Poking around amongst sheep’s droppings …

A couple of kilometres away from the stream featured in the previous post is an old quarry that we visit each year as part of this course (see “Nosing around for blue-green algae …”).   In a damp flush at the edge of the quarry floor, we found some patches of what looked, from a distance, like sheep droppings.   A useful strategy, shared by most of the human race, is to assume that anything that looks like a sheep’s dropping probably is a sheep’s dropping, and not to go prodding at this with a finger.   However, a curious soul in the distant past with a sense of adventure ignored this precept and discovered that a few of these were, in fact, growths of Cyanobacteria.   Most natural historians, wisely, focus their attention on more spectacular aspects of life on earth; however, a few of us have retained this childish instinct to poke at anything that looks like sheep’s droppings.

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Scytonema sp. from a flush at Whitbarrow Quarry, Cumbria, May 2015

A small part of one of these growths, teased out and mounted on a cover slip, reveals itself to have the characteristics of the genus Scytonema although, today and despite a long hunt, I could not find any filaments that presented themselves in a suitable position to photograph. The illustration below, therefore, is of a growth of Scytonema from another calcareous site in Cumbria. The Cyanobacteria is, you may remember, the modern name for the “blue-green algae” which is often confusing as many Cyanobacteria are not blue-green in colour.   What we can see here is a chain of cells (a “trichome”) which are surrounded by a thick sheath (“trichome” plus “sheath” equals “filament”, in Cyanobacteriological lore).   The sheath is a yellow-brown colour, due to a pigment called “scytonemin” which acts as a sunscreen, absorbing ultra-violet radiation and, in the process, obscuring the blue-green colour of the trichome within.

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Scytonema sp. from a calcareous flush at Sunbiggin Tarn, May 2005.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

Two other characteristics of Scytonema are visible in the photograph.   Both the left and right hand pictures shows “false branches”: if the trichome breaks for any reason, either or both of the broken ends keep growing and break out of the filament. The left hand illustration is a single false branch and, just to the left of the branch you can see the distinct “heterocyst”, a cell where nitrogen fixation takes place.   The wall of the heterocyst is thicker than that of other cells, as nitrogen fixation can only occur in the absence of oxygen.

Walking back from the quarry towards the minibus, Allan pointed up at dark patches on the cliffs looming over us.   It was Gloeocapsa alpina, the same species that we met a short while ago in a cave on Malta (see “The mysteries of Clapham Junction …”).   The cliffs at Whitbarrow are, in effect, a vertical “desert” from the point of view of any organism that aspires to live there. These Cyanobacteria, with their ability to “re-boot” on those occasions when conditions are favourable for growth, have an advantage here.   One guesses that the damp climate of north-west England is slightly more forgiving than that of a Mediterranean hillside but it is still a tough habitat in which to survive.

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Growths of Gloeocapsa alpina (arrowed) on the cliffs at Whitbarrow Quarry, May 2015.

Love and sex in a tufa-forming stream …

The reason behind my trip to the Lake District a couple of weeks ago was to teach a short course on identification of freshwater macroalgae with Allan Pentecost (see “Heatwave? What heatwave” and subsequent posts for more about last year’s course).   One of the sites we visit with the students is a small stream flowing off Whitbarrow, a Carboniferous limestone outcrop in southern Cumbria.   The bed of the stream is covered with tufa, formed from calcium carbonate precipitated from the water. We bring the students here because there is usually a good variety of cyanobacteria for them to learn to recognise in the field and to sample for later investigation in the laboratory.   Amongst these cyanobacterial growths, however, we also saw a few patches of green filaments on the stream bed, which we also took back with us.

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Sampling Whitbarrow tufa stream in May 2015.

These filaments turned out to be growths of the green alga Oedogonium. You may remember that I wrote a post last year with the title “The perplexing case of the celibate alga …” in which I commented that Oedogonium, though a common genus in freshwaters, is difficult to identify to species because this requires the reproductive organs which are rarely seen in the wild.

Our population of Oedogonium, however, was fertile, and this enabled us (Allan, to be strictly honest, as he knows the algae of tufa-forming streams extremely well) to name it.   The images below show the distinctive swollen oogonia within filaments of narrow cells (compare these with the much broader cells observed in “A case of mistaken identity?”). These oogonia look as if they have already fused with the male antheridia to form zygotes, which will eventually be released. These zygotes can lie dormant for a long time, which makes sexual reproduction a useful technique for overcoming adverse conditions (see also: “The River Ehen in March”). Not very romantic, I know, but that’s the reality of life at the unprepossessing end of biodiversity.

