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.

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Diatoms in the Herbarium

The diatom, Lyrella sp., photo from Fish and Wildlife Research Institute

The diatom, Lyrella sp., photo from Fish and Wildlife Research Institute

Diatoms are microscopic aquatic organisms that have glass-like shells.  These structures are symmetrical, and this symmetry combined with the transparency of the “glass” and the variety in their shapes makes them aesthetically awesome.  Their diversity and ubiquity in both fresh and saltwater environments also make them very important ecologically.  A few months ago, I visited the Diatom Herbarium at the Academy of Natural Sciences in Philadelphia, now part of Drexel University. Maria Potapova, the assistant curator in charge of the herbarium, gave me a tour and showed me some of the hundreds of thousands of specimens in the collection.  The usual metal herbarium cabinets are filled with box after box of glass microscope slides. In addition, there are vials and small bottles containing with the sandy remains of diatoms. Potapova explained why it is such a slow process to digitize the information about the specimens. A single vial may hold a number of diatom species, and individual species from these vials may have been mounted on a number of different slides. However, the links between slides and vials may not have been noted. Making these connections is possible through dates, collectors, etc., but it takes time.

Potapova also showed me several rare exsiccatae that contain many type specimens. The oldest, that of the German botanist Friedrich Traugott Kützing—Algarum Aquae Dulcis Germanicarum (1833-1836)—is in 16 notebooks with specimens wrapped in tiny paper envelopes or on glass. Another is the set of 22 small leather-bound book-like boxes filled with slides. The title on each spine is Diatomées de Belgique (1882-1885), and the collection was assembled by the Belgian botanist Henri Van Heurck. These are among the treasures of the collection, which includes specimens gathered in the 1800s by members of the ANS, many of whom were wealthy men who had microscopy as a hobby. The origins of the collection was in the Microscopical Society of Philadelphia, founded 1858. The first evidence of interest in diatoms at the ANS were three papers on marine and freshwater diatoms published by F.W. Lewis in 1860s. The Microscopical Society eventually merged with the Biological Section of the ANS to form the Biological and Microscopical Section.

Women could not become ANS members until well into the 20th century. In fact, Ruth Patrick (1907-2013), who had a doctorate based on diatom research at the University of Virginia, wanted to volunteer at the ANS but was kept out for several years. She finally become a volunteer in 1935, serving first as a virtual servant to the 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 had become 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 of rivers and streams, especially in terms of using diatoms to gauge water quality. She collaborated with the noted ecologist Evelyn Hutchinson among others. Her influence lives on in the ANS’s Patrick Center for Environmental Research. Potapova and her associates continue to do both taxonomic and ecological studies on diatoms.  The importance of this work became evident when Potapova mentioned that diatoms are responsible for 20-25% of the earth’s organic carbon fixation.