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