Focus editorial: new contributions in diatom research

This issue contains the contributions selected from those presented at the thirty-seventh Symposium of the Association des Diatomistes de Langue française (ADLaF, Cocquyt, Ector, and Van de Vijver (2018)) held in the Meise Botanical Garden, Belgium. This French-speaking society promotes diatom research and organizes an annual symposium for facilitating the exchange of information between diatom experts. Moreover it gives young researchers the opportunity to present their work and to be introduced to the scientific diatom world. Despite the largely recognized role of diatoms for the planet (e.g. Schoefs, Hu, and Kroth (2017)), the total number of taxa remains elusive and very regularly new genera and species are described (e.g. Dodinet, Nadot, and Schoefs (2019)). Diatom taxonomy mostly relies on the frustule organisation, including its ornamentation pattern. These morphological characteristics are crucial for the identification of taxa making a precise description of this ornamentation compulsary. The aim of the paper by Wetzel, Beauger, and Ector (2019) in this issue is to clarify the taxon Cocconeis rouxii Héribaud & Brun (Héribaud 1893) from samples collected in the Auvergne region (France) using light and scanning electron microscopy whereas the contribution by Wadmare et al. (2019) describes two new aerophilic species of Stauroneis in the Eastern Himalayas of India. The correct identification of diatom taxa is of prime importance for many aspects of diatom research, including biodiversity studies, the description and the evolution of diatom assemblages (e.g. Lai et al. (2019)), (paleo)distribution and (paleo)ecology (e.g. Prokopenko and Khursevich (2014)). Several papers published in this issue are related to these fields of research. First of all, Cocquyt, Mambweni Makaya, and Ngendja Kabitoma (2019) discuss the analysis of material collected in a river of the Congo basin (Democratic Republic of the Congo). Surprisingly but interestingly, they could identify an unusual Eunotia species as Eunotia enigmatica, a species that was recently described from Brazil (Costa et al. 2017). The relative abundance of the collected taxa revealed that E. enigmatica was a rather infrequent though not rare taxon. Beauger et al. (2019) discovered a new species of Craticula in a pond in Northern Senegal. Many aspects of diatom life have still to be studied in details to understand their ecology and distribution. This includes answers to nutrient deficiency (e.g. Heydarizadeh et al. (2019)) and the capacity to move. It is generally accepted that diatoms can move (Bertrand 2008), sometimes in a response to a stimulus (Bondoc et al. 2016), or remain immobile, for instance as epiphytes (e.g. Bertrand et al. (2016)). In this issue, Roubeix and Chalié (2019) report a unique case of epiphytism: the phoretic epithytism of Fallacia helensis on Nitzschia sigmoidea. This original work opens new avenues for investigating the intra-relationships between the diatom species, phoretic epiphytism and its role in the dissemination of diatoms. In this frame the finding by Heudre et al. (2019) of two diatoms in a lake in the Vosges (France), presenting an unusual biogeographical distribution, is of interest because it makes us wonder how these rare taxa managed to disperse and develop in this peculiar environment.