Imagine being stranded on a tropical island, hacking your way through the lush jungle, when - out of nowhere - a polar bear lunges at you! Terrifying and totally bizarre but, as followers of the TV show LOST will attest, you also can't keep yourself from wondering: how the heck did it end up in this patently wrong place?!
Well, this is akin to the strange reality marine microbiologists have been living for the past few decades, albeit less life-threatening than misplaced polar bears. They've been finding thermophilic (literally, "heat-loving") bacteria in permanently cold arctic sediments. Granted, these thermophilic bacteria are a minuscule part of the total community in arctic sediments, and they aren't metabolically active. Many of them have gone through endosporulation, which allows them to persist rather indefinitely in a dormant state. But still!
Is this more evidence in support of the theory (nicely phrased by Dutch microbiologist Baas-Becking in 1934) that in the microbial world, "everything is everywhere but the environment selects"? Recently, deep sequencing has revealed the diversity of microbial communities, including the "rare biosphere" which is comprised of extremely low-abundance taxa; it's also true that in almost every environment, the more you look the more rare microbial species you'll turn up. These rare organisms are a seed bank of diversity, probably ready to spring forth and multiply should environmental conditions change in their favor.
On the other hand, there must be some sort of biogeography of bacteria... some have argued we just can't distinguish between bacteria well enough to pick up on the biogeographic differences. We do have some examples of bacterial biogeography, like populations in salt marsh sediments that are more similar in community composition the closer they are geographically. But what about things like mountains, islands, currents and wind patterns? It seems like some features of the earth would variously divide or transport bacteria, as they do animals and plants.
Not much of an explanation has been offered for the occurrence of thermophilic arctic bacteria, so an international team of scientists decided to look at the diversity, abundance, and distribution of these organisms in permanently cold Svalbard sediments. They carefully extracted the sediments, and then incubated them at a range of temperatures to see whether (and at what temperature) the thermophilic spores were viable and chemically assess their metabolic activities. Finally, time to get some answers!
They found two populations of sulfate-reducing bacteria that grew when the sediments were heated - one population grew between 0 and 32C ( likely active in the environment) and one population that grew between 41 and 62C (definitely NOT active in situ). Bingo! At a sweltering 50C (thats 122F, or about 20 degrees hotter than your jacuzzi) the thermophiles happily consumed organic matter through sulfate reduction, fermentation and hydrolysis - which also happen to be the dominant metabolic processes in the natural population at cold temperatures. The closest known relatives of these thermophiles have been isolated from deep hot crustal fluids, from the crusts themselves, and from subsurface petroleum reserves. Hmmm...
Some back calculations derived from the cores and measuring sediment accumulation rates at the sites showed that these bacterial spores, though a small part of the community, are astonishingly abundant for the environment. With about 105 spores per cm3 and a sediment accumulation rate of 0.19 cm/yr, the 1000km2 of fjords and coastal shelf in this arctic area would require an annual supply of 1017 thermophiles per year! To sustain this kind of flux, there must be a hot, anaerobic source with sufficient distribution, magnitude and strength to populate arctic regions this way.
The authors suggest that fluids flowing through the seabed are actually what govern the biogeography of these thermophilic spore-forming bacteria. Studies have shown that crustal fluids and basaltic aquifers fuel microbial communities deep with the earth, which are referred to as the "deep subsurface biosphere." We've even caught seafloor vents spewing flocculated bacteria on camera. Thus, the deep subsurface biosphere, with diverse temperature and chemical gradients and specialized microbial communities, may indeed be a reservoir from which bacteria are released as crusts spread on the ocean floor or as fluid spouts at vent sites. Upon suspension into the water, spores could surf the slow, abyssal currents and rain down in the poles - where they are collected from the sediments by puzzled polar microbiologists.
It's another reminder that life on the smallest scales can be governed by some of the biggest processes - and that life goes on, even beneath the surface of the earth.
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The research discussed here was published recently in Science:
Hubert C et al. (2009) A Constant Flux of Diverse Thermophilic Bacteria into the Cold Arctic Seabed. Science 325(5947): 1541 - 1544
Bacteria in salt marshes:
Horner-Devine CM, Lage M, Hughes JB, and Bohannan BJM. (2004) A taxa-area relationship for bacteria. Nature 432, 750-753
For more on the "rare biosphere" and "deep subsurface biosphere" check out:
- Sogin, ML et al. (2006) Microbial diversity in the deep sea and the underexplored "rare biosphere" PNAS
- D'Hondt S. et al. (2004) Distributions of Microbial Activities in Deep Subseafloor Sediments. Science 306(5705):2216-2221
