Monday, September 16, 2013

Questions about lichens: Form? Function? Biology?

Some years ago, I had the opportunity to take several series of electron micrographs of lichens. I was focusing on a project on the Cladonia lichens of Australasia and thanks to the National Science Foundation and the National Geographic Society I had the chance to explore far and wide, documenting my collections at the Australian National University in Canberra. 

It was an ambitious project. You might even call it audacious. Because work like this hadn't been undertaken with lichens before. The more time I spent with my colleague at ANU, Roger Heady, who was in charge of the electron microscope laboratory, the more we started to detect interesting patterns and processes in the lichen samples I brought to be photographed. Roger was an intrepid worker and he encouraged me to look close, to look closer, and to look a lot. We spent hours getting lost in growth patterns of lichens that were evident at a very small scale, about 100 microns (1/10 of a millimeter).

I published lots of data from that exploratory time, but by no means all of it. The academic lichen community, small and conservative, found it hard to swallow what was obvious to me: lichens attain their shape in a programmed, controlled, and predictable manner. 

Lots of other questions arose as well. I call them "biological questions" because they go beyond the questions of pure form and form-making in lichens. Pursuing lichens in the field I became ever more curious about their ecology. Lichens are part fungus and part photosynthetic organism (algae or photosynthetic bacteria). But very often I would find evidence of the lichen fungus (without its photosynthetic partner) in wood or on leaves. 

The first photo below shows a diffuse lichen body growing on a moss. Is the lichen growing in a purely epiphytic manner (using the moss as a kind of "platform") or have its fungal hyphae invaded the moss body?

The second picture shows part of a lichen colony growing on a rainforest leaf. Since many plants in the rainforest retain their leaves over a number of years, lichens, which are relatively slow growing, can establish there. Again, is this lichen growing epiphytically or have some of its hyphae (elongated cells) penetrated the leaf? The visual evidence for penetration is not particularly strong. But the situation is possible.

I asked these questions because I took several pictures of fungal hyphae, unmistakably belonging to the lichen, in rotting wood and in deposits of leaf litter, shown in the bottom photo. In this photo we see the hyphae ramifying throughout the cellulose-rich substrate. I haven't done experiments to show whether the fungal cells are also digesting the leaf litter, but their abundant presence is demonstrated in these pictures.

As a parting thought I encourage scientists and non-scientists as well to think beyond the boundaries of their discipline. Looking at the lichen "model" as a fungus and its photosynthetic partner, and not allowing any doubt to sneak in, is a too-narrow interpretation. Maybe we don't have the evidence yet, but maybe there are other nutritional modalities that lichens experience, maybe for specialized times during their life cycle. 

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