The subsurface sediment collected from Integrated Ocean Drilling Program (IODP) Site U1385 of the Iberian Margin was examined via metagenomics.Examination of the assembled metagenomes across different depths showed abundant signatures of Aerophobia and Bacteroidia, with additional lineages in lower abundance.We find that predicted functions, such as formaldehyde oxidation and the reverse TCA cycle, decrease with depth, whereas acetogenesis and hemicellulose debranching increase with depth.Genomic signatures of iron reduction and nitrate reduction were widespread in the sediment column.Once binned into metagenome-assembled genomes (MAGs) across all depths, six MAGs were of sufficient quality for characterization, and taxonomically assigned to the classes Promethearchaeia, Thorarchaeia, Atribacterota, Aerophobia, Bacteroidia and SM23-21.We examined how these MAGs varied in relative abundance across sediment depths and how predicted function varied among MAGs and compared to geochemistry.Coverage signatures of Promethearchaeia decreased with depth, whereas signatures for other lineages, particularly the Atribacterota and Bacteroidia, increased with depth.Functions like beta oxidation and glycolysis were well represented across MAGs.Very few genomic signatures of methane and sulfate metabolic cycling were observed in MAGs.This analysis indicates that while the Iberian Margin sediment is a well-preserved site for paleoclimate which suggests limited microbial or chemical diagenesis, it also contains signatures of subsurface microbes that suggest activity in-situ due to depth dependent abundance trends.
Russell et al. (Tue,) studied this question.