Microbiologists in the CSUN Department of Biology have carefully sifted the microbial community living in permafrost — the permanently frozen layer of soil below Arctic tundra — to understand how bacteria cope with this extreme environment. Their results were just published online ahead of print in the journal Applied and Environmental Microbiology.
Microbial samples from permafrost, and other environmental samples, can be a mix of bacterial cells in a wide range of conditions, from fully live and active to dormant forms like endospores, to dead cells. The paper, led by recent CSUN Master’s graduate Alex Burkert with Associate Professor Rachel Mackelprang and a collaborator at the US Army Cold Regions Research and Engineering Laboratory, Thomas Douglas, details how the team separated microbial samples to identify not only the diversity of bacteria present in different permafrost samples, but to compare bacterial cells in different states within samples.
Burkert et al. examined a “chronosequence” of permafrost samples taken from different depths at a site north of Fairbanks, Alaska. The deeper a sample’s source, the older the permafrost it contained — the team worked with samples corresponding to 19, 27, and 33 thousand years ago. They stained extracted bacterial cells with a dye that colors dead and live cells differently, and counted each under a microscope. They used a centrifugation procedure to separate out live and dead cells, and another procedure to isolate cells in a dormant endospore state. They then sequenced the 16S rRNA gene diversity in the live, dead, and endospore samples, and compared them to determine whether bacterial taxa with differing 16S sequences were found in different states, or different timepoints in the chronosequence.
The results clearly showed differences between the bacterial diversity recorded in cold-preserved DNA from dead cells, and the community of live and active bacteria in the permafrost; as well as differences in bacterial community over the different samples in the chronosequence. The authors also identified bacterial taxa that were more likely to be present as endospores than others.
The full article is online at the journal website.
Image: Katherine Hammack, assistant secretary of the Army for Installations, Environment and Energy and Miranda Ballentine, assistant secretary of the Air Force for Installations, Environment and Energy tour the Cold Regions Research and Engineering Laboratory’s Permafrost Tunnel Research Facility, the source of the samples used in the new study, in 2015. (Wikimedia Commons)