The Global Turbidity Current Pump and Its Implications for Organic Carbon Cycling

Comparative metagenomics reveals the metabolic flexibility of coastal prokaryotic microbiomes contributing to lignin degradation

Coastal wetlands are rich in terrestrial organic carbon. Recent studies suggest that microbial consortia play a role in lignin degradation in coastal wetlands, where lignin turnover rates are likely underestimated. However, the metabolic potentials of these consortia remain elusive. This greatly hinders our understanding of the global carbon cycle and the "bottom-up" design of synthetic consortia to enhance lignin conversion. Here, we developed two groups of lignin degrading consortia, L6 and L18, through the 6- and 18-month in situ lignin enrichments in the coastal East China Sea, respectively. Lignin degradation by L18 was 3.6-fold higher than L6. Using read-based analysis, 16S rRNA amplicon and metagenomic sequencing suggested that these consortia possessed varied taxonomic compositions, yet similar functional traits. Further comparative metagenomic analysis, based on metagenomic assembly, revealed that L18 harbored abundant metagenome-assembled genomes (MAGs) that encoded diverse and unique lignin degradation gene clusters (LDGCs). Importantly, anaerobic MAGs were significantly enriched in L18, highlighting the role of anaerobic lignin degradation. Furthermore, the generalist taxa, which possess metabolic flexibility, increased during the extended enrichment period, indicating the advantage of generalists in adapting to heterogenous resources. This study advances our understanding of the metabolic strategies of coastal prokaryotic consortia and lays a foundation for the design of synthetic communities for sustainable lignocellulose biorefining.

Circulation of hydraulically ponded turbidity currents and the filling of continental slope minibasins

Natural depressions on continental margins termed minibasins trap turbidity currents, a class of sediment-laden seafloor density driven flow. These currents are the primary downslope vectors for clastic sediment, particulate organic carbon, and microplastics. Here, we establish a method that facilitates long-distance self-suspension of dilute sediment-laden flows, enabling study of turbidity currents with appropriately scaled natural topography. We show that flow dynamics in three-dimensional minibasins are dominated by circulation cell structures. While fluid rotation is mainly along a horizontal plane, inwards spiraling flow results in strong upwelling jets that reduce the ability of minibasins to trap particulate organic carbon, microplastics, and fine-grained clastic sediment. Circulation cells are the prime mechanism for distributing particulates in minibasins and set the geometry of deposits, which are often intricate and below the resolution of geophysical surveys. Fluid and sediment are delivered to circulation cells by turbidity currents that runup the distal wall of minibasins. The magnitude of runup increases with the discharge rate of currents entering minibasins, which influences the amount of sediment that is either trapped in minibasins or spills to downslope environs and determines the height that deposits onlap against minibasin walls.