The Algal World: Diatoms

haeckel-diatom

Diatom plate from Haeckel’s Art Forms in Nature

I was first attracted to diatoms by their exquisite beauty. When I studied aesthetics many years ago, beauty was often defined in terms of categories such as symmetry and form, and diatoms are definitely exemplars of both. They are one-celled algae, each encased in a glassy silica shell that varies with species. These structures can be elongated, triangular, circular, square, or more elaborately shaped. There is no better introduction to them than the diatom plate [shown above] from Ernst Haeckel’s Art Forms in Nature (1904). There are also great microscope photographs of diatoms on the web, at sites such as Micropolitan University. If you want more than just images, the Natural History Museum, London has Diatoms Online and the Academy of Natural Sciences (ANS) in Philadelphia (now part of Drexel University) has a Diatom Herbarium, both a real and a virtual space.

I visited the diatom collection at ANS two years ago and was drawn into a very different kind of herbarium world. Yes, there are metal cabinets, but they are filled with boxes of microscope slides, not sheets of white paper in folders. This collection was begun in the mid-19th century by members of the ANS who were interested in microscopy. At the time, this was, like seaweed, a hobby for many people who had the money to have leisure time and to buy a microscope. Some were physicians who had some familiarity with microscopes through their profession; others included bankers and industrialists who simply became fascinated with what couldn’t be seen with the naked eye. Like seaweed collecting, this was an area of interest in Britain, and also on the Continent, and had begun in the 17th century (Stafford, 1996). By the mid-19th century, microscope optics had improved and the instruments were easier to use. Many of the ANS microscopists were interested in fossilized diatoms found in diatomaceous earth, which could be found in areas around Philadelphia. This fine, sandy material is used in polishing among other things and represents the remains of organisms that lived in great numbers millions of years ago. Since diatoms are responsible for 20-25% of the earth’s carbon fixation, it’s difficult to overestimate their abundance, both now and in the past.

Eventually, the microscopists’ diatom collections morphed into the ANS Diatom Herbarium, which now houses the second largest such assemblage in the world. Along with slides, there are small glass bottles filled with diatomaceous earth collected in various locations. These are particularly difficult to catalog because each sample contains many species. In some cases, small portions of these sands have been separated out with individual species mounted on slides, but as Maria Popanova, the curator of the collection, notes the bottle that was the source of a particular mount wasn’t always recorded on the slide. There are ways of backtracking using dates and collection sites, but it’s time-consuming work and slows down digitization of the collection. However, 63,000 specimens are now available online. Also at ANS are rare 19th-century exsiccatae that contain many type specimens. These are store in book-like boxes with specimens either mounted on slides or in tiny envelopes. A counterpoint to these historically important items are posters on the walls of scanning electron microscope images of diatoms revealing an even more elaborate detail than that provided by a light microscope. The images are more expensive to produce so not every diatom receives this attention, but these images highlight the complexity of these minute structures.

I could easily dwell on the aesthetic aspects of these creatures, but I want to also stress their scientific significance. There are good reasons why the herbaria such as the ANS and NHS, among many others, maintain diatom collections. The cells can tell us a great deal about aquatic life of the past, the present, and the future. Diatoms serve as useful markers of aquatic ecosystem health. Their shells remain after death, providing stable evidence of water quality. A water sample’s use in monitoring usually deteriorates with time as organisms die, but this is a lesser problem with diatoms. Also, they are ubiquitous, found all over the world in both fresh and salt water. The species present at a site depend upon the presence or absence of pollution, among other factors.

Part of the research done at ANS involves water monitoring studies and having a rich diatom collection, including many type specimens, as reference adds weight to the findings. This work has a long history at the ANS, and the person most responsible for building its stature was Ruth Patrick (1907-2013). She had a doctorate based on diatom research from the University of Virginia and wanted to volunteer at the ANS in the 1930s. She was kept out for several years because they didn’t accept women. She finally became a volunteer in 1935, serving first as a virtual servant to the Microscopy Section, setting out specimens for their meetings among other duties. She eventually became the first woman member of the ANS. In the late 1940s, after she became a paid employee, Patrick founded the ANS Limnology Department. Through her work, the ANS developed a focus on freshwater diatoms; before that it had collected mostly fossils and saltwater species. She directed studies on rivers and streams, especially in terms of using diatoms to gauge water quality, and her influence lives on in the ANS’s Patrick Center for Environmental Research.

