Molecular and morphological characterization of the <i>Achnanthidium minutissimum</i> complex (Bacillariophyta) in Petuniabukta (Spitsbergen, High Arctic) including the description of <i>A. digitatum</i> sp. nov.

Recent morphology-based investigations of freshwater Arctic diatoms suggest that many species remain to be discovered, reflecting a unique polar flora. During a survey of the freshwater diatom flora of northern Billefjorden, including the Petuniabukta fjord region (Spitsbergen), several morphodemes belonging to the <i>Achnanthidium minutissimum</i> species complex were recorded. Molecular phylogenies based on <i>rbc</i>L, 28S and 18S sequences, including single cells from Canada and strains from Marion Island (sub-Antarctica) and GenBank revealed the presence of 12 distinct <i>A. minutissimum</i> complex lineages, of which three contained strains from Spitsbergen. One Arctic lineage is described as a new species. <i>Achnanthidium digitatum</i> sp. nov. is morphologically characterized by narrow, linear to only slightly lanceolate valves and usually two areolae per stria. The two remaining Arctic lineages are in need of a more complete morphological and molecular comparison with other representatives of the <i>A. minutissimum</i> complex to clarify their taxonomic identity. It is argued that implementation of molecular data in the taxonomy of <i>Achnanthidium</i> will be essential to solve the taxonomic problems associated with this group, eventually resulting in a better understanding of the biogeography and niche differentiation of different species belonging to the <i>A. minutissimum</i> complex. Ideally, this should be based on more variable genes than the currently widely used 18S, which does not have a species level resolution in the <i>A. minutissimum</i> complex.