What is loosely known as pond life, (freshwater life is a better term), especially the microscopic flora & fauna, can provide enough interest to last a lifetime. My main interest is in the algae, particularly the desmids. The best way to study algae, with the exception of the diatoms, is to examine living material. Algae fixed and made into fluid mounts can be useful for reference purposes, but unlike the diatoms which can be cleaned and mounted in a resin mountant, fluid mounts are rarely 100% permanent.
Although I study desmids, now and again another alga of special interest is found. In 1980 one of my samples had a coenobium (a colony of unicellular organisms) of Sorastrum americanum [Bohlin] Smidle, a member of the Chlorophyta (green algae) which I identified from Prescott (1954). My Fig.1. is re-drawn from his figure. I thought little of this until another coenobium turned up in a sample from Sutherland, N.W.Scotland in 1992. Realising this was only the second colony I had seen in twelve years, I looked it up in West & Fritsch (1904-1927) not available to me in 1980 when the first specimen was seen. They state 'it occurs in the Outer Hebrides' no figure of this species is shown and no mention of it being found in mainland Britain. Lund (personal communication) informs me he has no records of S.americanum for England or Wales since West & West (1905), although it is known from Africa, America & Europe. It is more common in the tropics than in the temperate zone where it is very rare. This is where an amateur like myself can make a useful contribution to the knowledge of algae by recording the occurrence and distribution of rare species at The Freshwater Biological Association, Windermere, England.
Sorastrum americanum is quite distinctive although it bears a superficial resemblance to some Pediastrum species i.e. the colony is only loosely attached, not compact, and it is globose not flat like Pediastrum. There are commonly 16-64 heart shaped cells; there are out of focus cells coming forward and away from the focal plane in the figure (Fig.1.). The only other British species S.spinulosum Naeg. has bean shaped cells with small processes, described by West & West as a rare species found in boggy pools etc. Lund however states 'S.spinulosum is cosmopolitan, somewhat common here since it is recorded from several places in England, Ireland & Scotland though not, apparently, recently.'
Another alga I found interesting, very common in Britain frequently found in boggy pools where desmids are abundant, is Asterococcus superbus [Cienk] Scherffel (1908) =(Pleurococcus superbus Cienk; Glococytis infusionum W.& G.S.West) The only British species, it has large cells, single or in groups up to 8, 25-30 microns in diameter, and a striking lamellated mucilage envelope often reaching 180 microns in diameter the concentric rings of which first attracted me to the species. (It has a nice species name I can actually pronounce, I am still looking for another, the desmid Cosmarium superbum so far only recorded from a few sites in North America, Prescott et.al.). The rings can often be seen under the microscope in normal transmitted light, but not as clearly defined as shown in Fig.2. drawn from life the rings have been over emphasised for reproduction in this article. With some specimens you may find the only way the rings can be seen is by staining or using phase contrast. With my phase 'DM' type the rings appear white on a grey background.
I experimented by adding diluted black Indian ink to a sample on a slide, the mucilage does not take up the ink, thus showing up white on a dark grey background. To a specimen on another slide I added Alcian blue. The mucilage, invisible or almost so, that surrounds or is extruded by most desmids will absorb this stain and be revealed. The Astrococcus mucilage must be different or it has a skin or pellicle preventing the stain being absorbed, only the outer ring stained and became easier to see.
To yet another slide I added Lugols iodine diluted to the colour of pale sherry, (sometimes used as an algae preservative). This must have penetrated the mucilage envelope, because the chloroplast of the cell turned almost black, deep purple usually indicates the presence of starch, the mucilage was not coloured by the iodine. NOTE the iodine, stains etc. were added by placing a few drops around the cover glass, these spread through the sample by capillary attraction.
Finally I tried to see the starch grains using polarised light and a 100:1 oil immersion objective. This failed, probably because the lamp a 6 volt 15 watt; ordinary tungsten not halogen on my microscope, a Nikon Skt. is not powerful enough to cope with crossed polars at this high magnification.
West & Fritsch state this organism has an eye spot and a motile phase although neither was seen during my observation.
This article first appeared in Balsam Post The Newsletter of the Postal Microscopical Society (British) 1992. It has been revised for the W.W.W.
Thanks are due to Mr Alan Joyce for the samples from Scotland, and to Dr J.W.Lund CBE.DSc.CBiol.FRS. of The Freshwater Biological Association, Windermere, England, for his information on Algae records and figures from the Fritsch Collection of Algal Illustrations.
Prescott G.W. (1954) How to Know the Freshwater Algae. Wm.C.Brown Co.Dubuque, Iowa.
Prescott G.W. Croasdale H.T. Vinyard W.C. & Bicudo C.De M. (1981) A Synopsis of North American Desmids Pt. 2. Section 3. University of Nebraska Press.
West G.S. (1904) revised Fritsch F.E. (1927) A Treatise on the British Freshwater Algae. Cambridge University Press. London.
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