References

Haeckel, E. (1904). Art Forms in Nature (Vol. 1974 ed.). New York: Dover.

Stafford, B. M. (1996). Artful Science: Enlightenment Entertainment and the Eclipse of Visual Education. Cambridge, MA: The MIT Press.

Uses of Herbaria: Biogeography and Climate Change

screen-shot-2017-01-29-at-5-36-27-pm

iDigBio is an NSF-funded program to digitize US natural history collections

Alexander von Humboldt (1759-1769) is often considered the father of biogeography because of his crucial work on this subject in South America, and his writings and diagrams showing the link between terrain and vegetation (Humboldt & Bonpland, 2009). However, even during the Renaissance it was becoming obvious that terrain and climate greatly influence plant life. Many of the plants of northern Europe turned out to be different from the Mediterranean plants described by ancient authors including Theophrastus and Dioscorides. Place mattered and as botanists went on field trips and collected specimens, they become more aware of local differences in plant habitats. Now plant distribution is a major focus for botanists and ecologists, and herbaria are important in documenting what grows where. Species distribution maps are a staple of floras.

Today, with the combination of GPS coordinate mapping and digitization of specimen data, it’s possible to generate distribution maps relatively easily from online herbarium data. However, there is a question as to how accurate these maps are. As with any output, the answer depends upon the input: the accuracy of the localities. There is software such as GEOLocate and the MaNIS Calculator that will generate the probability of a plant being within a particular radius. Doing georeferencing well is time-consuming, and there are millions of sheets requiring attention. Even if all the data were optimal, there is the question of what percentage of the sheets are available online? If the species distribution maps are based on online data, and that only represents, say, a quarter of the specimens, then how accurate is it? In addition, there is collection bias. In other words, rarely are regions sampled uniformly. Studies show that areas accessible via roads, rivers or railroads are more likely to be thoroughly canvased than are more remote locales, so all the error can’t be blamed on computers (Vetter, 2016). Some of the best studies are those that combine herbarium data with direct observation in an area (Martin et al., 2014; Mohandass & Campbell, 2015).

At the present there can’t be any discussion of biogeography without bringing up climate change. At the moment, interest in this topic is driving not only research on herbarium collections but also their digitization. Herbaria are among the few places where there are records that range over more than 100 years, in some cases 200-400 years, and they become more valuable by the day. Records can be studied in a variety of ways to glean information on climate and habitat characteristics in the past. The area that has been developed most substantially is phenology, the study of natural events that can be pinpointed in time. For plants this is often budding, flowering, or seed setting. Though this has not always been true in the past, it is now standard herbarium practice to collect specimens that are in flower or have seeds or fruit. Since collection data are always recorded for specimens, researchers can track flowering or fruiting over the years to detect differences that might be related to climate change. Again, if digitized and imaged specimens are used, the study is limited by the richness of the sample, but it does make for more efficient investigation. There have been several studies validating the methods used in this research (Davis et al., 2015; Spellman et al., 2016). To date, changes in flowering times have been noted in orchids (Molnar et al., 2012), eucalypts (Rawal et al., 2014), and perennial herbs (Matthews & Mazer, 2015) among others; changes were also recorded in leaf-out times for trees (Everill et al., 2014; Zohner & Renner, 2014). With projects like NSF’s iDigBio, which has resulted in a portal with access to over 75 million natural history specimens, the problems of small sample sizes are dwindling, but still significant.

