The emplacement of subaqueous gravity-driven sediment flows imposes a significant physical and geochemical impact on underlying sediment and microbial communities. Although previous studies have established lasting mineralogical and biological signatures of turbidite deposition, the response of bacteria and archaea within and beneath debris flows remains poorly constrained. Both bacterial cells associated with the underlying sediment and those attached to allochthonous material must respond to substantially altered environmental conditions and selective pressures. As a consequence, turbidites and underlying sediments provide an exceptional opportunity to examine (i) the microbial community response to rapid sedimentation and (ii) the preservation and identification of displaced micro-organisms. We collected Illumina MiSeq sequence libraries across turbidite boundaries at ~26 cm sediment depth in La Jolla Canyon off the coast of California, and at ~50 cm depth in meromictic Twin Lake, Hennepin County, MN. 16S rRNA gene signatures of relict and active bacterial populations exhibit persistent differences attributable to turbidite deposition. In particular, both the marine and lacustrine turbidite boundaries are sharply demarcated by the abundance and diversity of Chloroflexi, suggesting a characteristic sensitivity to sediment disturbance history or to differences in organic substrates across turbidite profiles. Variations in the abundance of putative dissimilatory sulfate-reducing Deltaproteobacteria across the buried La Jolla Canyon sediment–water interface reflect turbidite-induced changes to the geochemical environment. Species-level distinctions within the Deltaproteobacteria clearly conform to the sedimentological boundary, suggesting a continuing impact of genetic inheritance distinguishable from broader trends attributable to selective pressure. Abrupt, <1-cm scale changes in bacterial diversity across the Twin Lake turbidite contact are consistent with previous studies showing that relict DNA signatures attributable to sediment transport may be more easily preserved in low-energy, anoxic environments. This work raises the possibility that deep subsurface microbial communities may inherit variations in microbial diversity from sediment flow and deformation events.
Discrete biological community signatures were identified in individual sub‐annually deposited sedimentary laminae of anoxic lake sediments from two lakes in the Minneapolis–St. Paul (Minnesota, U.S.A.) urban area. Recognizing variation in microbial communities associated with discrete millimeter scale sedimentary horizons was made possible using a freeze‐coring method to recover bacterial DNA for amplicon iTag sequencing and Terminal Restriction Fragment Length Polymorphism analyses. Variation in 16S rRNA gene composition between laminae suggests that seasonal changes in cell transport from the water column impart a residual molecular signature on subsurface communities. Direct comparison of frozen‐in‐situ core samples to ambient temperature sediment indicates that freeze coring methodology imposes no significant bias on DNA‐based community fingerprints. This work further supports previous observations demonstrating the efficacy of freeze coring for high‐resolution analysis of microbial communities, but here it is applied to resolving molecular signatures derived from sedimentary laminae.