More about mucilaginous algae

Following my post about the increase in algal “gloop” (see “Is algal gloop on the increase?”), I thought it would be interesting to show some of the organisms responsible for this “gloop” at higher magnification.   Chris Carter provided the images.

First of all, there are some images of the green alga Coccomyxa confluens, one of the organisms mentioned in my previous post.   Under the microscope, the mucilaginous masses can be seen to consist of large numbers of solitary cells, each with a single chloroplast wrapped around the margin, embedded in the mucilage.   But what is particularly interesting about these specimens is the fungal hyphae that weave through the mucilage.   Coccomyxa can be found both as a free-living alga but also as the algal partner in a lichen. I wrote about lichens last year (see “Discovering a liking for lichens …” and “It takes two to tango …”) and speculated about their origins.   The image of fungal hyphae weaving through the mucilaginous mass re-ignites this curiosity. Broadly speaking, the lichen symbiosis is not a meeting of equals: the fungal partner cannot live without the alga but the alga can survive without the fungus. The fungal partner is capable of sexual reproduction but once the spores arrive at a suitable habitat, they need to find some algal cells in order to thrive. A free-living mass of Coccomyxa is, therefore, the perfect place for a tiny wind-blown lichen spore to land.   Maybe that is what we are seeing here: the start of a beautiful relationship?


Coccomyxa confluens, photographed by Chris Carter. Left: macroscopic view of mucilaginous masses; right: microscopic view showing individual cells along with fungal hyphae.

The second representative of the “gloop”-forming algae is the desmid Mesotaenium macrococcum.   We’ve met desmids a few times in this blog (see “Hunting for desmids in Upper Teesdale”) but Mesotaenium does not have the elegant, symmetrical outline of most desmids with a central constriction.   It belongs to a group called the “saccoderm (or false) desmids”, which is a family in the same sub-order as the filamentous algae Spiroygra and Mougeotia which we’ve met elsewhere in this blog.   By contrast, most of the desmids such as Closterium and Micrasterias, which we met in Upper Teesdale, belong to the “placoderm” (or true) desmids”.   The exception is Netrium, which is also a saccoderm desmid.   In Upper Teesdale, I was hunting for desmids amongst Sphagnum in bog pools; Mesotaenium, by contrast, is a species that can live in situations where it is not permanently submerged. Once again, the mucilage offers protection against desiccation.


Mesotaenium macrococcum, photographed by Chris Carter: left: medium-power image showing mucilaginous masses; right: high power image showing the simple chloroplast.

Finally, another green alga, though we are not sure what this one should be called.   Chris and I contemplated the genus Palmella but this seems to be a name that has fallen out of favour recently.   Species formerly included in Palmella can be found in both the Cyanobacteria (now species of Aphanothece, Chlorogloea and Gloeothece) and Chlorophyta (now Trochiscia) so we are clearly not the first people to struggle.   However, Chris has highlighted a small eye-spot and also the beginnings of a flagellum, which suggest that this is the resting stage of a motile Volvocale rather than a permanently sessile organism. Chris commented that this specimen came from a wetter habitat than the Coccomyxa and it may be that this is the resting stage of an organism which, when conditions are favourable, lives freely in the water, rather than forming mucilaginous masses.


Resting stage of a Volvocalean alga, photographed by Chris Carter; left: low-power image showing mucilaginous masses; right: high power image with (inset) cells with eye spot and the start of a flagellum highlighted.


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