Still other ways to document climate change with specimens include counting the stomata on leaves. This is obviously more time-consuming, but the number of studies in this area suggest that it’s considered a fertile area of investigation. Stomata are the pores in a leaf through which plants absorb CO2, the fuel for photosynthesis. Since global warming is precipitated by the increase in atmospheric CO2 due to fossil fuel combustion, it’s not surprising that in several species, there has been a decrease in stomatal density. Plants just don’t need as many pores to absorb the same amount of CO2. In addition, leaves can give evidence of other types of environmental change. An alteration in leaf area over the past three centuries has been noted for some species (Peñuelas & Matamala, 1990). Also, there was a marked decrease in leaf sulfur levels in the years after the clean air act was passed and an increase in nitrogen with increased use of fertilizers (Peñuelas & Filella, 2001). Someone has even studied the distance over time between leaf teeth in the mulberry Hedycarya angustifolia over the past 160 years to see if an increase in temperature had affected this trait. However, despite the painstaking analysis, researchers couldn’t find any significant change (Scarr & Cocking, 2014). This work shows how many ways climate change can affect plants, and how clever botanists and ecologists employ varied plant characteristics in collecting data on this phenomenon. Having different types of data strengthens the case for the significant impact of climate on plant growth and also on the survival of species. I’ll present more evidence for this in the next post.

References

Davis, C. C., Willis, C. G., Connolly, B., Kelly, C., & Ellison, A. M. (2015). Herbarium records are reliable sources of phenological change driven by climate and provide novel insights into species’ phenological cueing mechanisms. American Journal of Botany, 102(10), 1599–1609.

Everill, P. H., Primack, R. B., Ellwood, E. R., & Melaas, E. K. (2014). Determining past leaf-out times of New England’s deciduous forests from herbarium specimens. American Journal of Botany, 101(8), 1293–1300.

Humboldt, A. von, & Bonpland, A. (2009). Essay on the Geography of Plants. (S. T. Jackson, Ed., S. Romanowski, Trans.). Chicago, IL: University of Chicago Press.

Martin, M. D., Zimmer, E. A., Olsen, M. T., Foote, A. D., Gilbert, M. T. P., & Brush, G. S. (2014). Herbarium specimens reveal a historical shift in phylogeographic structure of common ragweed during native range disturbance. Molecular Ecology, 23(7), 1701-1716.

Matthews, E. R., & Mazer, S. J. (2015). Historical changes in flowering phenology are governed by temperature × precipitation interactions in a widespread perennial herb in western North America. The New Phytologist, 210(1), 157-167.

Mohandass, D., & Campbell, M. J. (2015). Assessment of Roscoea population size in the Central Himalayas based on historical herbarium records and direct observation for the period 1913-2011. Journal of Biological Records, 0022015, 10–16.

Molnár, A., Tökölyi, J., Végvári, Z., Sramkó, G., Sulyok, J., Barta, Z., & Bronstein, J. (2012). Pollination mode predicts phenological response to climate change in terrestrial orchids: A case study from central Europe. Journal of Ecology, 100(5), 1141–1152.

Peñuelas, J., & Filella, I. (2001). Herbaria century record of increasing eutrophication in Spanish terrestrial ecosystems. Global Change Biology, 7(4), 427–433.

Peñuelas, J., & Matamala, R. (1990). Changes in N and S leaf content, stomatal density and specific leaf area of 14 plant species during the last three centuries of CO2 increase. Journal of Experimental Botany, 41(9), 1119–1124.

Rawal, D. S., Kasel, S., Keatley, M. R., & Nitschke, C. R. (2014). Herbarium records identify sensitivity of flowering phenology of eucalypts to climate: Implications for species response to climate change. Austral Ecology, 40(2), 117-125.

Scarr, M. J., & Cocking, J. (2014). Historical responses of distance between leaf teeth in the cool temperate rainforest tree Austral Mulberry “Hedycarya angustifolia” A. Cunn. from Victorian herbarium specimens. Victorian Naturalist, 131(2), 36–39.

Spellman, K. V., & Mulder, C. P. H. (2016). Validating herbarium-based phenology models using citizen-science data. BioScience, 66(10), 897–906.

Vetter, J. (2016). Field Life: Science in the American West during the Railroad Era. Pittsburgh, PA: University of Pittsburgh Press.

Zohner, C. M., & Renner, S. S. (2014). Common garden comparison of the leaf-out phenology of woody species from different native climates, combined with herbarium records, forecasts long-term change. Ecology Letters, 17(8), 1016-1025.