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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More pleasures in my own backyard

Back in early July I wrote about a visit to a pond in a local nature reserve (see “Pleasures in my own backyard”) and ended with the hint that there was one other abundant alga there that I was unable to name at the time.  I was reticent about naming it, as it seemed to be a rare alga and the habitat where I had found it did not match the locations where it had been found to live.

I’ve now shown it to Brian Whitton and he has joined me on another excursion to the same pond, and I can confirm that it is, in fact, Chroothece richteriana, a freshwater red alga.   We’ve met (and even eaten) red algae several times over the lifetime of this blog (see “More from the Lemanea cookbook …”), but Chroothece is different in that it does not form filaments or thalli, but lives in mucilaginous masses.   The individual cells, each of which are ovoid, with a single star-shaped chloroplast, live embedded within this mass.

Chroothece_Crowtrees_July16

A colony of Chroothece richteriana growing on marl-encrusted rocks from Crowtrees Pond, County Durham, July 2016.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

This is a species that was, until recently, known only from two very old records. However, searches over the past few years have found it growing at a number of different locations.  There are now half a dozen locations in the UK, plus one in the Isle of Man.   Interestingly, the population at Crowtrees matches these other records in respect to the underlying geology – limestone – which yields very hard water, but differs in being permanently submerged.  The other records are from seepages and other semi-aerial habitats.   The population at Crowtrees formed a thin film that was firm to the touch due to the deposition of calcite crystals within the matrix.   There were also some cyanobacterial filaments mixed in amongst the Chroothece, as well as the diatoms that I mentioned in the previous post.  I suspect that the snails that I observed on my earlier visit were scraping up a mixture of all these species from the thin surface layer that had not yet had time to become hardened by calcite crystals.

One theory for the success of Chroothece here is that habitats such as this are naturally low in phosphorus, an essential nutrient that is naturally scarce but which is relatively insoluble and consequently is precipitated out of the water along with the calcite.   Studies in Spain (in a river, rather than a seepage or pond) showed that Chroothece shares the characteristic of several other algae from this type of habitat, of producing enzymes that can scavenge phosphorus from tiny particles that are suspended in the water.  The enzymes are thought to be concentrated in the mass of mucilage (which is actually formed from the organism’s stalks)

Ironically, our excursions to Crowtrees Nature Reserve have become more frequent over the past year or so as our usual running and walking beats in the countryside around Bowburn have been changed as a local quarry expands its activity (seen in the gouge in the skyline in the picture below).  The pond, itself, looks natural, but local drainage is strongly influenced by mining and quarrying.  The area around here, especially associated with the Permian limestone, abounds in nature.   But whether or not this nature is natural is a topic for another day …

view_from_Crowtrees_July16

The view from Crowtrees Nature Reserve towards the Tarmac quarry, July 2016.

References

Aboal, M., García-Fernández, M.E., Roldán, M. & Whitton, B.A. (2014).  Ecology, morphology and physiology of Chroothece richteriana (Rhodophyta, Stylonematophyceae) in the highly calcareous Río Chícamo, south-east Spain.  European Journal of Phycology 49: 83-96.

Pentecost, A., Whitton, B.A. & Carter, C.F. (2013).  Ecology and morphology of the freshwater red alga Chroothece in the British Isles.  Algological Studies 143: 51-63.

Pleasures in my own backyard

Crowtrees_pond_July16

One of the delights of my part of County Durham is the range of natural history that is available without the need to travel great distances.  That, indeed, has been the theme of this blog right from the start (see “Cassop”) and today’s post continues the theme of nature on my doorstep, with a visit to a local nature reserve within walking distance of my house.  Like Cassop Pond, it is at the foot of the Magnesian Limestone escarpment and, at this time of year, the grassland is rich with Northern marsh and Common spotted orchids.   It is, of course, the ponds that draw my attention: they are rich in aquatic plants including, once I start to look closely, beds of the alga Chara, which I’ve written about before (most recently in “Everything is connected …”).  And then, once my eyes are adjusted to looking at natural history at this more intimate scale, I can see that the stones on the bottom of the pond are covered with tiny snails (probably Hydrobiidae) with shells coiled in the shape of an ice-cream cornet.  Freshwater snails crawl across submerged surfaces rasping off attached algae with their tough radula so I started to wonder what snails in this particular pond might be feeding upon.

Crowtrees_stones_July16

Submerged stone from the pond at Crowtrees Nature Reserve, County Durham, covered in Hydrobiidae snails (left: the stone is about 10 cm across) and (right) a stone removed from the bottom of the pond showing the marl-covered part that was exposed and the marl-free part that was buried in the sediment. 

Viewed from just above the water, the surface of the stone looked as if it could be an algal film but, when I picked it up, the stone did not have the yielding texture that I associate with such films, but was a hard, mineral-rich marl.  More intriguingly, it was only present on the exposed surfaces, possibly, I suspect, due to the subtle interactions between chemistry and biology that I wrote about in “Everything is connected …”.

The calcite crystals make it hard to get a good view of the material under the microscope, but I managed to see a number of diatoms, mostly Gomphonema pumilum, or a relative, but also a good number of tiny, slightly asymmetric cells of a species of Encyonopsis, a genus that was, until recently, included in Cymbella, and which is usually a good indication that the water is about as untainted by human influences as it is possible to get.   It is, however, hard to get a really clear view of these under the microscope as they were scooting around.   With valves that are barely more than a hundredth of a millimetre long, I really needed to use an oil immersion objective to see them clearly, but the calcite crystals on the slide made it almost impossible to get a clear view of the live cells.  Not surprisingly, most of what we know comes from studies of carefully-cleaned preparations of the empty frustules.   Encyonopsis shares with Tyrannosaurus Rex the distinction of being an organism better known dead than alive.   It is rather ironic, given that healthy populations are living so close to my house, but that’s very often the case with diatoms.

There was one other abundant alga living amidst the rock (and, indeed, probably the major food source of the snails), but I am having some problems giving it a name, so a full account of that one will have to wait until another day.

Crowtress_diatoms_July16

Diatoms at Crowtrees Nature Reserve, July 2016: a.-d.: Gomphonema (possibly G. pumilum) in girdle and valve views; e.-g.: Encyonopsis sp.   Scale bar: 10 micrometres (= 1/100th of a millimetre).