Some time ago, we asked this Talmudic Question: Can you think of a place on Earth where there is free water but no microbes? (A sterile flask of nutrient broth in a lab, the insides of the body, or an IV bag in a hospital don't count.) Someone answered that perhaps deep in Antarctica there would be a buried lake that was sterile. Not so. The early returns from the deep drilling of the buried Vostok Lake indicated the presence of microbes even there. But other sites in that continent come close to being just such an environment because their microbial population is unusually thin. So, let’s rephrase the question: “Can you think of natural places where there is free water but, comparatively speaking, few microbes?” (Pick your own limit, but <105 cells/ml seems thin to me.)
Fig. 1. Deep Lake. Credit: University of New South Wales. Source.
In answer, I point to Deep Lake, a most unusual environment in East Antarctica. Despite its low temperature, it never freezes. The reason? Its waters contain 27% salt (w/v, almost 5M of NaCl), just a tad below the salinity of the Dead Sea (33.7% w/v) and about ten times that of the oceans. That salinity keeps the lake liquid despite temperatures being as low as -40° C for the ambient air and -20° C throughout its depths. What stalwart microbes do you find here? Mainly, some unusual salt-loving Archaea, aka haloarchaea. They have been studied in some detail, with some unexpected findings. Four species make up about three quarters of the total population. They belong to a different genus each, all within the family Halobacteriaceae (the ‘-bacteriaceae’ being a holdover from older, “pre-archaeal” taxonomy). Like other haloarchaea, some of these isolates harbor two or three chromosomes, or replicons, totaling nine replicons among them. Each species has its own predilection for particular depths. They can be cultivated, as long as you have a lot of patience. One of them grows at temperatures from -1° C to 40° C, fastest at 33° C. When cultured in the laboratory at -1°C, the temperature of the top of the lake, the cells double six times per year. At that rate, the authors calculate that it would take 10 years for a single cell to fully populate the lake.