Algae of the World Unite—You Have Nothing to Lose But Your Hyphae!
An old pick-up truck sprouts grey growth on its roof, oaks gently toss pale green threads in the foggy wind, the high rising cliffs of Pinnacles National Monument are covered in orange and yellow crusts, and the rocks along the coast are painted black: lichens, lichens, lichens everywhere. From the high mountaintops and the snow-covered plains of the Antarctic, from the depths of the tropical rain forest to the immensely hot dry desert sands, on rocks and even inside them, you will find lichens. Within each lichen is a fungus. Indeed, lichens are a combination of a fungus with something else, but the combination is more than the sum of its parts.
A huge variety of fungi, mainly Ascomycetes, can form lichens together with another diverse group of photosynthesizing organisms. While green algae are the partners most commonly encountered, cyanobacteria (formerly called blue algae), or brown or golden algae, can also be encapsulated by fungi to form a different organism than the two parts grown separately. This theme has even more variations: one lichen can be made up of a fungus, a green alga and a cyanobacterium species, a ménage à trois; or, one fungal species can do it with different algal species in different parts of its geographical range; and, the same fungus can have either a green alga or a cyanobacterium as partner. There are about 14,000 fungal species involved and slightly more than 200 photobionts (the general word for the photosynthesizing partner) are known, but this latter number might enormously be underestimated. The fungi cannot maintain a separate existence, but their partners are often found growing on their own.
So, what's the deal? The photobiont makes sugars from carbon dioxide and water, a process for which sunlight delivers the energy and chlorophyll provides the catalyst. The fungus takes these sugars from the photobiont, in various ways; it will even send out its hyphae to penetrate the algal cells. The fungus provides the structure in which this happens, and the water necessary for the photosynthesis. The photobiont lives inside and gets some protection against predators and UV light. Many lichens look like leaves, with the photobionts in the same sheltered position as the chlorophyll in a leaf. If the photobionts were on the surface they would easily be damaged by too much light, especially UV light. The fungi are good at making secondary compounds, called lichen substances or lichen acids, which are often the bright pigments we find so characteristic of lichens and provide subtle colours for dyers. The orange parietin and dark melanin are such pigments, which act as sunblock for the photobiont.
Water is an essential for photosynthesis, and can be taken by many fungi from the air—hence the growth of lichens in fog and spray zones. Nevertheless, without water these organisms are able to survive for month after month. High and also low temperatures can be endured in dry conditions, while with water they would either be cooked or frozen to death. However, the lichens that are found in dry hot areas are different from the ones in cold wet or cold dry places, and from the ones on the leaves of trees in the tropical rain forests.
Every generation the symbiosis has to be remade. Lichens often form little packages containing both fungal and algal material; fragmentation is another way of starting a new organism, but these are vegetative ways of reproduction. The fungus does form fruitbodies, just like the free-living Ascomycetes. The fungal spores have to germinate and find photobiontic partners in the wild. Mites may play a role in facilitating this: they crawl over the surface of the lichen, eating their way through it, both spores and algal cells, and everything ends up in their droppings, ready and viable to start a new life together.
The lichen symbiosis was always considered as the ultimate mutualistic way of life: a harmonic and idyllic life style in which both partners profit and none suffers, a welfare state in a nutshell (sometimes literally). Present day lichenologists describe it much more as a capitalistic society, in which the fungus is the only one making a profit and doing it on the backs of the poor algae and cyanobacteria. These cannot escape, their growth and division rates are highly controlled and they can forget about sex. The fungi, of course, do produce sexually, and they grow as far as they can, albeit often quite slowly.
This profit making goes far. Lichens with both a green alga and a cyanobacterium make the most of both. The green algae deliver the sugar and the cyanobacteria are forced to produce more of their nitrogen fixing cells to churn out nitrogen for the fungi. Nitrogen is a limiting factor for fungal growth (it is an important substance of fungal cell walls), and also necessary for cell maintenance. It is thought that the above-mentioned lichen substances are formed because the fungus has too much carbon and too little nitrogen, and has to do something creative with the sugars. This symbiotic relationship originated not just once in the history of life on earth, but several times, in independent events. Recently it a saprotrophic life to being a parasite or a symbiont. Options remain open; everything is possible.
And that seems to be the theme of the lichens: everything is possible, all combinations of players, all kinds of forms and shapes, and an incredibly wide range of habitats and places.
An eye-opening book to get a taste for lichens is:
- Brodo, I.M., S.D. Sharnoff & S. Sharnoff, 2001. Lichens of North America. Yale University Press, New Haven & London. 795 pp.
And some related websites: