Comparison of the microbial diversity at different depths of the GISP2 Greenland ice core in relationship to deposition climates.

This study presents comparative geochemical, microbiological and molecular analyses of Greenland GISP2 ice core samples representing different depths, ages, deposition climates, in situ temperatures, and gas and ionic compositions. Our goal was to determine whether specific organisms, preserved at different depths, correlate with past climate characteristics recorded chronologically in ice layers. Three clear ice samples were selected from 2495, 2545 and 2578 m to represent distinct climatic periods with milder (-45 degrees C), colder (-51 degrees C) and warmer (-39 degrees C) deposition temperatures, and two Marine Isotope Stages, MIS3 (2495 m) and MIS4 (2545 and 2578 m). Results showed higher microbial abundance in ice deposited during colder climates with higher in situ ion content. The constructed universal SSU rRNA gene clone libraries were dominated by Gram-positive sequences (55-65%), and had fewer Proteobacteria (6-9%) and Archaea (1%). The 2495 m library differed from the other two by being dominated by Actinobacteria (55%) rather than Firmicutes. Fungi were more prevalent in the colder climate (40%). For comparison, a library was constructed from an older silty ice sample (3044 m) possibly originating from underlying permafrost with different in situ characteristics (high temperature, high methane and higher cell numbers). It showed significantly different diversity not found in the clear ice libraries. The bacterial and fungal isolates from the clear ice samples were related to organisms originating from Asian deserts, marine aerosols and volcanic dust, suggesting these environments as sources of deposited microorganisms. The observed differences in microbial diversity patterns, especially with the 2495 m library, support the idea that local climate conditions and global atmospheric circulations at different time periods have influenced the origin and composition of the microbial populations preserved at different depths of Greenland ice. Further investigations may lead to the development of microbial 'markers' for identifying specific deposition climates.