Diatom researchers often ask questions like: “What diatoms are here?,” “why do these diatoms live here?,” and “what role do these diatoms play in the ecosystem?” We usually answer these questions by going out to a stream, lake, or pond; collecting a biofilm, water, or sediment sample; and then identifying the diatoms through a microscope or DNA sequencing. Sometimes, we ask “How did these diatoms get here?” And we usually answer this question by inferring diatom dispersal between waterbody A and waterbody B with mathematical models of diatom community similarity and geographic location – not by sampling the air or feathers of birds.
In an article recently released (Schulte et al. in press), my colleagues and I report on catching diatoms in the act of dispersing to answer how they are dispersed, to what extent they are dispersed, and what that dispersal means for all the diatoms in the area. Why? To learn more about the different processes that influence diatom diversity. To do this, I traveled to Antarctica and listened to a lot of Bob Dylan for inspiration.
Bob Dylan (born Robert Allen Zimmerman), is an American singer-songwriter (Figure 1) whose celebrated works include “Blowin’ in the Wind” and “The Times They Are…” Oh, this is a diatom blog? Okay, okay…
Figure 1. Bob Dylan, blowing diatoms into a passive wind sampler.
Not too long ago, the prevailing idea was that something as small as a diatom could be dispersed across the entire world via wind or hitchhiking on a bird. Whether a diatom would settle where it landed had to do with its chances of survival and of thriving in its new environment. This is the common interpretation of Dutch microbiologist Baas Becking’s hypothesis “Everything is everywhere, but the environment selects” (Baas Becking 1934). While this seems to be true for some microorganisms (Finlay 2002), recent studies show that the composition of many diatom metacommunities (communities of diatoms connected via dispersal across landscapes / waterscapes) can only be explained by considering environmental conditions alongside variation in dispersal capability (Vyverman et al. 2007).
To estimate the extent to which diatoms are dispersed by wind, my colleagues and I visited the largest ice-free region of Antarctica – the McMurdo Dry Valleys. The Dry Valleys is a network of glaciers, seasonally active meltwater streams, perennially ice-covered lakes and ponds, and soils – most of which are hydrologically disconnected from each other. Here, winds are strongest in the winter and move sediment and other material around across the landscape. Benthic diatoms are the most diverse eukaryotic organisms, and the largest creatures are mosses, tardigrades (water bears or – even better – moss piglets), nematodes, and the occasional lost seal.
Diatom diversity in the Dry Valleys is strongly associated with environmental conditions like streamflow intermittency, nutrient availability, and water temperature (Stanish et al. 2012, Kohler et al. 2016). However, simulations on the stream diatom metacommunity in the Dry Valleys suggest that the observed metacommunity composition needs to take into account both diatoms floating downstream and flying from place to place (Sokol et al. 2020). Therefore, dispersal across hydrologically disconnected waterbodies must be happening – but is it really by wind, and is it all that important?
Yes and maybe, according to our work. We set up 5 passive wind samplers across the Dry Valleys that collected anything <2 mm diameter that was blowin’ in the wind at 50 cm off the ground during the winter and summer. We did this in 2013-14 and again in 2018-19. We then looked for diatoms under the microscope and with high-throughput DNA and RNA sequencing.
And… there were diatoms! A heckuva lot of them, all things considered. More during the winter, when the winds are strongest, than the summer. Were any alive, you ask? We didn’t try culturing any of them, but we did look for intact chloroplasts under the ‘scope and for RNA (which degrades quickly in the environment and can be used to indicate recently active individuals). And, based on this, we found that, up to 15% of intact cells, could be viable (about half of what we see in living stream communities). This means that up to 34,000 ‘living’ diatoms could be delivered via wind to a single 2 cm2 in a year (!). When you scale that up to the entire 4,800 km2 area of the Dry Valleys, it’s practically raining down diatoms (diatoms being the ‘rain,’ of course, since it doesn’t rain water in the Dry Valleys… yet) (Figure 2).
Figure 2. Wait, that’s not a Bob Dylan song… How about Rainy Day Diatoms #12 & 35?
