landscape change

Biological responses to landscape change in the McMurdo Dry Valleys, Antarctica


The McMurdo Dry Valleys, Antarctica, are experiencing rapid landscape scale change including increased glacial melt, the expansion of water tracks, thermokarst formation, an increase in the extent of the soil active layer, lake level rise, and altered stream flow. The impacts of these changes for biological communities are currently unknown. The goal of this study was to conduct surveys and experiments in three Dry Valley soil habitats that are expected to undergo change: water tracks, lake margins, and active layer profiles. Specifically, samples were collected from: 1) transects spanning dry-wet-dry soils in two water tracks in different lake basins, including three transects from each of three reaches for each water track, 2) experiments in which water and varying levels of salt were added to dry soils immediately adjacent to water tracks, 3) transects in lake margin soils in two different lake basins, including four transects perpendicular to each lake that span wet-to-dry soils, 4) experiments in which sterilized and non-sterilized lake water were added to dry soils adjacent to lakes, 5) the soil active layer from a total of eight soil pits in two different lake basins that were sampled at 5cm increments of depth, from the surface to 30cm or the permafrost, whichever was encountered first, and 6) experiments in which soils from the bottom of the soil active layer were moved to the upper layer and vice versa. Bacterial communities were sequenced for each sample and edaphic characteristics were measured for a subset of samples.

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Bacterial 16S rRNA Gene Sequencing: DNA from 0.7 g of soil was extracted using the cetyltrimethylammonium bromide (CTAB) method. Dual-index paired-end amplicon sequencing of 16S rRNA genes was performed using V6 universal bacterial primers 939F 5’ TTG ACG GGG GCC CGC ACA AG-3’ and 1492R 5’-GTT TAC CTT GTT ACG ACT T-3’ on an Illumina MiSeq. The 16S rRNA gene sequences were trimmed and quality filtered using Sickle and paired-end reads were aligned and merged via PANDAseq. The Quantitative Insights into Microbial Ecology (QIIME) pipeline was used to analyze the reconstructed gene sequences. Unique 16S rRNA gene sequences or operational taxonomic units (OTUs) were identified by the 97% DNA identity criterion using UCLUST. A representative sequence was picked from each OTU and aligned using the PyNAST aligner and the Greengenes core set (v. 13_8) and given taxonomic assignments using the Ribosomal Database Classifier program.Edaphic Chemistry: Soil pH was determined on 1:2 soil/deionized water extracts using an Orion pH probe. Electrical conductivity of 1:5 soil/water extracts was measured with a Yellow Springs Instrument 3100 conductivity meter. Approximately 20g of material were subsampled weighed and dried for 24 hours at 100 degrees C. Soil moisture content was calculated by dry mass – wet mass / dry mass. Dry soil samples were ground with a mortar and pestle into a fine powder and then weighed into tin capsules (~15 mg) for isotope analysis. A step-wise acidification process was utilized to remove carbonates from the soils. Briefly, 50 mL of 1% HCl was added to each open capsule. In desiccators, the capsules were exposed for 24 h to the vapor from 100 mL of 32% HCl in a beaker. All samples were again wetted with 50mL of 1% HCl and put in the desiccator, where they were exposed for 32 h to the vapor from 100mL of 37% HCl. Samples were then dried in a drying oven for 3 days at 35°C–40°C. Carbon (δ13C) isotope values were measured using a Costech 4010 elemental analyzer coupled to a Thermo Scientific Delta V isotope ratio mass spectrometer at the University of New Mexico Center for Stable Isotopes (Albuquerque, NM). Stable isotope data are expressed as δ values using the equation δ13C = [(RSample  − RStandard)/RStandard] x 1000, whereRSampleand RStandard  are the ratios of 13C /12C for each sample and standard. The internationally accepted standard for δ13C is Vienna Pee Dee Belemnite (V-PDB). The units are expressed as permil (‰). Internal lab reference materials included organic sediment standards with δ13C values (± SD) of -24.5 ± 0.6. Analytical precision was estimated via repeated (within-run) measurements of these reference materials calibrated to internationally accepted standards; within-run standard deviation for all reference materials was ≤ 0.2‰.

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