A strew slide of Navicula granulata. No makers name or location. No mountant information, but as this was so low contrast I suspect it is Canada Balsam. Labeled as ‘Very rare’. Olympus BHB microscope using 450nm LED light. I selected a couple for imaging at higher resolution. 63x Leitz Pl Apo 1.4 objective, oil immersion. Olympus Aplanat Achromat condenser, oil immersion, oblique lighting. 2.5x Nikon CF PL photoeyepiece. Monochrome converted Nikon d850 camera. Images stacks prepared in Zerene (Pmax). On the nice clean image, this was just the top valve – the stack had both valves in it, but the bottom one just made the details on the top one blurry. About 1-2% of the diatoms were ‘speckly’, so I imaged one of these as well. More on this in a minute.
Name time. The slide says Navicula granulata. There are a lot of variants of N. granulata and I came across a paper with some nice images – Helen M. Jones, Gillian E. Simpson, Alan J. Stickle & David G. Mann (2005), Life history and systematics of Petroneis (Bacillariophyta), with special reference to British waters, European Journal of Phycology, 40:1, 61-87, DOI: 10.1080/09670260400024675. Based on this I think it looks like either Petroneis monilifera, Petroneis latissima or Petroneis humerosa. All of these link back to being described as N. granulata at some point.
Petroneis humerosa (Brébisson ex W.Smith) Stickle & D.G.Mann 1990. Published in: Round, F.E., Crawford, R.M. & Mann, D.G. (1990). The diatoms. Biology and morphology of the genera. pp. [i-ix], 1-747. Cambridge: Cambridge University Press. Page 674. A synonym is Navicula granulata var. humerosa (Brebisson) Carruthers 1864.
Petroneis monilifera (Cleve) Stickle & D.G.Mann 1990. Published in: Round, F.E., Crawford, R.M. & Mann, D.G. (1990). The diatoms. Biology and morphology of the genera. pp. [i-ix], 1-747. Cambridge: Cambridge University Press. Page 674. Heterotypic synonym: Navicula granulata Brébisson 1858.
Petroneis latissima (W. Gregory) Stickle & D.G.Mann 1990: 674. Published in: Round, F.E., Crawford, R.M. & Mann, D.G. (1990). The diatoms. Biology and morphology of the genera. pp. [i-ix], 1-747. Cambridge: Cambridge University Press. Page 674. A homotypic synonym is Navicula granulata var. latissima (Gregory) Carruthers 1864.
Coming back to the unusual looking ones which made up 1-2% of the diatoms present, the Jones paper from 2005 mentioned above mentioned ‘dividing cells’ being frequently enclosed within a thick mucilage sheath. Could these be an example of that? Maybe, not sure, however it’s a reasonable working hypothesis.