Antarctic wind-driven bacterial dispersal paper published

Frost flowers over young sea ice in the central Arctic Ocean. Photo Mattias Wietz.

Frost flowers over young sea ice in the central Arctic Ocean. Photo Mattias Wietz.

I’m happy to report that one of the appendices in my dissertation was just published in the journal Polar Biology.  The paper, titled Wind-driven distribution of bacteria in coastal Antarctica: evidence from the Ross Sea region, was a long time in coming.  I conceived of the idea back in 2010 when it looked like my dissertation would focus on the microbial ecology of frost flowers; delicate, highly saline, and microbially enriched structures found on the surface of newly formed sea ice.  Because marine bacteria are concentrated in frost flowers we wondered whether they might serve as source points for microbial dispersal.  This isn’t as far-fetched as it might seem; bacteria are injected into the atmosphere through a variety of physical processes, from wind lofting to bubble-bursting, and frost flowers have been implicated as the source of wind deposited sea salts in glaciers far from the coast.

At the time we’d been struggling to reliably sample frost flowers at our field site near Barrow, Alaska.  Frost flowers form readily there throughout the winter, but extremely difficult sea ice conditions make it hard to access the formation sites.  We knew that there were more accessible formation sites in the coastal Antarctic, so we initiated a one year pilot project to sample frost flowers from McMurdo Sound.  By comparing the bacterial communities in frost flowers, seawater, sea ice, terrestrial snow, and glaciers, we hoped to show that frost flowers were a plausible source of marine bacteria and marine genetic material to the terrestrial environment.  Because the coastal Antarctic contains many relic marine environments, such as the lakes of the Dry Valleys, the wind-driven transport of bacteria from frost flowers and other marine sources could be important for continued connectivity between relic and extant marine environments.

Frost flowers are readily accessible in McMurdo Sound throughout the winter, however, this does not mean that one can simply head out and sample them.  While the ice conditions are far more permissible than at Barrow, Alaska, the bureaucracy is also far more formidable.  The can-do attitude of our Inupiat guides in Barrow (who perceive every far-out field plan as a personal challenge) was replaced with the inevitable can’t-do attitude at McMurdo (this was 2011, under the Raytheon Antarctic Support Contract, and does not reflect on the current Lockheed Antarctic Support Contract, not to suggest that this attitude doesn’t persist).  Arriving in late August we were initially informed that our plan was much to risky without helicopter support, and that nothing could be done until mid-October when the helicopters began flying (we were scheduled to depart late October).  Pushing for a field plan that relied on ground transport ensnared us in various catch-22’s, such as (paraphrased from an actual conversation):

ASC representative: You can’t take a tracked vehicle to the ice edge, they’re too slow.

Me: Can we take a snowmobile to the ice edge?  That would be faster.  We do long mid-winter trips in the Arctic and it works out fine.

ASC representative: No, because you have to wear a helmet, and the helmets give you frostbite.  So you can only use a snowmobile when it’s warm out.

Ultimately we did access the ice edge by vehicle several times before the helicopters started flying, but the samples reported in this publication all came from a furious two week period in late October.  What we found really surprised us.

Sampling frost flowers is as easy as scraping them from the ice surface with a clean shovel into a sterile plastic bin.

Sampling frost flowers on a warm day in September, 2011, in McMurdo Sound.

Pull very short ice cores from young sea ice after sampling frost flowers. You can see one of the short cores in the lower right of the image.

Sampling some of the frost flowers used in this study over much thinner ice north of Ross Island in October, 2011.

Rotor wash on a smooth ice surface leads can lead to lots of scattered gear. On any tenous surface the helicopter unloads us "hot", without shutting down, meaning that the first few minutes at a sampling site are a loud, confusing scramble to get everything unloaded and protected before the helicopter takes off. Here we sort ourselves out after the helicopter departs. Photo: Shelly Carpenter.

Sampling “blue ice” on Taylor Glacier a few days earlier.  Comparing various terrestrial ice environments with the marine samples gave us some interesting insights on how bacteria are dispersed by strong winter winds in the coastal Antarctic.

There is ample evidence for the wind-driven transport of bacteria in this region but, contrary to our hypothesis, most of that material is coming from the terrestrial environment.  The major transportees were a freshwater cyanobacterium from the genus Pseudanabaena and a set of sulfur-oxidizing Gammaproteobacteria (GSO).  The cyanobacterium was pretty easy to understand; it forms mats in a number of freshwater lakes and meltponds in the region.  In the winter these freeze, and since snow cover is low, ice and microbial mats are ablated by strong winter winds.  Little pieces of mat are efficiently scattered all over, including onto the sea ice surface.

Microbial mat in Lake Chad, 2005 (photo by A. Chiuchiolo), taken from http://www.montana.edu/priscu/antarcticimages/dry_valley_lakes.html.

Easily dispersed: desiccated microbial mat in Lake Chad, 2005
(photo by A. Chiuchiolo), taken from http://www.montana.edu/priscu/antarcticimages/dry_valley_lakes.html.

The GSO threw us for more of a loop; the most parsimonious explanation for their occurrence in frost flowers is that they came from hydrothermal features on nearby Mt. Erebus.  We did some nice analysis with wind vectors in the region and while you don’t get a lot of wind (integrated over time) to move material from Mt. Erebus to our sample sites, you do get some occasional very strong storms.

Wind velocity and magnitude for the study region in October, 2011. Taken from Bowman and Deming, 2016.

Wind velocity and magnitude for the study region in October, 2011. Taken from Bowman and Deming, 2016.

What all this means is that, consistent with other recent findings, there is high regional dispersal of microbes around the coastal Antarctic.  While I’m sure there are some endemic microbes occupying particularly unique niches, in general I expect microbes found in one part of the coastal Antarctic to be present in a similar environment in a different part of the coastal Antarctica.  There are however, quite a few ways to interpret this.  Bacteria and Archaea can evolve very fast, so the genome of a clonal population of (for example) wind deposited Pseudanabaena newly colonizing a melt pond can diverge pretty fast from the genome of the parent population.  This has a couple of implications.  First it means that the coastal Antarctic, with all it’s complex topography yet high degree of microbial connectivity, is an excellent place to explore the dynamics of microbial adaptation and evolution, particularly if we can put constraints on the colonization timeline for a given site (non trivial).  Second, it raises some questions about the propriety of commercially relevant microbes obtained from the continent.  The commercialization of the continent is probably inevitable (I hope it is not), perhaps the potential ubiquity of Antarctic microbes will provide some defense against the monopolization of useful strains, enzyme, and genes.

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