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Novel clades of intriguing Nitzschia species from marine plankton

The paraphyletic genus Nitzschia Hassall comprises morphologically distinctive pennate diatoms. The principal diagnostic characters for delimitation of Nitzschia species include valve shape, the position of the raphe, and specific morphometrics such as size, stria density and fibula density. These often show considerable overlap between taxa, making species identification very difficult when based solely on morphology. Additionally, this genus is currently probably the largest in the diatoms (approx. 1000 species described) and occurs in fresh, brackish and marine habitats. Improving the taxonomy of Nitzschia species is therefore a major challenge, especially because their diversity is considerably better known in freshwaters compared to marine environments. Nitzschia species have been thought to be scarce in marine phytoplankton and are rarely reported ; they usually adhere to surfaces and do not have morphological adaptations to planktonic existence, unlike the related genera Pseudo-nitzschia and Fragilariopsis. However, as a result of ongoing global expeditions such as Tara Oceans or the Global Ocean Survey, more and more information about planktonic Nitzschia species is becoming available. During a BIOTA (Bio-tracing Adriatic Water Masses) cruise conducted in March 2016 in the open waters of the South Adriatic Pit we isolated several (11) small pennate strains into cultures from net samples and fractioned seawater samples taken from range of different depths (30m, 100m, 250m, 400m). Morphological analyses comprised LM, SEM and TEM observations, which, along with specific morphometrics, allowed us to distinguish 4 different Nitzschia morphotypes. These findings were congruent with molecular analyses performed on rbcL, SSU and psbC genes. Two distinct clades, represented by a total of 9 strains of Adriatic Nitzschia species, represent new lineages in the rbcL tree of Bacillariaceae, one being sister to Psammodictyon and one corresponding to a novel clade that is very distinct from other Nitzschia species sequenced so far. The remaining 3 Nitzschia strains corresponded to Nitzschia cf. incerta and two more unknown species.

Nitzschia, marine plankton, morphology, phylogeny

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