Diatoms as Bark Epiphytes in the Tropical Lowlands of Panama

Diatoms are of major importance in marine and freshwater systems, but their occurrence in terrestrial situations is generally thought to be exceptional. Following up on the accidental discovery of epiphytic diatoms on bark samples in an unrelated study, we investigated their presence in the tropical lowlands of Panama more systematically using scanning electron and light microscopy. We sampled inundated and aerial bark portions of Annona glabra, a tree that grows along the shore of Lake Gatun, and took bark samples from other tree and liana species at c. 1.5 m height in the forest understory. In total, we found 70 diatom taxa in 28 genera. Species numbers and composition differed among the three microhabitats with the largest numbers on inundated bark portions, but even in the forest understory, we found 12 taxa with densities of up to 900 frustules per mm−2 of bark. Our data set is still quite limited in scale but the results suggest the possibility that hitherto unacknowledged assemblages of epiphytic diatoms may be quite common in wet tropical forests, which clearly warrants further study.