After the recent revisions of the diatom genus Skeletonema, our knowledge on its diversity has greatly improved, although its taxonomy has become more complicated, especially with the descriptions of cryptic or semicryptic species, which make species recognition difficult. The combination of different observation techniques (LM, SEM and TEM) and/or the use of molecular tools are now required for an accurate identification. In this paper, we studied numerous phytoplankton samples collected with nets or bottles from locations along the Mexican Pacific, and provided observations on the morphospecies of Skeletonema. Four taxa were found using LM, SEM and TEM: Skeletonema sp. aff. japonicum and Skeletonema cf. marinoi, S. pseudocostatum and S. tropicum. Some features of the chains, cells, valves, terminal or intercalary fultoportulae and rimoportulae, and cingular bands, are given for each species. The taxon named here Skeletonema cf. marinoi showed particular characters (ribs-like structures), not previously described, on the fultoportulae of terminal valves, which arise suspects of the presence of new species within the species complex. We annotate Skeletonema pseudocostatum as a new record from the Mexican Pacific, where only three species have been previously recorded. The species Skeletonema costatum, traditionally considered as abundant, frequent and widely spread all over the region, was not recognized in our samples.

Fragilaria pinnata Ehrenberg is one of the most commonly reported taxa in floristic and ecological works from around the world. Yet, the images published for this taxon reveal that its identity is not well defined and that several morphological variants have been ascribed to it. The present manuscript includes a preliminary analysis of the literature and type material housed in the Ehrenberg Collection, Berlin, Germany. The nomenclatural history of the taxon, a critical examination of original drawings, and results of the examination of type material lead to the conclusion that Fragilaria pinnata (current synonym: Staurosirella pinnata (Ehrenberg) D.M.Williams & Round) is not an araphid diatom, but rather an organism with links to the genus Denticula Kützing. The history of another taxon, Staurosira pinnata Ehrenberg, was also investigated to determine its actual relationship to Fragilaria pinnata since recent publications present these two names as synonyms. Staurosira pinnata also has an entangled history and a dubious current concept. Although, we have not examined type material for this taxon yet, based on examination of the nomenclatural history and original drawings, we were able to conclude that it is indeed an araphid diatom, but that the current synonymy with Fragilaria pinnata is incorrect.

The ultrastructure of Gomphonema gautieri (Van Heurck) Lange-Bert, et Metzeltin was studied based on a small population from a small Swedish, slightly acid, eutrophic river. The observations were compared with the holotype slide of G. augur var. gautieri Van Heurck, as the species was originally described in 1885. The valves are characterized in having a typical clavate outline with the largest width near the broadly rounded headpole. The species can be separated from G. augur Ehrenb. by its larger valve dimensions, the presence of broad shoulders on the headpole and the lower number of areolae per stria. Based on these differences, a separation of both taxa is justified.

Two new Geissleria species are described from Portugal based on their morphological and ultrastructural characteristics. Geissleria tagensis Novais et Ector sp. nov. has rhombic-elliptical valves with broadly rounded to slightly protracted apices and lacks an isolated pore. Geissleria lusitanica Novais et Ector sp. nov. has linear-elliptical to elliptical valves with slightly to markedly protracted ends and two isolated pores. The type analysis of similar species (Geissleria acceptata (Hustedt) Lange-Bertalot et Metzeltin, G. schoenfeldii (Hustedt) Lange-Bertalot et Metzeltin and Navicula modica Hustedt) allowed their better characterization and illustration, supporting the delimitation of the new Portuguese species. A new taxon present in the type material of G. schoenfeldii, G. hinziae Novais et Ector is also described. Three new combinations are also proposed: Geissleria capitata (H. Kobayasi in Kobayasi et Ando) Ector et Novais comb, nov., G. modica (Hustedt) C.E. Wetzel, Novais et Ector comb. nov. and G. taishanica (Y.Q. Guo et S.Q. Xie) Ector et Novais comb. nov. A checklist of the species and a thorough survey of the literature available on Geissleria are provided.

We detail under light and scanning electron microscopy the type materials of three poorly known diatom species first described by Hustedt from aerial habitats in Germany. Navicula parsura Hustedt (synonym: N. obscura Hustedt), N. obsoleta Hustedt, and N. evanida Hustedt type materials were observed and compared with populations from Luxembourg. Ultrastructural analysis revealed the presence of striae composed by areolae occluded externally by hymenes and a silica line at the valve face-mantle junction along the apical plane. These taxa also present a naviculoid raphe, which besides overall valve outline, conforms to the genus Chamaepinnularia Lange-Bertalot et Krammer. Therefore, we propose two new combinations: Chamaepinnularia obsoleta (Hustedt) C.E. Wetzel et Ector comb. nov. and C. parsura (Hustedt) C.E. Wetzel et Ector comb. nov. Excepting C. evanida (Hustedt) Lange-Bertalot, which is often observed in European rivers and already placed in the genus Chamaepinnularia, the remaining two taxa are poorly known and autecological data are scarce. Our results suggest that these species, present in stream drift during flood events, might partially derive from near-stream riparian or upslope terrestrial habitats. In addition, based on literature data, three new combinations are also proposed.

Tools that may be used to characterize the state of aquatic ecosystems and the impacts associated to pesticides are currently requested for the monitoring of ecosystems. Chemical analysis alone is not sufficient for such purposes and biomonitoring is essential. Many bioindicators based on diatoms are commonly used to assess the quality of aquatic environments but have not been developed to specifically address the impacts of pesticides. It is therefore crucial to develop dedicated tools. Our research aimed at performing a field study (analysis of monitoring network data) in order to evaluate the responses of different metrics towards pesticide pressure.

For diatom communities, toxic pressure associated to pesticides, especially herbicides, had a weak effect on species composition and on the various tested metrics. Nutrient and organic matter concentrations (and also typology) are the main influencing factors. When trophic and saprobic levels are controlled, it is possible to identify the effects of herbicides on some metrics. The only responding metric is the abundance of the “high profile” guild, which corresponds to taxa presenting an important contact surface with water flow. Its abundance decreases when herbicide toxic pressure increases. It could therefore be used to assess the impact of herbicides on diatom communities, but in a framework where confounding factors variability (e.g., nutrients, organic matter, river order) is reduced.

Diatom development depends on several environmental factors, including the availability in metals. When micronutrients are present of adequate amount, cells exhibit a strong fitness and develop at their maximum growth rate. In many circumstances, the optimal metal amount in the cell environment is disrupted and cells experience starvation or excess for one or more elements. The metals in excess interfere with biochemical and cellular processes triggering a dysfunctioning that reduces growth and may ultimately lead to cell death. The ability of diatoms to adapt/resist to environmental changes has ecological consequences in term of biodiversity. To survive, diatoms activate defence mechanisms, such as the production of antioxidants or/and metal chelators. In this contribution, the diatom requirements for cadmium, copper, zinc and sodium are briefly reviewed. Then the impacts of an excess or a deprivation in one of these elements on diatom physiology is discussed from the molecular and biochemical point of views. The defence mechanisms enabling diatoms to overcome the metal stress are presented. At the end of this contribution, an assay on the integration of the defence mechanisms is presented.