Recording of climate and diagenesis through sedimentary DNA and fossil pigments at Laguna Potrok Aike, Argentina

VUILLEMIN, A.; BUNTING, L.; ARIZTEGUI, D.; PASADO SCIENCE TEAM; LEAVITT, P. R.

BIOGEOSCIENCES

COPERNICUS PUBLICATIONS

Lugar: Gottingen; Año: 2016 vol. 13 p. 2475 - 2492

1726-4170

Aquatic sediments record past climatic conditions while providing a wide rangeof ecological niches for microorganisms. In theory, benthic microbial communitycomposition should depend on environmental features and geochemical conditionsof surrounding sediments, as well as ontogeny of the subsurface environment assediment degraded. In principle, DNA in sediments should be composed of ancientand extant microbial elements persisting at different degrees of preservation,although to date few studies have quantified the relative influence of eachfactor in regulating final composition of total sedimentary DNA assemblage.Here geomicrobiological and phylogenetic analyses of a Patagonian maar lakewere used to indicate that the different sedimentary microbial assemblagesderive from specific lacustrine regimes during defined climatic periods. Twoclimatic intervals (Mid-Holocene, 5 ka BP; Last Glacial Maximum, 25 ka BP)whose sediments harbored active microbial populations were sampled for acomparative environmental study based on fossil pigments and 16S rRNA genesequences. The genetic assemblage recovered from the Holocene record revealed amicrobial community displaying metabolic complementarities that allowedprolonged degradation of organic matter to methane. The series of Archaeaidentified throughout the Holocene record indicated an age-relatedstratification of these populations brought on by environmental selection duringearly diagenesis. These characteristics were associated with sedimentsresulting from endorheic lake conditions and stable pelagic regime, highevaporative stress and concomitant high algal productivity. In contrast,sulphatereducing bacteria and lithotrophic Archaea were predominant insediments dated from the Last Glacial Maximum, in which pelagic claysalternated with fine volcanic material characteristic of a lake level highstandand freshwater conditions, but reduced water column productivity. Comparison ofsedimentary DNA composition with that of fossil pigments suggested thatpost-depositional diagenesis resulted in a rapid change in the initial nucleicacid composition and overprint of phototrophic communities by heterotrophicassemblages with preserved pigment compositions. Long DNA sequences (1400?900bp) appeared to derive from intact bacterial cells, whereas short fragments(290?150 bp) reflected extracellular DNA accumulation in ancient sediments. We concludethat sedimentary DNA obtained from lacustrine deposits provides essentialgenetic information to complement paleoenvironmental indicators and tracepost-depositional diagenetic processes over tens of millennia. However, itremains difficult to estimate the time lag between original deposition oflacustrine sediments and establishment of the final composition of thesedimentary DNA assemblage.