Marine bacteria in deep Arctic and Antarctic ice cores: a proxy for evolution in oceans over 300 million generations

[摘要] Using fluorescence spectrometry to map autofluorescence of chlorophyll (Chl)and tryptophan (Trp) versus depth in polar ice cores in the US National IceCore Laboratory, we found that the Chl and Trp concentrations often showed anannual modulation of up to 25%, with peaks at depths corresponding tolocal summers. Using epifluorescence microscopy (EFM) and flow cytometry(FCM) triggered on red fluorescence at 670 nm to study microbes fromunstained melts of the polar ice, we inferred that picocyanobacteria may havebeen responsible for the red fluorescence in the cores. Micron-size bacteriain all ice melts from Arctic and Antarctic sites showed FCM patterns ofscattering and of red vs. orange fluorescence (interpreted as due to Chlvs. phycoerythrin (PE)) that bore similarities to patterns of cultures ofunstained picocyanobacteria Prochlorococcus andSynechococcus. Concentrations in ice from all sites were low, butmeasurable at ~ 1 to ~ 10³ cells cm⁻³. Calibrationsshowed that FCM patterns of mineral grains and volcanic ash could bedistinguished from microbes with high efficiency by triggering on scatteringinstead of by red fluorescence. Average Chl and PE autofluorescenceintensities showed no decrease per cell with time during up to 150 000 yrof storage in glacial ice. Taking into account the annual modulation of~ 25% and seasonal changes of ocean temperatures and winds, wesuggest that picocyanobacteria are wind-transported year-round from warmerocean waters onto polar ice. Ice cores offer the opportunity to studyevolution of marine microbes over ~ 300 million generations byanalysing their genomes vs. depth in glacial ice over the last 700 000yr as frozen proxies for changes in their genomes in oceans.

[学科分类] 地球化学与岩石