A few algae lift my soul when I see them under the microscope through their beauty. To see such intricate yet symmetrical organisation in something too small to be visible with the naked eye never ceases to amaze and delight me. One of the genera that has that effect is the green alga Pediastrum, which forms cog-like colonies: flat plates of cells whose outer members bear horn-like projections. One of its representatives, Pediastrum boryanum, was common in the River Wear when I visited recently (see previous post). You can see, from the illustration above (the scale bar is 20 micrometres – 1/50th of a millimetre – long), the characteristic disc-like arrangement of cells, always in multiples of four (there are 16 in the colony above). There are many species of Pediastrum, differing in the shape of both the inner and marginal cells, and the number and length of the horns.
I have found Pediastrum on many occasions in the Wear in the past, but never quite as abundant as it was in my most recent samples. Pediastrum boryanum is the species I find most often, here and elsewhere, but other species occur too. I have also found Pediastrum in some unusual places, including deep in lake sediments when I was searching for fossil pollen grains and there is evidence that the cell walls of Pediastrum contain both silica and a sporopollenin-like material (sporopollenin is the extremely tough material found in the outer walls of pollen grains (which probably explains why it had survived the fierce mix of chemicals that we used to prepare the lake sediments for pollen analysis). I am guessing that the sporopollenin and silica both add some structural integrity to the cells. There are references in the literature to Pediastrum being planktonic but I often find it in samples from submerged surfaces and associated with submerged macrophytes, so I suspect that it is one of those species that moves between different types of habitat. It should not really be a surprise that a relatively large colonial alga with a payload of silica and sporopollenin in addition to the usual cellulose cell wall, is going to be common in benthic films in a river towards the end of a long, dry summer.
Pediastrum is another genus that has been shaken up in recent years as a result of molecular studies. According to these, Pediastrum boryanum should now be called Pseudopediastrum boryanum although the Freshwater Algal Flora of the British Isles continues to use the old name. Not everyone agrees with this split (see McManus and Lewis’ paper in the list below) but the divisions suggested by molecular data do also seem to match differences in morphological characteristics of the group (see Table below).
Pediastrum is part of the family Hydrodictyaceae and, as I was writing this, it occurred to me that I have never written about another interesting member of this family, Hydrodictyon reticulatum. As I like to accompany my posts with my own photographs, I spent part of yesterday afternoon tramping around a location where I have found Hydrodictyon in the past. All I got for my troubles, however, was two damp feet, so that post will have to wait for another day.
Differentiating characteristics of Pediastrum and similar genera (after Krienitz & Bock, 2011).
| Genus | Features |
| Pediastrum | Flat coenobia with intercellular spaces, marginal cells with two lobes |
| Lacunastrum | Flat coenobia with large intercellular spaces, marginal cells with two lobes |
| Monactinus | Flat coenobia with large intercellular spaces, marginal cells with one lobes |
| Parapediastrum | Flat coenobia with intercellular spaces, marginal cells with two lobes, each divided into two projections |
| Pseudopediastrum | Flat coenobia without intercellular spaces, marginal cells with two lobes, each with a single projection |
| Sorastrum | Three-dimensional coenobia, each cell with two or four projections. |
| Stauridium | Flat coenobia without intercellular spaces, marginal cells “trapezoid”* with deep incision to create two lobes, each with a concave surface, though the lobes are not really extended into “projections” |
* not all of the illustrations show marginal cells that are strictly “trapezoid” (e.g. with at least one pair of parallel sides).
References
Buchheim, M., Buchheim, J., Carlson, T., Braband, A., Hepperle, D., Krienitz, L., Wolf, M. & Hegewald, E. (2005). Phylogeny of the Hydrodictyaceae (Chlorophyceae): inferences from rDNA data. Journal of Phycology 41: 1039-104.
Good, B.H. & Chapman, R.L. (1978). The ultrastructure of Phycopeltis (Chroolepidaceae: Chlorophyta). I. Sporopollenin in the cell walls. American Journal of Botany 65: 27-33.
Jena, M., Bock, C., Behera, C., Adhikary, S.P. & Krienitz, L. (2014). Strain survey on three continents confirms the polyphyly of the genus Pediastrum (Hydrodictyaceae, Chlorophyceae). Fottea, Olomouc 14: 63-76.
Krienitz, L. & Bock, C. (2012). Present state of the systematics of planktonic coccoid green algae of inland waters. Hydrobiologia 698: 295-326.
McManus, H.A. & Lewis, L.A. (2011). Molecular phylogenetic relationships in the freshwater family Hydrodictyaceae (Sphaaeropleales, Chlorophyceae), with an emphasis on Pediastrum duplex. Journal of Phycology 47: 152-163.
Millington, W.F. & Gawlik, S.R. (1967). Silica in the wall of Pediastrum. Nature (London) 216: 68.
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