Entomoneis in three dimensions

I’ve written about the genus Entomoneison a few occasions in the past (see “A typical Geordie alga …”).   It is a challenging species to understand partly because the cells often do not survive digestion in the strong oxidizing agents that we routinely use to understand the structure of diatom cell walls, and partly because of its unusual three-dimensional architecture.   I’ve commented on this before, using some of Chris Carter’s photos to illustrate this (see “The really rare diatom show”).  Now, thanks to yet more careful work from Chris, we have a new set of photos with which to understand this species.

The underlying problem of a complicated geometry (the frustule [cell wall] is actually twisted in two planes) is compounded by the shallow depth of field that is available when viewing organisms at high magnifications. The first of Chris’ images shows how most diatomists will encounter Entomoneis: as a cleaned cell mounted on a slide and shows how the girdle bands bands (the silica “spacers” between the two valves) seem to present a particular problem.  Look, in particular, at the arrangement of these in the left-hand image, focused on the top of the cell, and note how they appear to cross over one another.  Compare this to image that is focused on the bottom of the cell.  By contrast, a cell that has not been subjected to the strong oxidising agents that we use to “clean” diatoms prior to observation presents quite a different view, as seen in the second set of three photographs.   The contrast is poorer here, as the cell is not mounted in a high-resolution mountant (the reason diatomists “clean” their samples in the first place) but we can, nonetheless, see the girdle bands.   When Chris focuses on the top of the cell. the girdle bands are clearly visible, not criss-crossed, and diagonal across the cell. At the other extreme (focus on bottom of cell) the bands are still just visible, sloped the other way somewhat obscured by the cell contents but, most importantly, not presenting a gaping hole.

B Entomoneis naphrax mount.jpg

A cell of Entomoneisthat has been cleaned and mounted in Naphrax before being photographed at three focus levels using simple brightfield microscopy.  The left-hand image is focussed on the top of the cell and shows how the girdle bands appear to cross one another whilst the right hand image is focussed on the bottom of the cell and shows a chasm in the centre of the cell where the girdle bands have collapsed. The middle image shows an intermediate focal plane where the apices are in focus: this is where the girdle bands are attached.

C Entomoneis alcohol mount.jpg

A cell of Entomoneisthat has been mounted in alcohol before being photographed at three focus levels. The contrast is much poorer here but at one extreme (focus on top of cell ie towards observer) the bands are clearly visible, not criss-crossed, and diagonal across the cell. At the other extreme (focus on bottom of cell) the bands are still just visible, sloped the other way but somewhat obscured by the cell contents.

What we think is happening is that the girdle bands are so weak that they collapse as soon as the frustule is dried or hits hot Naphrax; this collapse can be either towards the observer or away from the observer, creating a slightly different optical effect in each case.   Most of the time, however, the bands detach completely leaving isolated valves – sometimes with some straggly bits attached.  Chris thinks that almost all the published images of this taxon are misleading: usually flattened either optically or by software in order to give a sharp image for presentation and, in the process, disguising this detail.

These images all show us what Entomoneis looks like in girdle-view, the way we are most likely to encounter an intact cell when looking down a light microscope.  The next two plates show it from above (“valve view”) and in apical view (i.e. looking at the cell from one end), both of which are not often seen during routine observation.    The pair of valve views show the outline at different focal levels, and we can see how the thin wing (keel) is twisted towards the viewer; this twist is also present in the main (cylindrical) part of the cell but is not visible in these photographs.   The series of photographs in the next plate takes this further: the sequence along the top shows an apical view at several points of focus.  Some particulate matter is caught within the open structure of the frustule and acts as a reference point when comparing the two views. The thin keel with its thickened edge (containing the raphe) shows clearly. The body of the cell is not symmetrical because of the twist; the girdle band section is at the bottom of the inverted U section and is demarcated by ridges associated with each band: the number of bands can be estimated as shown on the enlarged fourth section. The other valve must have detached without holding onto any girdle bands.

A Entomoneis valve view in alcohol.jpg

Valve view of an alcohol mounted celul of Entomoneisat two focus levels.

D Entomoneis semicell in apical view in alcohol.jpg

An alcohol mounted semicell of Entomoneis caught in both apical (top row, showing several points of focus) and girdle views (bottom right).  The image at the bottom left shows a slightly magnified version of the fourth apical view indicating the location of the girdle bands on the opposite sides of the valve (indicated by the vertical red lines).

Entomoneis highlights the limitations of using two-dimensions to portray algae.  The answer, Chris and I agree, would be a three-dimensional model (see “Taking desmids to the next dimension …”) that we could pick up and view from all angles.  Another option is to use a scanning electron micrograph (SEM), and the two references at the end of this article contain several useful images.   However, most of us are still going to encounter Entomoneisprimarily via the light microscope.  Having a sense of the three-dimensional form of an alga lodged in your mind makes it much easier to interpret the flattened two-dimensional images that we routinely encounter.  Prior to the era of SEMs, the three-dimensional form of Entomoneis, and, indeed, its true taxonomic position, was very difficult to appreciate.   Both the 1930 and 1990s editions of the Süsswassflora von Mitteleuropaplaced it with Naviculawhereas we now understand enough about the form of the raphe to know that Entomoneis is more closely related to Surirella(see Round et al.,referenced below).  It is a good reminder that the study of diatoms has always been limited by the technology available.   Our toys may have changed enormously over the past hundred years but the gaps in our understanding remain …

References

Round, F.E., Crawford, R.M. & Mann, D.G. (1990).  The Diatoms: Biology and Morphology of the Genera.  Cambridge University Press, Cambridge.

Dalu, T., Taylor, J.C., Richoux, N.B. & Froneman, P.W. (2015).  A re–examination of the type material of Entomoneis paludosa(W Smith) Reimer and its morphology and distribution in African waters.  Fottea15: 11-25.

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