Genetic and Physiological Diversity in the Diatom Nitzschia inconspicua

Nitzschia inconspicua is an ecologically important diatom species, which is believed to have a widespread distribution and to be tolerant to salinity and to organic or nutrient pollution. However, its identification is not straightforward and there is no information on genetic and ecophysiological diversity within the species. We used morphological, molecular (rbcL and LSU D1–D3), ecophysiological and reproductive data to investigate whether N. inconspicua constitutes a single species with a broad ecological tolerance or two or more cryptic species with shared or different ecological preferences. Molecular genetic data for clones from upstream and deltaic sites in the Ebro River basin (Catalonia, Spain) revealed seven N. inconspicua rbcL + LSU genotypes grouped into three major clades. Two of the clades were related to other Nitzschia and Denticula species, making N. inconspicua paraphyletic and suggesting the need for taxonomic revision. Most clones were observed to be automictic, exhibiting paedogamy, and so the biological species concept cannot be used to establish species boundaries. Although there were morphological differences among clones, we found no consistent differences among genotypes belonging to different clades, which are definable only through sequence data. Nevertheless, separating the genotypes could be important for ecological purposes because two different ecophysiological responses were encountered among them.