Ultrasmall bacteria from Antarctic Lake raise questions about the limits of life

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Credit: Wikimedia Commons

Credit: Wikimedia Commons

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.

Approximate location of Lake Vida, Antarctica. Credit: Wikimedia Commons

Approximate location of Lake Vida, Antarctica. Credit: Wikimedia Commons

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.

Europa, Jupiter's icy moon, has excited astrobiologists as a potential site for extraterrestrial life in our solar system. Credit: Wikimedia Commons

Europa, Jupiter’s icy moon, has excited astrobiologists as a potential site for extraterrestrial life in our solar system. Credit: Wikimedia Commons

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.

ResearchBlogging.org

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

Frozen bacteria repair their DNA at -15ºC

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Bacteria encased in ice can be resuscitated after thousands, perhaps even millions of years. How these hardy bugs manage to survive deep freeze is something of a mystery.  If nothing else, the low levels of radiation hitting Earth’s surface should cause any ice-bound bacterium’s DNA to break apart over time, eventually leading to irreparable damage. Some scientists think bacteria survive cryosleep by encasing their DNA in protective shells known as spores and entering a state of dormancy. Following spore formation, a bacterium can withstand harsh environmental conditions, including desiccation, strong acids, heat and UV radiation.

Spores don't actually have eyes like they do in the popular video game, but they can resist drought, fire, ice, acid, and even antibiotics.

Spores don’t actually have eyes like they do in the popular video game, but they can resist drought, fire, ice, acid, and even antibiotics.

But other researchers think we aren’t giving enough credit to the ice dwellers. Recent studies have shown that some psycrhophiles– technical-speak for cold-loving bacteria – are able to maintain basic metabolic functions at subzero temperatures. Could psychrophiles trapped in ice be repairing their DNA faster than the UV radiation bombarding our planet pulls it apart? Microbiologist Markus Dieser at Lousiana State University was interested in finding out. In a study published  in the journal Applied and Environmental Microbiology, Dieser and colleagues show for the first time that one bacteria- Psychrobacter arcticus– can repair it’s DNA at temperatures as low as -15ºC, or 5ºF. Moreover, it can do so 100,000 times faster than damage occurs.

P. articus is an innocuous little bacteria that is famous for one thing: it really likes the cold. It can grow and metabolize at -10 ºC, making it one of the most psychrophilic organisms on Earth. To investigate P. articus’’s ability to repair DNA in deep freeze, Dieser and colleagues isolated viable P. articus cells from Siberian permafrost that has been frozen for 20 to 30 thousand years. In the lab, the researchers dosed their cell cultures with a large pulse of ionizing radiation- roughly equal to what P. articus might experience over 225 thousand years of field exposure. By using such an intense burst of radiation, the team hoped to induce many “double-strand breaks”, or breaks that cause small DNA fragments to separate off from P. articus’s main chromosome.They incubated the irradiated cultures at -15ºC and monitored their survival over the course of 505 days.

Rather astoundingly, the scientists found no significant difference between the survival rates of irradiated and non-irradiated bacteria over the year and a half long study. While this finding alone suggests P. articus can repair its DNA at subzero temperatures, Dieser and colleagues wanted direct evidence.  They used pulse-field electrophoresis, a technique which separates DNA fragments by size, to determine how may DNA double-strand breaks occurred after radiation exposure, and whether the DNA fragments reassembled themselves over time. Like Humpty Dumpty rebuilding himself, the scientists could literally watch P. articus reassemble its genome. On average, P. articus was able to patch thirteen double-strand DNA breaks over the course of the study-  quite close to the roughly sixteen breaks inducted by radiation.

DNA ligase repairing a DNA molecule that has suffered a double-strand break. Credit: Wikipedia

DNA ligase repairing a DNA molecule that has suffered a double-strand break. Credit: Wikipedia

Not only can P. articus repair its DNA at subzero temperatures, it can do so really fast. Using annual radiation exposure data collected in the field, Dieser estimates that P. articus can repair double-strand breaks 100,000 times faster than they occur. The discovery has important implications for the survival of life in extreme environments, including cold extraterrestrial environments. For instance on the surface of Mars, where radiation levels are ~400 times greater than the Siberian permafrost, P. articus can still patch DNA breaks 280 times faster than they would accrue. As scientists continue exploring the “cold limit” to essential cellular functions such as DNA repair, they will continue to refine, and perhaps expand, our understanding of the fundamental boundaries for life.

ResearchBlogging.org

Markus Dieser, John R. Battista, & Brent C. Christner (2013). DNA Double-Strand Break Repair at −15°C Applied and Environmental Microbiology DOI: 10.1128/AEM.02845-13