Notes on the genus Humidophila R.L. Lowe et al. from the USA

Interest in the aerophilic diatom genus Humidophila R.L. Lowe et al. remains high, with descriptions of new species and expanded biogeographical distributions. We evaluate the taxonomy of Humidophila species and explore aspects related to ecology and habitat preferences. After examination of material from various U.S. locations, especially wet wall environments, we provide a taxonomic update and discussion on ultrastructure supporting survival in environments with low water availability. We document and discuss the morphological variability of H. perpusilla (Grunow) R.L. Lowe et al., particularly across the external surface of valve face, and update the size range for U.S. populations. An additional survey of samples from running waters across several states in the United States revealed minimal to no occurrences of Humidophila, in further support of the aerial and subaerial habitat preference of this genus. We formally transfer Navicula keeleyi R.M. Patrick to Humidophila keeleyi (R.M. Patrick) Furey, Manoylov, & R.L. Lowe comb. nov. and place Diadesmis fukushimae Lange-Bert., Werum & Broszinski in synonymy with H. keeleyi.