Imagine you were forced to live in perpetually subzero temperatures, with no oxygen, no light, and way more salt than your system could handle. How would you manage? One way might be to get extremely small. At least, that seems to be what’s happening in a frozen Antarctic lake that’s cut off from the rest of the world by 27 meters of perennial ice.
Lake Vida, Antarctica, has come under biological scrutiny recently. It’s an fascinating environment for a number of reasons. For one, it represents a unique combination of extreme conditions. Vida’s high salt concentrations keep the lake’s water liquid at -13.4ºC, or 7.9 ºF. And, even more intriguing, this super-chilled salt bath has been cut off from the outside world for nearly 3,000 years.
The microbial inhabitants of Lake Vida have had a unique opportunity to evolve in complete isolation. For microbial ecologists, this means a potential goldmine of novel adaptations and genetically unique organisms.
So far, Lake Vida’s microbes have lived up to expectations. In a study published recently in the journal Applied and Environmental Microbiology, Dr. Alison Murrary and colleagues find Lake Vida’s brine is teeming with some very tiny critters. These ultrasmall microbes, or ultramicrocells, are roughly 200 nanometers in diameter, just undercutting the theoretical “lower size limit” for a single-celled organism. In addition, these tiny critters display some fascinating adaptations for handling the stress of life in cold, salty brine.
Murray and colleagues used several techniques to characterize the ecology of Lake Vida brine samples collected in 2010, including scanning electron microscopy, spectroscopy, and x-ray diffraction.
In their recent study, the scientists observed two cell populations in Lake Vida’s brine. One population of rod-shaped bacteria ranged in size from ~0.4-1.5 µm, while a smaller class of spherical bacteria were approximately ~0.2 µm, or 200 nanometers, in diameter. This second class, designated the “ultrasmalls”, was 100 times more abundant than their larger counterparts. Even smaller particles that ranged in size from 20-140 nanometers were also abundant.
Further analysis using x-ray spectroscopy indicated that both ultrasmalls and nanoparticles had granular, iron-rich surface coatings. Interestingly, these coatings resemble iron oxide minerals found in old, weathered soils. It was also common for ultrasmalls to possess exopolysaccharides- long, filamentous proteins- connecting them to the nanoparticles.
Exopolysaccharides can serve many functions for microorganisms. In this case, the scientists speculate exopolysaccharides act as a nucleation site for iron particles- that is, a surface to which iron particles can precipitate in solid form. The resultant “iron exoskeleton” may be a unique adaptation for protection against extreme cold.
The nanoparticles remain something of a mystery, but the scientists hypothesize these may also be a part of an elaborate ultrasmall survival strategy. The size and morphology of the nanoparticles suggests they may, in fact, be extracellular membrane vesicles- pieces of cells that have popped off their parent and become self-contained storage units . Other scientists have found that microbes produce such vesicles in response to temperature stress. Like a storage unit, vesicles allow microbes to sweep their house clean, removing unnecessary clutter. One sort of unwanted baggage for the Lake Vida ultrasmalls may be misfolded proteins. Protein misfolding is a common problem in subzero environments. Harboring useless misfolded proteins represents a drain on valuable cellular resources.
Lots of open questions remain regarding the ecology of Lake Vida’s ultrasmalls. Perhaps the biggest question is why exactly these microbes are so tiny. There are a number of possibilities to be explored. Smallness is a response to stressful environments across all domains of life. Hyperosmotic stress- the result of being bathed in a super salty liquid- may result in water loss and cell shrinkage. Or ultrasmalls may be expending so much energy dealing with the cold that they don’t have the extra resources required to grow bigger.
Answering these questions will help scientists understand how microbes may cope with extreme environments not only on Earth, but on icy extraterrestrial worlds as well.
Kuhn, E., Ichimura, A., Peng, V., Fritsen, C., Trubl, G., Doran, P., & Murray, A. (2014). Brine Assemblages of Ultrasmall Microbial Cells within the Ice Cover of Lake Vida, Antarctica Applied and Environmental Microbiology, 80 (12), 3687-3698 DOI: 10.1128/AEM.00276-14