Okay, diatoms are blowin’ in the wind down in Antarctica. So what? We then looked to see if there were differences in which diatoms were being blown around. First, we found that 36 of the 50 diatoms known in the region were present in aeolian material. Cool – dispersal could be important for a majority of the species in the Dry Valleys! Second, we found that species composition at sites close to each other was more similar, and that the collectors atop glaciers were more different from those on the ground. This indicates that the bulk of near-ground aeolian diatom dispersal happens at a pretty small spatial scale and that some dispersal limitation is possible. After all, if there were no limitations on dispersal like most macro-organisms, we would expect each wind sample to look basically the same.
Finally, we wanted to know what all of this diatom dispersal meant for the regional metacommunity. So we compared the composition of aeolian diatom communities with that of streams, lakes, ponds, and glaciers across the Dry Valleys. And, overall, we found that aeolian samples were more similar to each other than to any water body. These results suggest that other processes – like different local environmental conditions across water bodies – affect diatom diversity in addition to variable wind dispersal.
Okay, whatever, diatoms are blowin’ in the wind. What did we actually learn from this? Well, one, diatoms are blowin’ in the wind. Wind dispersal of diatoms has long been assumed, but this is one of the first studies to comprehensively confirm it happens! Two, there appear to be some barriers to aeolian dispersal of diatoms in the Dry Valleys, providing further evidence that everything is not everywhere when it comes to diatoms. And, three, wind dispersal doesn’t act alone in shaping the diversity of diatoms across a landscape. Altogether, “the answer, my friend, is blowin’ in the wind” … in part.
Nick Schulte is a Postdoctoral Fellow at University of Colorado Boulder and a Research Scientist at Jonah Ventures in Boulder, Colorado. He is a co-editor of the Diatom of the Month blog. You can email Nick at Nicholas.Schulte[at]colorado.edu or leave a message in the comments section if you have any question about the blog post.
- Baas Becking, L.G.M. 1934. Geobiologie of inleiding tot de milieukunde. The Hague, the Netherlands: W.P. Van Stockum & Zoon (in Dutch).
- Finlay, B. J. 2002. Global dispersal of free-living microbial eukaryote species. Science 296:1061-63.
- Kohler, T. J., Van Horn, D. J., Darling, J. P., Takacs-Vesbach, C. D. & McKnight, D. M. 2016. Nutrient treatments alter microbial mat colonization in two glacial meltwater streams from the McMurdo Dry Valleys, Antarctica. FEMS Microbiol. Ecol. 92:fiw049.
- Schulte, N. O., Khan, A. L., Smith, E. W., Zoumplis, A., Kaul, D., Allen, A. E., Adams, B.J., & McKnight, D. M. In press. Blowin’in the wind: Dispersal, structure, and metacommunity dynamics of aeolian diatoms in the McMurdo Sound region, Antarctica. Journal of Phycology. https://doi.org/10.1111/jpy.13223
- Sokol, E. R., Barrett, J. E., Kohler, T. J., McKnight, D. M., Salvatore, M. R. & Stanish, L. F. 2020. Evaluating Alternative Metacommunity Hypotheses for Diatoms in the McMurdo Dry Valleys Using Simulations and Remote Sensing Data. Front. Ecol. Evol. 8:295.
- Stanish, L. F., Kohler, T. J., Esposito, R. M., Simmons, B. L., Nielsen, U. N., Wall, D. H., Nemergut, D. R. & McKnight, D. M. 2012. Extreme streams: flow intermittency as a control on diatom communities in meltwater streams in the McMurdo Dry Valleys, Antarctica. Can. J. Fish. Aquat. Sci. 69:1405-19.
- Vyverman, W., Verleyen, E., Sabbe, K., Vanhoutte, K., Sterken, M., Hodgson, D. A., Mann, D. G., Juggins, S., Van de Vijver, B., Jones, V., Flower, R., Roberts, D., Chepurnov, V. A., Kilroy, C., Vanormelingen, P. & De Wever, A. 2007. Historical processes constrain patterns in global diatom diversity. Ecology 88:1924-31.