Fenestrella antiqua, a fossil diatom from Fur, Denmark

[Beatsy]

Hello folks,

Anne Gleich kindly sent me a sample of cleaned diatomite from Fur, Denmark (thanks Anne). It contained several Fenestrella antiqua frustules, a large species up to 300µm diameter but quite fragile. These valves are usually badly broken in Fur material, but there were several complete ones in this particular sample. Very nice!

Here's one of those complete ones. It's ~200µm diameter and 80µm thick. Shot with an Olympus UPlanSApo 40x/0.9 objective using BF and 405nm (violet) light for more resolution.


Like Anne, I was intrigued by the short "arcs" near the edges - top left and bottom right in the image above. So I switched to 365nm UV illumination (in case there was finer detail to resolve) and shot an epic stack using an Olympus UPlansAPO 100x/1.4 objective. Step size was 100nm, requiring 675 images to cover the full DoF. They were each 4-shot pixel-shift exposures too so 2,700 images were captured in total. Oof!


Well, that was disappointing! I expected more, nicely structured detail but got a jumbled mess in the arc. It turned out the valve was damaged after all. So I looked at some broken specimens I'd picked out and found one with a much better-looking arc. I broke out the relevant piece so I could mount it with the arc closer to the coverslip and shot a shallower stack with the 100x/1.4 lens, but using violet light instead of UV (it's quicker to capture).


That's more like it. The inset on the right shows a closer view of small slots around the rim, spaced in between the arcs. The arc in the left inset shows some interesting, albeit very fragile structure now. My theory is that this area is a serious weakness which is easily crushed under pressure as the diatomite deposit forms. Hence the jumbled mess in an otherwise undamaged valve. Broken valves probably distribute the forces differently such that the arcs aren't compressed so much from the sides - and therefore less prone to being crumpled. Lesson learned.

Does anyone know what these "arc" structures are called, and what they might be for?

Thanks for looking.

Cheers
Beats

[anne]

Dear Steve,
"with a little help from my friend" I got these images.
Perhaps they will help to clarify a bit the function of this special arcs.
best
anne

[Beatsy]

Thanks for posting these Anne. Very interesting.

These are all inside views so we see the pore pattern of the separate sieve plate that sits under the outer surface of the valve. I saved a few fragments of that (not from the arc region) but haven't mounted or imaged those. My (optical) outside views only show the external pore pattern which is quite different. Curiouser and curiouser...

Cheers
Beats

[anne]

Dear Steve,
yes from outside Bernd told me there is "nothing". Just holes.

best
anne