We sampled pond microbial mats from the McMurdo Dry Valleys and Ross Island, Antarctica, using a brass cork borer. Samples were preserved in ~5% formalin (final concentration), and shipped to the University of Colorado for analysis. Samples were digested using heat and hydrogen peroxide (H₂O₂) and rinsed several times with MilliQ water until a neutral pH was achieved. Digested material was dried onto cover slips and permanently mounted on glass microscope slides with Zrax® (W. P. Dailey, University of Pennsylvania) mounting medium and analyzed by light microscope at 1250x magnification. We also analyzed 14 of the original West & West (1911) microscope slides prepared from material collected during Shackelton’s Nimrod expedition, and are stored at The Natural History Museum, London. Because of the preparation method used in making the historical slides (some had thick coverslips), several of these historical slides were counted at 650 or 850x (eg. Green Lake, Lake West side of McMurdo). For both modern and historical material, at least 300 valves were counted from each slide, and relative abundance of species were calculated (slides were also scanned afterward for rare species). Scanning electron microscopy (SEM) images were made from some contemporary samples at the Charles University in Prague after being air-dried onto aluminum stubs, which were then sputter-coated with 50 nm of Au and photographed on a JEOL JSM-6380LV at 25 kV. Diatom taxonomic identifications were performed according to the species list in Sakaeva et al. (2016), the Antarctic Freshwater Diatoms database, along with the original West and West (1911) publication.

Sakaeva, A., Sokol, E.R., Kohler, T.J., Stanish, L.F., Spaulding, S.A., Howkins, A., Welch, K.A., Lyons, W.B., Barrett, J.E. and McKnight, D.M. 2016. Evidence for dispersal and habitat controls on pond diatom communities from the McMurdo Sound Region of Antarctica. Polar Biology 39(12): 2441-2456. doi: 10.1007/s00300-016-1901-6

West, W. and West, G.S. 1911. Freshwater algae. British Antarctic Expedition (1907–1909) Science Report, Biology 1: 263-298.