Amoeboid protists isolated from ancient Siberian permafrost

Study Extent

The samples were collected in the field and stored constantly frozen during all periods of transportation and processing. In the lab, a part of each sample was used to isolate protist strains. The remaining part has never been melted and is stored at −18 °C. The isolation was done in sterile conditions. Obtained samples were cloned and maintained as bacterized or axenic cultures on plastic or agar with liquid overlay.

Sampling

The drilling was performed using a mobile drilling rig (core-drilling machine) UKB-12/25 (V.V. Vorovsky Machine-Building Plant, Moscow, Russia) operated without flushing and blowing. Flushing and blowing were shown to cause contamination of the cores by modern soil microorganisms. Cores were removed each 30—70 cm of the drilling. The core diameters were 115 to 75 mm, depending on the well depth (the deeper the well the smaller). A removed core was enveloped in a one-centimeter-thick coat of half-melted cuttings. This coat was removed with a knife, showing a completely frozen inner part. After a short lithological and glaciological description of the sediments, the core was passed to a clean field lab organized in a tent. Operations in the lab were conducted behind a gas-fired burner using disposable materials and gloves, following general microbiological practice. In the lab, the core was shaved with a sterile scalpel so that an approximately five mm outermost layer was removed. This remaining core was four to six cm in diameter, depending on the initial value. Immediately after shaving, the core was put into a sterile plastic bag and placed in a portable freezer, a cave dug into an ice wedge, or an empty borehole used as a freezer. In total, the “outdoor” stage of the process lasted five to ten minutes, depending on the current well depth. The “indoor” lab stage generally took around five minutes. All collected cores were kept at negative temperatures during the whole period of transportation to the stationary lab. Outcrops are natural exposures of ancient permafrost sediments formed at sea and river banks. The advantage of sampling from the outcrop wall is the possibility of visual inspection and description of the whole sediment layer. Samples from outcrops were taken from the frozen surface of the outcrop wall after the removal of melted material. In the wall, a hole of about 40 cm deep was made with either a hand-held drill, a chisel, or a knife. A cylinder sample of about 5 cm in diameter was carefully carved or drilled out from the bottom of the hole, treated with 95% ethanol, and immediately placed into a sterile plastic bag. As with the cores, all collected outcrop samples were kept frozen during the whole period of transportation. Buried terminal nesting chambers of the ground squirrel (Citellus) burrows are unique paleontological objects of Pleistocene Ice Complex sediments. They usually contain animal supplies made of seeds of surrounding grasses. Usually frozen in the living state, they are very well preserved. From the tissue of a Silene stenophylla seed, a viable flowering plant was grown (Yashina et al. 2012). Chambers also contain a diverse community of protists and fungi. Chambers were cut from the outcrop wall in one or several pieces ten or more cm in each dimension, put immediately in sterile plastic bags, and kept frozen until processing in the lab.

Quality Control

During the development of the permafrost microbiological sampling technique, several tests for contamination of the core interior have been established at different phases of sampling and storage. Gilichinskiy et al. (2010) and Shi et al. (1997) used Serratia marcescens cultures (Bacteria) producing easy-to-notice red colonies. A drilling barrel was covered with a culture suspension 2 h prior to drilling. In a parallel experiment, frozen samples were seeded with the same suspension already in the lab and left intact at negative temperature for several hours to several months. In both experiments, the distribution of S. marcescens cells in a core has been investigated during the sample processing. In all tests, bacteria have been found exclusively in the outer layer and never inside the core. Later, Juck et al. (2005) used fluorescent latex beads (microspheres) 0.5 μm in diameter and a transformed Pseudomonas strain expressing the green fluorescent protein (GFP). Both beads and transformed bacteria were applied to the drilling equipment before drilling, similar way as described above for S. marcescens suspension. Fluorescence microscopy showed that neither beads nor bacteria penetrate into a sample to a depth exceeding one cm from the surface. Additionally, PCR revealed no amplification of the GFP gene from the inner part of the cores. Based on the negative results obtained for bacteria and fluorescent beads, i.e. particles around two μm in diameter or less, we consider it proven that resting protist cysts, which are at least five times bigger, can’t move inside the frozen ground and thus could not penetrate the sediments considerably later than they were deposited. Same way, the contamination of the inner part of the samples during sampling and laboratory processing is not possible either.

Method steps

1. Field sampling (see above)
2. Isolation of protists in the lab For the isolation of protists, 1—5 grams of the sediment from the inner part of the frozen sample were placed in 90 mm Petri dishes filled with autoclaved mineral Prescott and James (PJ) medium (Prescott and James 1955). Negative controls (same procedures without sample inoculation) were performed simultaneously. The isolation was performed in a laminar flow hood using disposal or sterilized equipment. After 7—10 days of incubation, samples were examined using a Nikon TMF100 inverted microscope. Detected cells were transferred to the fresh medium by a glass capillary. Strains were cloned and further cultured in 60 mm Petri dishes using 0.01% Cerophyl infusion made on PJ medium (Smirnov and Brown 2004).

Permafrost samples for the strain isolation were obtained in North-Eastern Eurasia, from Yamal Peninsula to Chukotka.