechanisms of evolution within life beneath the surface of the Earth. The population bottleneck that microbes in deep marine sediment experience implies that mutational and population genetic forces could lead to higher levels of relaxed selection and an increase in pseudogenes. To investigate this hypothesis, a group of Thalassospira strains were isolated from subseafloor sediment that is 3 to 6 million years old, as reported by Orsi and colleagues in 2021. These isolates, representing lineages that have been buried for millions of years, offer an excellent opportunity to study the evolution of life beneath the seafloor over a long period. The existence of closely related strains from environments on the surface of the Earth enabled us to examine the impact of selection within each group. We discovered that isolates from beneath the seafloor show lineage-specific similarities to Thalassospira from the surface world, both in the overall intensity of selection on the genome and in the specific genes affected by mutation. We found no signs of increased relaxed selection or other notable genomic changes in the genomes of the Thalassospira isolates from beneath the seafloor, suggesting that these subseafloor isolates were awakened from a million-year near-stasis. The unique genomic characteristics of each Thalassospira lineage from beneath the seafloor must then reflect genetic changes that surface-inhabiting decendants acquired in the past 3-6 million years. Remarkably, Thalassospira lineages beneath the surface appear to have stably maintained their genomes in the midst of metabolic dormancy and extremely long generation times.

The Phylogeny and Metabolic Potentials of a Lignocellulosic Material-Degrading Aliiglaciecola Bacterium Isolated from Intertidal Seawater in East China Sea

Lignocellulosic materials are composed of cellulose, hemicellulose and lignin and are one of the most abundant biopolymers in marine environments. The extent of the involvement of marine microorganisms in lignin degradation and their contribution to the oceanic carbon cycle remains elusive. In this study, a novel lignin-degrading bacterial strain, LCG003, was isolated from intertidal seawater in Lu Chao Harbor, East China Sea. Phylogenetically, strain LCG003 was affiliated with the genus Aliiglaciecola within the family Alteromonadaceae. Metabolically, strain LCG003 contains various extracellular (signal-fused) glycoside hydrolase genes and carbohydrate transporter genes and can grow with various carbohydrates as the sole carbon source, including glucose, fructose, sucrose, rhamnose, maltose, stachyose and cellulose. Moreover, strain LCG003 contains many genes of amino acid and oligopeptide transporters and extracellular peptidases and can grow with peptone as the sole carbon and nitrogen source, indicating a proteolytic lifestyle. Notably, strain LCG003 contains a gene of dyp-type peroxidase and strain-specific genes involved in the degradation of 4-hydroxy-benzoate and vanillate. We further confirmed that it can decolorize aniline blue and grow with lignin as the sole carbon source. Our results indicate that the Aliiglaciecola species can depolymerize and mineralize lignocellulosic materials and potentially play an important role in the marine carbon cycle.