It has been a while since I have had a diorama to show in my blog but I was inspired by the lush growths of Audouinella in the River Ehen back on 12 October (“at last … a red alga that is really red”) and, unhappy with how my previous attempt to portray Audouinella had turned out (so bad, in retrospect, that I am not giving you the link), set to work with pencil, paint and pastel again.
A three-dimensional representation of the Audouinella-dominated community in the River Ehen, October 2013. The pinkish-red filaments of Audouinella form a dense matrix of filaments within which we can see colonies of the diatom Gomphonema truncatum on the left (with Achnanthidium minutissimum as an epiphyte) along with some chains of another diatom, Tabellaria flocculosa, on the right. Filaments of the cyanobacterium Heteroleibleinia rigidula can be seen as epiphytes on the Audouinella.
Notice, in the picture, how the Audouinella has a few short side-branches which end in long, colourless hairs. These, we think, are adaptations that exude enzymes and which allow the alga to extract phosphorus from organic matter in the river. And note how the different diatoms all have distinct habitats within the mass of Audouinella filaments. When we prepare diatoms for microscopy, we pay homage to the statistical gods and strive for a random strew of diatoms across our slide. This is, however, far from the situation in the river itself, where Tabellaria and Gomphonema seemed to occupy different niches within the Audouinella. Note, too, how there are no reproductive bodies on these filaments, in contrast to those I photographed back in February.
My dissatisfaction with my earlier diorama was partly my failure to capture the structure of the chloroplasts. It was hard to reconcile what I could see under the microscope with the description in the Flora of “several parietal ribbon-like chloroplasts”. The ones I could see were far from “ribbon-like”. I checked other Floras without adding much to my understanding of the freshwater forms then, for contrast, I looked at the descriptions of the 33 marine species of Audouinella found in Britain and Ireland. These seem to have a range of chloroplast forms, from a single, lobed chloroplast to many discoid chloroplasts. There was, however, a suggestion that, in some species at least, the single chloroplast fragmented into several smaller chloroplasts as the cell aged, which might explain why I could see several non-ribbon-like chloroplasts in my specimens. I’m still not entirely happy with my representation of the Audouinella but, having spent several hours on this image, it will have to do for now. Maybe I should return to the subject in the spring, when the carpospores re-appear?
Dixon, P.S. & Irvine, L.M. (1977) Seaweeds of the British Isles Volume 1 Rhodophyta Part 1 Introduction, Nemaliales, Gigartinales. British Museum (Natural History), London.