A hitchhiker’s guide to algae …

One of the recurring themes of this blog is the hidden delights of natural history for anyone prepared to take a closer look at unprepossessing locations, so it is appropriate that we have found some quite rich habitats within walking distance of our home in County Durham.   I’ve written before about visits to Crowtrees, a local nature reserve (see “More pleasures in my own backyard” and “Natural lenses”) and Heather is also writing a series of posts about the ever-changing flora of this small vale at the foot of the Permian limestone escarpment (see “Crowtrees LNR July 2018 part 2: gentians to grasses” for the most recent and links back to previous ones).   I visited again last week, taking Brian Whitton along for company.

His interest was the red alga Chroothece ricteriana, which I described in one of my earlier posts about Crowtrees but we did not find it on this particular visit.   Instead, my eye was drawn to soft clouds of green filaments that floated just above the bed of the pond.   When I looked closely under my microscope, I saw that these were thin filaments of Oedogonium.  Typically, these had no reproductive organs, so cannot be named (see “Love and sex in a tufa-forming stream” for a rare exception), but all showed characteristic “cap cells” (see lower illustration).

Growths of Oedogonium in Crowtrees pond, August 2018.   The frame width is about 30 centimetres.   The photograph at the top of the post shows Brian Whitton searching for algae during our visit.

The diatom Achnanthidium minutissimum was growing on small stalks attached to the Oedogonium filaments, often alone but also in pairs and stacks of four, as the diatom cells divided and re-divided.  Oedogonium is a rougher alga to the touch than filamentous genera such as Draparnaldia, Stigeoclonium and Spirogyra, and often carries epiphytes, and I presume the lack of mucilage is a factor in this.   Achnanthidium minutissimum is a diatom that is very common on the upper surface of submerged stones in both lakes and rivers, but it is not fussy and I often see it as an epiphyte if conditions are right.  In this case, I suspect that the very hard water of Crowtrees Pond is a factor: calcium carbonate is constantly being precipitated from the water to create a thin layer of “marl” (see photo in “Pleasures in my own backyard”).   This makes life difficult for a tiny diatom that cannot move, so hitch-hiking a ride on the back of a filamentous alga that floats about the lake bottom makes a lot more sense.

Oedogonium filaments with epiphytic Achnanthidium minutissimum, from Crowtrees pond, August 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Oedogonium is an adaptable genus.  It is also common in the River Ehen (soft water, low nutrients) and I also find it in lowland polluted rivers too.  Being able to name the species would, I am sure, help us to better understand the ecology but this is, as I have already mentioned, problematic (see “The perplexing case of the celibate alga”).   However, in each of the cases I’ve mentioned, the epiphytes are different (Achnanthidium minutissimum here, Tabellaria flocculosa and Fragilaria species in the Ehen, Rhoicosphenia and Cocconeis placentula in enriched lowland rivers) and I suspect that these might offer an easier way to interpret the habitat than the filaments themselves, at least until someone finds a stress-free way of naming them.

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