The aim of the present work was to present the ecological and morphological characteristics of species from the genus Achnanthes Bory sensu stricto, which develops in terrestrial mosses near the Rio de Janeiro Botanic Garden, Brazil. A literature comparison was made with other similar species, including the LM and SEM analysis of original material bearing Achnanthes inflata (Kützing) Grunow housed at the Grunow Collection in Vienna, and data from the available literature. Samples were collected from clumps of moss growing on tree trunks, and from a concrete wall within the botanic garden. Four taxa from the genus Achnanthes were recorded: A. coarctata, A. inflata var. gibba, A. mauiensis and Achnanthes pseudoinflata sp. nov. The main morphological differences between these taxa were cell dimensions (length and width), striae and areolae density. The most common diatom species found in these samples were Humidophila sp. (90%), Humidophila contenta (74.8%), Luticola moreirae (17.9%), and Achnanthes pseudoinflata sp. nov. (7.4%).

The samples for this study were collected from terrestrial mosses and lichens growing on palm tree trunks and concrete walls in the city of Rio de Janeiro, south-eastern Brazil. During the investigation on diatom diversity, a new species from the genus Luticola was found. The new species occurred individually at all sampling sites. The aim of this paper is to provide a morphological and ecological description of Luticola minutissima sp. nov. from an aerophytic environment. The species is characterized by a small valve size (5.2–16.8 μm length and 3.7–4.4 wide) and abruptly hooked proximal raphe endings (ca. 90-degree angle). Additionally for comparison, type material of the most similar species, Luticola neglecta Zidarova, Levkov & Van de Vijver, was studied and new information for the ultrastructure of the latter species is provided as well.