Evidence for Longevity of Seeds and Microorganisms in Permafrost

Résumé

The problem of whether seeds and microorganisms are able to remain viable in permanently frozen ground during a very long span of years was recently considered by Porsild et al. who germinated Lupinus arcticus seeds from fossil rodent burrows in permanently frozen silt deposited in a placer mining area in the Yukon Territory. Based on the geology of the locality, Porsild et al. estimated the age of the seeds to be at least 10,000 years. Moreover, in that and in previous papers Porsild suggests that seeds of Rorippa barbareaefolia, Descurainia sophioides, and Senecio congestus may be able to survive in permanently frozen silt as these species apparently are restricted to freshly disturbed soil within the placer mining districts. Discussing in general the problems of dating viable seeds in old deposits Godwin questions the dating of the Lupinus arcticus seeds because of the indirect method applied. In view of these studies and the investigations made by Ødum on the presence and age of viable seeds in Danish soils, Ester Creek placer mine west of Fairbanks, Alaska, was visited 16 to 19 July 1968. Some observations on the vegetation on the exposed silt and disturbed soil were made and soil samples were taken for further examination .... The contents of microorganisms in the soil samples were examined by using the dilution plate technique: 10 g of each sample were mixed with 90 ml of sterile water and placed on a shaking table for 20 minutes. Serial dilutions were made, and 0.1 ml of each of 10**-1, 10**-2, 10**-3 and 10**-4 suspensions were plated on Bengal Rose-, Sabouraud-, Cook-, and V-8-agar. Besides 1 ml of each suspension was added to test tubes with NIB. After one week of incubation at 4°C, 24°C, and 37°C the colonies were counted and the different strains isolated on slants with V-8-agar. The sample from the top soil showed c 10**3 fungi per g soil at 24° C, representing the following genera, known from similar localities: Cladosporium herbarum, Mortierella sp., Mucor circinelloides, Penicillium spp., Trichoderma viride, and unidentified, sterile imperfects and phycomycetes. ... Only a few investigations have been carried out to find living microorganisms in permanently frozen soil, and generally only the upper layers have been studied, because of the difficulties of sampling through permafrost. However Becker and Volkmannlo recovered 8 bacteria at 20 to 60 feet below the surface in permanently frozen soil near Fairbanks. Boyd and Boyd also studied permafrost soils from Barrow, Alaska, near Bison Lagoon, and found living bacteria at 8 to 15 feet. The distribution of Descurainia sophioides within the placer mine area and its extremely high frequency on the exposed muck does strongly support Porsild's theory. However, the fact that the seeds of the species are lacking in the investigated samples may lead to the alternative possibility that the ecology of the species is rather specific. Which factor or combination of factors is responsible for the germination and growth of the species on that particular substratum, ... cannot be determined without field experiments at the locality and further analyses; the high water content of the silt may appear to be a factor of importance. The investigation does not prove that viable seeds of Descurainia sophioides, Rorippa barbareaefolia, Senecio congestus or any other species are not present in permanently frozen soil; the negative result of the investigation, however, indicates that on this locality it is not very likely that they are present. ...