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.

Scytonema_Whitbarrow_May15

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.

Scytonema_Sunbiggin_Tarn

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.

Gloeocapsa_alpina_Whitbarro

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

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It takes two to tango …

One of the striking features of my recent introduction to lichens at Malham was the amount of algae that we saw in terrestrial habitats. I’ve written twice over the last few days about Trentepohlia, but there were others, including several patches of dark jelly-like material on vertical limestone faces This a unicellular species of blue-green alga (Cyanobacteria), probably Gloeocapsa sp., which we met last year (“More reflections from the dawn of time …”). The individual cells of this species are set in mucilaginous matrix and, looking at these patches, I saw, perhaps, how the lichen symbiosis may have evolved. The alga secretes mucilage which forms a jelly-like mass which protects the alga against desiccation. Many algae and Cyanobacteria produce mucilage and, indeed, Gloeocapsa is not a genus associated with fungi.  Its proximity to lichens at Malham, however, gives me a starting point for some speculations …

Gloeocapsa_sp_Malham

Gloeocapsa sp. on a vertical limestone face at Malham Tarn field centre. Right hand image shows the jelly-like masses in close-up (scale bar: 1 centimetre).

As the mucilage that algae produce is composed largely of carbohydrates, it is a potential source of energy for other organisms. So we could envisage a proto-lichen in which fungal hyphae grew into the mucilage produced by an alga that was already adapted to living in damp, if not fully terrestrial habitats. The fungus can utilise the algal carbohydrate as a source of energy but for a symbiosis to evolve, both partners must gain from the relationship. For a semi-terrestrial alga, maybe, the capillary action that a network of fungal hyphae would create is one further protection against the evaporation of water, balancing the loss of the carbohydrate that the alga has “donated” to the fungus. As the relationship evolves, so the fungi become preferentially located at the periphery of the algal mass, adding shade to the benefits received by the alga (and reducing the need for the alga to invest in the energetically-expensive production of “sun tan” compounds that we see in Trentepohlia. Our proto-lichen can now move into less damp and shaded environments than those where we find Gloeocapsa today.

Gloeocapsa_sp_Malham_#2

Gloeocapsa sp.: the gelatinous growths from the previous photograph shown at high magnification. Scale bar: 10 micrometres (100th of a millimetre).

All this makes sense up to a point. Except that the gelatinous material surrounding Gloeocapsa clearly has no fungi hyphae “borrowing” the energy that Gloeocapsa has won from the sun. The relationship between algae and fungi is clearly more complicated than I have just suggested, with evidence of specialised fungal filaments called ‘haustonia’ penetrating into the algal cells. All I am trying to do here is suggest a starting point. As the earliest lichens are recorded from the Devonian era, 400 million years ago, there has been a lot of time for the relationship between the two partners to evolve. But why has Gloeocapsa stayed immune to the advances of fungi? I have no idea, but it would be interesting to see if the mucilage produced by genera such as this has any anti-fungal properties. Gloeocapsa and relatives have also been around for a long time and they, too, will have had plenty of time to devise means of fighting off wandering hyphae.

Reference

Taylor, T.N., Hass, H., Remy, W. & Kerp, H. (1995). The oldest fossil lichen. Nature (London) 378: 244.