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

Standard
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

Cryogenics, gene popsicles and the oldest life on Earth

Standard

While the notion of “cryogenic freezing”, or putting a person into a state of frozen suspension, has been a common theme in science fiction for decades (think the Alien movies, Star Wars, Sleeper, Vanilla Sky) bacteria have probably been doing their own version of cryogenic sleep for billions of years.

Researchers studying ice cores from the Dry Valleys of Antarctica have found viable, frozen bacteria that are thousands to millions of years old. The ice in this region of the Dry Valleys ranges from modern to about ten million years old, making it some of the oldest known ice on earth. By analyzing the ice crystal structure and isotopic data, these researchers determined their ice samples had likely been permanently frozen (i.e., no thawing/refreezing), implying that the bacteria encased within the ice have been trapped since its formation.

Resuscitation

The scientists incubated meltwater from ice core samples at temperatures just above freezing for up to 300 days, adding supplemental nutrients to encourage bacterial growth. The samples they incubated represented a broad range of timescales, with ages ranging from 10,000 years to 8 million years. Astoundingly, bacterial growth was observed in all samples, though growth rates declined with sample age: bacteria that had been encased in ice for shorter periods of time grew much more rapidly than bacteria frozen for millions of years.

Caveats to cryogenic

The study concluded that even bacteria cannot maintain cryogenic preservation forever. In addition to slower growth rates for older bacteria, the study found an exponential decline in the size of the community DNA pool over time, suggesting the DNA is slowly degrading, even in a deep freeze. Very slowly. The estimated half-life for the reduction in DNA pool size (i.e., the amount of time it takes to reduce the amount of DNA in a sample by 50%) was 1.1 million years. (I think I just heard the microbial ecologists breathe a collective sigh of relief.) So, it may be perfectly reasonable to find frozen bacteria that are hundreds of thousands of years, even a couple million years old, that can still be resuscitated.

{An aside: why does DNA degrade, even in a deep freeze? The jury is still out, though one suspect in the present study is cosmic radiation (high-energy particles that bombard the Earth from space). Antarctica receives the highest levels of incoming cosmic radiation on the planet.}

Gene popsicles in a melting world

Bacteria encased in ice for thousands to millions of years are literally a gene bank. Collectively, the community DNA frozen in ice can be thought of as a “gene popsicle” that provides a snapshot into the past and another clue scientists can use to piece together ancient Earth environments. Moreover, it is well known that bacteria are capable of transferring genes amongst each other in a process known as lateral gene transfer. Could the periodic melting of ice sheets, due to shifts from glacial to interglacial periods, result in an influx of ancient genes into modern bacterial communities? Could genetic information perhaps be preserved for hundreds of millions, or even billions of years, through freezing, melting, and re-uptake of ancient genes by living bacteria?

And finally, the million dollar question: what are the implications for of melting gene popsicles on present-day Earth? As glaciers and ice sheets across the world continue to melt due to climate change, will hordes of ancient bacteria start to “wake up”? Could they plague the world with ancient diseases that no modern humans have resistance to? (Hmm…sounds like a good idea for a science fiction story 🙂 The answer to the former question is, probably yes, the latter, probably not. But time, and a lot more research on the microbial ecology of melting ice sheets is needed to answer these questions.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0702196104)