A floristic survey of marine tube-forming diatoms reveals unexpected diversity and extensive co-habitation among genetic lines of the <i>Berkeleya rutilans</i> complex (Bacillariophyceae)

Limited molecular data for marine tube-forming diatoms are available currently and this study provides the first molecular survey of these taxa. To conduct this survey, we used a molecular-assisted alpha taxonomy (MAAT) approach that utilizes DNA barcode data. We used three DNA barcode markers: the 3´ end of the large subunit of RUBISCO (<i>rbc</i>L-3P); the variable D2/D3 region of the nuclear large subunit ribosomal DNA (LSU D2/D3); and the internal transcribed spacer 2 (ITS2) to assign marine tube-forming diatoms from Canada to genetic species groups. The <i>rbc</i>L-3P analysis uncovered 29 genetic groups including representatives of <i>Haslea crucigera, Navicula bottnica, N. brunelii, N. ramosissima, Nitzschia fontifuga, N. tubicola, Parlibellus berkeleya</i> and <i>P. delognei</i> f. <i>elliptica</i>, as well as a complex of 14 closely related groups morphologically consistent with <i>Berkeleya rutilans</i>. We sequenced ITS2 for representatives of the <i>B. rutilans</i> complex; these data were consistent with the <i>rbc</i>L-3P genetic clusters for 86% of the colonies tested. The remaining 14% were in conflict, possibly indicating that more than a single <i>Berkeleya</i> genetic species was present in each colony. To investigate this hypothesis further, we developed ‘species-specific’ ITS2 primers and confirmed heterogeneity of <i>Berkeleya</i> genetic species in 64% of the colonies tested (<i>N</i> = 91). Therefore, a taxonomic assessment of tube-forming species that were originally described on the basis of colony morphology (e.g. <i>Berkeleya rutilans</i>) can only proceed using clonal cultures or single-cell analysis.