Morphology and identity of some ecologically important small Nitzschia species

Abstract

The taxonomy of several small-celled, ecologically significant Nitzschia species, which are frequently confused with each other or whose names are misapplied, is clarified. Following an examination of type material and modern samples by light and electron microscopy, it was concluded that N. frustulum (Kützing) Grunow, N. inconspicua Grunow, N. soratensis E. Morales & Vis and N. invisitata Hustedt are independent species. No morphological basis was found for separating N. frustulum var. subsalina Hustedt or N. boliviana E. Morales & Vis from N. inconspicua and they are therefore placed in synonymy with N. inconspicua. Nitzschia soratensis, described recently from Bolivia, has previously been misidentified in Europe, either as N. inconspicua (from which it differs most obviously in having more bluntly rounded poles, striae within the raphe canal that are composed of triplets, and fibulae that can be seen in light microscopy to widen at their bases) or as N. abbreviata Hustedt ex Simonsen (from which it differs in pore ultrastructure). Nitzschia frustulum resembles N. inconspicua in every morphological feature examined, but with wider valves and consistently higher maximum length.

Keywords:

• diatoms
• frustule ultrastructure
• Nitzschia
• Nitzschia frustulum
• Nitzschia inconspicua
• Nitzschia invisitata
• Nitzschia soratensis
• taxonomy
• type material

Acknowledgements

We are grateful for support from SYNTHESYS (financed by European Community Research Infrastructure Action under FP6 and 7) for research visits by David Mann (to the Naturhistorisches Museum, Vienna, in summer 2009) and Laia Rovira (to the Royal Botanic Garden Edinburgh in spring 2010). Support from European Distributed Institute of Taxonomy (EDIT) Research Grant (from the EDIT WP2, funded by EU FP6) made possible the visit of Rosa Trobajo to the Natural History Museum, London, in summer 2010. We are also very grateful to Alex Ball and Frieda Christie for help with SEM; Anton Igersheim, for generous help in the use of the Grunow Collection and for loans of material; Friedel Hinz for arranging loans from the Hustedt Collection; Eduardo A. Morales and Marina Potapova for providing the type material of Nitzschia soratensis and N. boliviana; and the two anonymous reviewers for their constructive comments, which helped to improve this paper.

The following supplementary material, comprising 4 supplementary plates, is available for this article accessible via the Supplementary Content tab on the article's online page.

Supplementary Material - Plate 1 Nitzschia inconspicua

Figs 1–14. Nitzschia inconspicua from modern populations of the Ebro Estuary. Figs 1–10. Light micrographs (bright field optics) illustrating size reduction, scale bar = 5μm. Figs 11–14. SEM micrographs. Fig. 11. Valve interior showing central nodule and fibula details. Figs. 12. Valve exterior with straight central raphe endings. Fig. 13. Detail of the central raphe endings, note the clearly visible hymenes in the raphe canal poroids and also in the stria poroids. Fig 14. Valve exterior with central raphe endings turned towards the mantle. Scale bars = 1 μm.

Supplementary Material - Plate 2 Nitzschia boliviana

Figs 1–22. Nitzschia boliviana from type material (Sorata Department, Bolivia, South America. ANSP GC 26804). Figs 1–20. Light micrographs (bright field optics) illustrating size reduction, scale bar = 10 μm. Figs 21–22. SEM micrographs. Fig. 21. General aspect of valve exterior. Fig. 22.Valve interior showing central nodule and fibula details. Scale bars = 1 μm.

Supplementary Material - Plate 3 Nitzschia soratensis

Figs 1–24. Nitzschia soratensis from type material (Sorata Department, Bolivia, South America. ANSP GC 26804). Figs 1–20. Light micrographs (bright field optics) illustrating size reduction. Figs 5, 9 and 15 illustrating both Nitzschia soratensis (a) and Nitzschia boliviana (b) co-occurring, scale bar = 10 μm. Figs 21–24. SEM micrographs. Fig. 21. Valve interior showing central nodule and fibula details. Fig. 22. Valve exterior showing pore arrangement on the valve face. Fig. 23. Detail of centre outside, showing the triplets of poroids on the valve mantle and valve face. Raphe interrupted at the central nodule. Fig. 24. Detail of a large specimen (centre) surrounded by accompanying flora, Reimeria seriata (W. Gregory) Kociolek & Stoermer (below) and Navicula sp. (above). Scale bars Figs 21-23 = 1 μm, Fig. 24 = 10 μm.

Supplementary Material - Plate 4 Nitzschia soratensis

Detail of the areolae illustrated in Fig. 92, showing hymen pores arranged in quincunx. Scale bar = 200 nm.