4:25 PM - 4:45 PM
[BBG01-09] Mechanisms of community assembly in the subseafloor biosphere
★Invited Papers
Keywords:Subseafloor sediment, Microbial community, Extreme environment
The subseafloor sediments, which cover 70% of the Earth's surface, contain vast numbers of microorganisms, up to 1029 cells, comparable to those in soils and oceans (Kallmeyer et al., 2012, PNAS). The environment of subseafloor sediments is not energy-rich, supported by primary production, as in other biospheres. This is because the supply of substrates to the sediments is limited to that provided by seawater, which is mostly consumed at the sediment surface. Microbial cells are also very difficult to multiply in this extreme environment, with doubling times estimated at up to a million years (Hoehler and Jørgensen, 2013, Nature Rev. Microbiol and references therein). In addition, the porosity between sediment particles and their connectivity is reduced deep below the seafloor, so the microbial community there is thought to be trapped in situ over geological timescales. However, this extremely low-energy environment is known to contain microbial communities with a diversity comparable to that found in soils and oceans(Hoshino and Inagaki, 2021, PNAS), which are thought to be adapted to extreme conditions. The assembly of these microbial communities and the mechanisms by which individual microorganisms adapt to their environment are thought to be very different from those in surface environments, where energy is abundant and cell division is active, and much remains unknown. In this study, microbial communities in sediments collected from the surface to 300 m below the seafloor off the Shimokita Peninsula were analyzed to elucidate the mechanisms of adaptation of subseafloor microbial communities to the environment.
The sediments off the Shimokita Peninsula used in this study consist mainly of diatomaceous silty clay that has been stratified without major disturbance from the past 500,000 years to the present down to 300 m below the seafloor. DNA was extracted from 31 samples taken every 10 m from the sediments and analyzed for 16S rRNA gene amplicons. The microbial communities obtained were typical of anaerobic environments rich in organic matter. To understand the microbial adaptation processes to the extreme environment below the seafloor, we looked for microbial species that were consistently present from the surface to the deepest depths and found that they belonged to taxonomic groups such as Atribacteria, Chloroflexi, Aerophobetes, and Asgardarchaeota. However, Asgardarchaeota, declined rapidly with increasing depth, suggesting that it is being eliminated by the environment. In addition, while Atribacteria showed little correlation between relative abundance and depth, some of the microbial species belonging to Chloroflexi and Aerophobetes showed an increase in abundance with depth. Considering that there are not enough energy sources or substrates in the sediments below the seafloor to allow cells to divide, it is possible that the Atribacteria are simply remaining as they were when they became dominant in the surface layers of the sediments. In other words, they are probably stasis microorganisms rather than adapted and living microorganisms. On the other hand, the latter two microbial species may have characteristics that allow them to use the few remaining substrates and energy sources in the subseafloor environment more efficiently than other microbial species, thereby increasing their abundance. To address these questions, metagenomic analyses have been conducted, yielding approximately 250 high quality MAGs and 1400 medium quality MAGs. In this presentation, the results of the metagenomic analyses will also be presented to further the discussion on the environmental adaptation processes of the microbial community present in the subseafloor sediment.
The sediments off the Shimokita Peninsula used in this study consist mainly of diatomaceous silty clay that has been stratified without major disturbance from the past 500,000 years to the present down to 300 m below the seafloor. DNA was extracted from 31 samples taken every 10 m from the sediments and analyzed for 16S rRNA gene amplicons. The microbial communities obtained were typical of anaerobic environments rich in organic matter. To understand the microbial adaptation processes to the extreme environment below the seafloor, we looked for microbial species that were consistently present from the surface to the deepest depths and found that they belonged to taxonomic groups such as Atribacteria, Chloroflexi, Aerophobetes, and Asgardarchaeota. However, Asgardarchaeota, declined rapidly with increasing depth, suggesting that it is being eliminated by the environment. In addition, while Atribacteria showed little correlation between relative abundance and depth, some of the microbial species belonging to Chloroflexi and Aerophobetes showed an increase in abundance with depth. Considering that there are not enough energy sources or substrates in the sediments below the seafloor to allow cells to divide, it is possible that the Atribacteria are simply remaining as they were when they became dominant in the surface layers of the sediments. In other words, they are probably stasis microorganisms rather than adapted and living microorganisms. On the other hand, the latter two microbial species may have characteristics that allow them to use the few remaining substrates and energy sources in the subseafloor environment more efficiently than other microbial species, thereby increasing their abundance. To address these questions, metagenomic analyses have been conducted, yielding approximately 250 high quality MAGs and 1400 medium quality MAGs. In this presentation, the results of the metagenomic analyses will also be presented to further the discussion on the environmental adaptation processes of the microbial community present in the subseafloor sediment.