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Oedogonium calcareum from Whitbarrow tufa stream, May 2015, showing oogonium. Arrows indicate position of “caps” (scar tissue from intercalary cell division) a. scale bar: 20 micrometres (= 1/50th of a millimetre); b. & c.: scale bar: 10 micrometres (= 1/100th of a millimetre).

More about red algae

One other alga that we saw at Burn Head (the location near Whitbarrow Quarry) was tucked away in a shaded area close to where the spring bubbled out from the base of a limestone cliff. At first glance, this was barely recognisable as a plant, as it looked more like a splash of red paint on a rock. It is, however, a thin crust of red algal cells, called Hildenbrandia rivularis. This is the only freshwater representative of the genus, although other species can be found on the seashore. Under the microscope, you can see polygonal cells, though we are actually looking here at the top of a short stack of cells.

I generally associate Hildenbrandia with good ecological conditions although, as for Batrachospermum, there are exceptions, as I have seen it growing at quite high nutrient concentrations in chalk streams and, I daresay, it thrives in enriched waters elsewhere. Here it was growing in very shaded conditions, and I have also seen it growing under quite thick patches of moss, which must also have trapped much of the light. However, I often see it growing in shallow, well-lit places as well.

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Hildenbrandia rivularis from Burn Head, southern Cumbria, May 2014.

So why are some red algae red and others not? The answer to this question lies in the pigments that they contain. Red algae, like Cyanobacteria, contain chlorophyll a (the common green pigment), plus two protein-based pigments, phycoerythrin (red) and phycocyanin (blue). The balance of these two pigments influences the final colour of the organism: those with more phycyoerythrin tend to be red; those with more phycocyanin have a blue-green or grey-green colour. Red-coloured algae have an advantage in deep water as it can absorb those wavelengths of light that penetrate furthest. Being able to absorb over a broader range of the light spectrum than would be possible if it just had green chlorophyll means that a plant is able to use the limited light more efficiently. Why we find some red-coloured algae in shallow, freshwater situations is a mystery. It may simply reflect the evolutionary history of the genera concerned. It may be significant that two of the reddest freshwater red algae (see also “At last … a red alga that really is red”) both come from genera that can be found in both freshwater and marine locations whereas the two olive-green genera we’ve met (Lemanea and Batrachospermum) are found exclusively in freshwaters.

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Looking down on a crust of Hildenbrandia rivularis, showing the tops of the polygonal cells. Photograph: Chris Carter.

Algae … cunningly disguised as frog spawn?

A couple of kilometres from Whitbarrow Quarry there is a spring that we always visit during the “Introducing Macroalgae” course because it usually yields a range of larger algae that we like to ensure that all the students can recognise. One of these forms tufts of filaments that are very slippery to the touch. There is a slight resemblance to frog spawn in both appearance and texture: under a hand lens the filaments can be seen to have a beaded appearance and this plus the texture creates a superficial resemblance to frog spawn. It is, in fact, another red alga, Batrachospermum which, like Lemanea (see “Lemanea in the River Ehen”) has an olive-green rather than red colour. I’ll explain more about that in the next post. I have also included one of Chris Carter’s photographs to show the structrure of Batrachospermum at higher magnification: the “beads” are composed of tufts of branchlets arising from a central filament.

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Left hand image: Batrachospermum sp. growing at Burn Head, near Whitbarrow in Cumbria; right-hand image: filaments of Batrachospermum in the palm of my hand. Each of the “beads” is about half a millimetre across.

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Batrachospermum sp. from Bodmin, Cornwall. Photograph by Chris Carter

I usually associate Batrachospermum with healthy ecological conditions: low nutrients, clear, cool water and diverse invertebrate communities. However, when I told the group on our course this, one of the participants said that he sometimes found it in quite polluted conditions. Interestingly, the same thing happened on a presentation of the course a few years ago and both the contradictory examples were from chalk streams in southern England. I went back to the published literature to reassure myself and, sure enough, these also referred to Batrachospermum as a species associated with good ecological conditions. There must be, however, some rare combination of conditions that enables Batrachospermum to occasionally proliferate in very enriched conditions. What we have, I suspect, is a common situation in ecology: we base our inferences about preferences on statistics rather than ecophysiology. This means that we assume that an association between a genus or species and a set of environmental conditions represents the realised niche of the species, without always understanding the nuances of ecology and physiology that determine these niches.

Next time: a red alga that really is red.