Having discussed some of the recent name changes in green algae in the previous post, I thought that I would continue this theme using some of the other taxa that I found in the samples I collected from the River Wear a couple of weeks ago. The plate below shows some specimens that, 20 years ago, I would not have hesitated to call Scenedesmus, characterised by coenobia of either four cells or a multiple of four cells arranged in a row. Over 200 species, and 1200 varieties and forms have been recognised although there were also concerns that many of these so-called “species” were, in fact, variants induced by environmental conditions. A further problem is that Scenedesmus and relatives do not have any means of sexual reproduction. This means that any mutation that occurs and which does not have strong negative effects on the organism will be propagated rather than lost through genetic processes. Working out what differences are really meaningful is always a challenge, especially when dealing with such tiny organisms.
Scenedesmus and Desmodemus species from the River Wear, Wolsingham, September 2018. a. and b. Scenedesmus cf ellipticus; c. Desmodesmus communis. Scale bar: 20 micrometres (= 1/50th of a millimetre).
The onset of the molecular era shed some new light onto these problems but, in the process, recognised differences within the genus itself that necessitated it being split into three, two of which are on the plate below. Scenedesmus, in this modern sense, has cells with obtuse (rounded) apices and mucilage surrounding the cells whilst Desmodesmus has distinct spines at the apices of marginal cells and, sometimes, shorter ones elsewhere too. In addition to these there is Acutodesmus, which is similar to Scenedesmus (i.e. without spines) but whose cells have more pointed (“acute”) ends and which does not have any surrounding mucilage. A further genus, Pectinodesmus, has been split away from Acutodesmus on the basis of molecular studies, although there do not seem to be any features obvious under the light microscope which can differentiate these.
The genera Ankistrodesmus and Monoraphidium present a similar situation. In the past, these long needle- or spindle-shaped cells would all have been considered to be Ankistrodesmus. Some formed small bundles whilst others grew singly and this, along with a difference in their reproductive behaviour, was regarded as reason enough for splitting them into two separate genera. Both were present in the Wear this summer, but only Monoraphidium presented itself to me in a manner that could be photographed. Two species are shown in the plate below. Recent molecular studies seem to not just support this division but also suggest that each of these could, potentially, be divided into two new genera, so we’ll have to watch out for yet more changes to come.
Monoraphidium species from the River Wear, Wolsingham, September 2018. a. and b.: M. griffthii; c. M. arcuatum. Scale bar: 20 micrometres (= 1/50th of a millimetre).
The final illustration that I managed to obtain is of another common coenobium-forming alga, Coelastrum microporum. Though the three-dimensional form makes it a little harder to see, cell numbers, as for Pediastrum, Scenedesmus and Desmodesmus, are multiples of four. I apologise if the picture is slightly out of focus, but it is a struggle to use high magnification optics on samples such as these. The oil that sits between the lens and the coverslip conveys the slight pressure from fine focus adjustments directly to the sample, meaning that the cells move every time I try to get a crisper view. That means it is impossible to use my usual “stacking” software. The answer is to use an inverted microscope so that the lens was beneath the sample. However, I do this type of work so rarely that the investment would not be worthwhile.
That’s enough for now. I’m off on holiday for a couple of weeks, so the next post may be from Portugal and perhaps I will also find time to sample the River Duoro as well as the products of the vineyards in it’s catchment…
Coelastrum microporum from the River Wear,Wolsingam, Septmber 2018. Scale bar: 20 micrometres (= 1/50th of a millimetre).
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