We just published the first analysis of a microbial community inhabiting natural frost flowers in the journal Environmental Microbiology Reports. Our results are a bit surprising, and I’ll get to them shortly. First, a brief look at what we did. Frost flowers have received quite a bit of media attention lately. If you missed the coverage frost flowers are delicate crystal structures that are nearly ubiquitous to the surface of newly formed sea ice (so long as the ice forms under a relatively cold atmosphere). Salt, organic material, and bacteria from the source seawater are concentrated in frost flowers during the process of sea ice growth and maturation.
Although chemical and biological reactions are typically suppressed by low temperatures, the high concentrations of all these organic and inorganic materials, and the presence of ample energy (from sunlight), means that some ecologically interesting things might be happening here. For example check out this paper by a group of atmospheric chemists connected with the OASiS (Ocean-Atmosphere-Sea Ice-Snow) project. Among other things they report a high concentration of formaldehyde in frost flowers, probably the result of the photolysis (sun-driven breakdown) of larger organic molecules.
Our group is interested in what biological processes might be happening in frost flowers. To develop testable hypothesis however, we needed to make some initial observations about the system. The most basic observation that a microbiologist typically wants to make is an assessment of community composition; what bacteria are present and their abundance relative to one another. To do this we relied on an analysis of the 16S rRNA gene, a commonly used taxonomic marker gene for prokaryotes. By comparing the 16S rRNA gene sequences in frost flowers with those of known bacteria in a database, we can approximate the composition of the frost flower microbial community.
The small lead from which frost flowers and young sea ice were sampled in April, 2010. Not the prettiest picture of frost flowers around, but representative of 90 % of the days in Barrow.
What we found really surprised us, in fact it took several months to wrap our heads around the results. The heatmap below shows the relative abundance of bacterial genera in our analysis, as determined by one of the four 16S rRNA gene identification methods we used. Black indicates bacterial genera below detection, the remainder are scaled from white to blue to red (most abundant). The columns are our samples; there are four frost flower (FF) and four young sea ice (YI) samples. Several microbial genera, in particular the Methylobacteria, Rhizobium, and Mesorhizobium, are enriched within frost flowers relative to the underlying young sea ice. These genera are all members of the order Rhizobiales, thus we posit that that Rhizobiales are, in general, enriched in these frost flowers.
Relative abundance of genera by microarray analyis of the 16S rRNA gene (Bowman et al. 2013). Genera belonging to the order Rhizobiales tend to be enriched in frost flowers relative to young sea ice.
This is pretty weird. The Rhizobiales are certainly not unheard of in marine waters, but are virtually unreported in sea ice and we can’t find any report of them dominating a marine environment. And they do truly dominate these frost flowers, in a more in-depth analysis of a single frost flower sample we found that 77 % of the 16S rRNA genes classified as Rhizobiales. The environment where Rhizobiales do typically dominate is (ironically) on the roots of real flowers, or at least on legumes. There they engage in a mutualistic relationship with the plant, providing fixed nitrogen in exchange for carbohydrates.
This fact might be a clue as to how Rhizobiales could come to dominate the sea ice surface. Although there isn’t a lot of evidence for algae (aka phytoplankton trapped in the ice) in these frost flowers, there is for the underlying young ice. Rhizobiales can be found in close associations with phytoplankton, just as they are with plants, so perhaps the Rhizobiales end up in ice because so many phytoplankton do. Once in the ice, the now stressed phytoplankton and bacteria might end their relationship (enough stress will test even the closest of relationships), leaving the Rhizobiales free to transport to the ice surface during brine rejection (a key element of sea ice growth).
Of course this is just a hypothesis, but hypothesis development was our goal with this study. We are left in the best possible situation, with an interesting observation and just enough data to develop further questions (okay, the best possible situation would be a definitive answer and a paper in Nature, but still…). To refine those questions we are continuing to work with these samples, in particular with metagenomes obtained from one of our frost flower and one of our young ice samples.
