Last autumn I went on vacation with my brother to sunny Italy. Along with marvelling at St. Peter’s Basilica and eating great food, I did what any other keen museum volunteer would do: collect Italian freshwater diatoms for the museum’s collection.
Clockwise from top-left: St Peter’s Basilica, Vatican City; gondolas in Venice; Pompeii with looming Mt. Vesuvius; Valley of the Temples, Agrigento, Sicily; citizen scientist Joe Holmes beside the Silvestri Crater, Mt Etna, Sicily; Isle of Capri and Tyrrhenian Sea. Image: Joe Holmes © Canadian Museum of Nature.
Diatoms are fascinating, microscopic, one-celled algae, with a thin silica shell, measuring just five to 150 microns, or millionths of a meter.
Rome’s Tiber River looking toward the Sisto Bridge and St Peter’s Basilica. Two samples were taken here. The collected diatoms indicate alkaline pH, some brackishness, and moderate levels of organic nutrients. Left: Tryblionella constricta (36 µm x 6 µm). Second: Ulnaria acus (68 µm x 5 µm). Third: Frustulia vulgaris (44 µm x 10 µm). Right: Gyrosigma acuminatum (145 µm x 17 µm). Image: Joe Holmes © Canadian Museum of Nature.
These free-floating phytoplankton are the foundation of the aquatic food web, converting sunshine into stored chemical energy, and carbon dioxide into oxygen. Diatoms are found in all fresh and marine waters. Biologists use them for water quality analysis and climate change research.
Florence’s Arno River Dam where a sample was taken at lower left. Top: Aulacoseira granulata (40 µm x 6 µm). Bottom-left: Gomphonema pala (23 µm x 10 µm). Middle: Diatoma vulgare (53 µm x 13 µm). Right: Encyonema prostratum (44 µm x 18 µm). Image: Joe Holmes © Canadian Museum of Nature.
Whenever I had free time during the trip, I collected various types of diatom samples, amassing a total of 41 from lakes, rivers, fountains and ponds across Italy, including Rome, Florence, Venice, and Sicily.
An irrigation canal near Pomposa on the Po River Delta. The diatoms were sampled using a mud retriever. They reveal the presence of higher levels of organic nutrients, likely from agricultural runoff. Left: Bacillaria paxillifera (55 µm x 4 µm). Second: Gomphonema insigne (57 µm x 11 µm). Top: Tryblionella levidensis (30 µm x 15 µm). Bottom: Cyclotella meneghiniana (12 µm x 12 µm). Image: Joe Holmes, © Canadian Museum of Nature.
Back at the lab in the museum’s collections facility, samples were processed, and specimens photographed and identified using an online Italian government diatom guide. Interestingly, the diatoms reveal a lot about Italian water quality and terrain at the sampled sites.
The Aquarium pond in the Orto Botanico di Palermo, Palermo, Sicily. Left: Navicula radiosa (70 µm x 10 µm). Top: Pseudostaurosira brevistriata (30 µm x 15 µm). Middle: Cocconeis placentula (22 µm x 14 µm). Bottom: Rhoicosphenia abbreviata (32 µm x 9 µm). Image: Joe Holmes © Canadian Museum of Nature.
For example, most diatoms at sample sites indicate a preference for alkaline pH reflective of the limestone, volcanic and carbonate rocks that make up most of the Italian Peninsula. Some species are indicators of water with moderate to high levels of organic nutrients, likely from nearby farms. Lastly, some specimens indicate traces of brackish water, perhaps due to salt in the Italian soil or proximity to the Mediterranean Sea.
Girgenti River Estuary, San Leone, Sicily. This is tidal water, with the Mediterranean Sea at left, so all the collected diatoms are marine species. Top: Achnanthes brevipes (27 µm x 8 µm) Second: Navicula ramosissima (43 µm x 7 µm) Third: Navicula longa var irregularis (48 µm x 9 µm) Bottom: Synedra fasciculata (31 µm x 6 µm). Image: Joe Holmes © Canadian Museum of Nature.
The trip yielded many of the same species of diatoms I’ve collected in Canada, Ireland and the Middle East.
Torrente Santa Venera, Giardini Naxos, Sicily, looking southeast toward the Ionian Sea. Top: Nitzschia sigma (61 µm x 7 µm). Second: Nitzschia communis (29 µm x 4 µm). Third: Nitzschia recta (69 µm x 7 µm). Bottom: Navicula apiculata (34 µm x 9 µm). Image: Joe Holmes © Canadian Museum of Nature.
For museum diatom researchers and students these new Italian specimens are proof that some scientific souvenirs can provide a lot more than just good memories!
Vigna di Valle, Bracciano Lake, the source of Rome’s water. The collected diatoms indicate that the water is relatively clean, with low nutrient levels. Top: Aneumastus tuscula (32 µm x 12 µm). Bottom-Left: Cocconeis neodiminuta (21 µm x 13 µm). Middle: Raphoneis surirella (30 µm x 8 µm). Right: Cavinula pseudoscutiformis (12 µm x 9 µm). Image: Joe Holmes © Canadian Museum of Nature.