Diatoms are good indicators of water quality because of their great diversity and wide ecological range. They are also interesting in assessing the impact of radioactivity on ecosystems because of their capacity to absorb radioelements. They also show radio-induced deformities, observed in many species. In order to assess more precisely the impact of natural radioactivity, diatom communities were monitored during a 9-month survey in two mineral springs characterized by radioactivity levels ranging within two orders of magnitude. The experimental data obtained were analyzed using different multivariate and clustering analyses. The richness was high with the dominance of Planothidium frequentissimum and Crenotia angustior. In the most radioactive spring known in Auvergne (above 4000 Bq L⁻¹ radon activity), the average deformation was 24.7%, in contrast with lower teratological rates observed in the less radioactive spring. In the absence of heavy metals, this result confirmed that radioactivity is an environmental stress for the diatoms living in these particular ecosystems. This conclusion was reinforced by the selection of springs from a database of 126 ecosystems comparable to our studied sites except the lower level of radioactivity. In these “mirror” springs, few deformations were observed.

Light microscopy (LM) and scanning electron microscopy (SEM) observations on a new species of the genus Pseudostaurosira from a spring of the Massif Central (France) are presented. The new species, with very small dimensions, presents similarities with other species previously observed in other parts of Europe. However, based on its morphology, the new species presents striae sometimes composed of only two areolae on the valvar surface: one big, round or elliptic near the mantle edge and the second smaller, slit-like or round (sometimes completely occluded externally) situated further toward the interior. Moreover, the size, the density of striae and the presence of volae allow the separation of Pseudostaurosira bardii from the other species. Morphologically similar species were also re-investigated in the current project, including the type material of Pseudostaurosira trainorii E.Morales, Pseudostaurosiropsis connecticutensis E.Morales and Pseudostaurosira cataractarum (Hustedt) C.E.Wetzel, E.Morales and Ector. They were also investigated and illustrated using SEM. The ecological preferences of Pseudostaurosira bardii are briefly discussed.

During a floristic survey of freshwater springs in the French Massif Central region, a new Sellaphora species (Bacillariophyceae) was found and is hereby described: Sellaphora labernardierei sp. nov. This new small species, with linear to linear-elliptical valves, slightly inflated in the middle, is formally described using light and scanning electron microscopy. The species has a single H-shaped chloroplast and striae usually biseriate tending to be uniseriate near the centre of the valve. The valve dimensions, the valve, pole and central area shapes, and the striae density and pattern allow their separation from similar taxa belonging to the genera Eolimna, Naviculadicta and Sellaphora. The ecological preferences of Sellaphora labernardierei are mineral waters with high nitrate concentrations. The examination of similar taxa belonging to the genera Eolimna and Naviculadicta and their characteristics lead to propose four new combinations of Sellaphora species.