Discussion of Lichen Biology (Nash 2008)
Chapter 1: Introduction (T.H. Nash)
Since this is my personal discussion and notes from the introduction to Lichen Biology, I’ll briefly introduce you, the reader of this website, to the lichen.
Firstly, lets start with definitions. What are lichens? They are symbiotic organisms, and there are between 13,500 and 17,000 estimated species worldwide. At a most basic level lichen are composed of 1) a fungal partner, called the mycobiont, and 2) a photosynthetic partner, called the photobiont. The photobiont is either a green algae (Plant Kingdom) and/or a cyanobacterium (Bacteria Kingdom). And a study last year by researchers out in Colorado shows that there is an additional symbiont within the lichen: species specific communities of bacteria that are distinct from the surrounding soil — but these bacteria are not photobionts or mycobionts, their role in the symbiosis has yet to be discovered!
The Lichen as an Emergent Property
The symbiosis between the mycobiont and the photobiont creates an organism that is more than the sum of its parts, in other words, a lichen is an emergent property. Lets take a step back to examine this statement. On the one hand, neither the photobiont nor the mycobiont can withstand intense UV radiation, dessication, or extreme temperatures. But on the other hand, when the photobiont and mycobiont work together within the context of the lichen symbiosis, they create an organism that can withstand living in outer space – thats more extreme temperature and radiation (not to mention vacuum exposure) than is experienced on Earth! Lichen can even grow within rocks (endolithic lichen)! These are conditions that would kill a fungus or algae — but a lichen is not just a mere sum of its parts, it is much more than that, and scientists are just starting to scratch the surface of what makes this symbiosis work so successfully.
The body created by the symbiosis is called a thallus, and it is a discrete entity. From the perspective of evolution and genetics, there are some issues with calling a lichen “an individual.” We’ll get more into these intriguing issues, but basically the taxonomy of lichen is currently based upon the mycobiont. So when you see the lichen genus and species name, such as Letharia vulpina, that is the name of the fungus. What about the photobiont? Well, the photobiont of Letharia vulpina is a green algae, and this particular algae is relatively ignored by the current system of lichen taxonomy. Such current taxonomical disregard, or “mycocentric bias” as my favorite lichenologist Trevor Goward would say, is not representative of the actual role that the photobiont plays in the symbiosis –in actuality, the photobiont plays a huge role not just in the function of the lichen, but in the morphological characteristics too.
The too-often Ignored Photobiont
Lets dig into the huge role that the photobiont plays in the lichen symbiosis. First off, as far as current research indicates, the photobiont creates all the food for the lichen symbiosis (green algal lichens create polyol carbohydrates, cyanolichens create glucose and fixed nitrogen). There is not yet any evidence showing that the mycobiont gathers substantial nutrients from the substrate. Secondly, the morphology of the lichen thallus has been proven to be very much determined by the photobiont: photosymbiodemes are lichens that look totally different even though they are composed of the same fungus, but a different photobiont. Nash mentions that the occurrence of photosymbiodemes “implies ontogenetic control by the photobiont” (! exclamation !). He refers to a paper by Armaleo and Clerc 1990: “Lichen chimeras: DNA analysis suggests that one fungus forms two morphotypes” in Experimental Mycology. Despite the large roles that the photobiont plays in the lichen symbiosis, there exists very little research on the photobionts. Surprisingly, we hardly even know the photobiont found in most species of lichen. Nash explains that this is “because the systematics at the species level of many cyanobacteria and unicellular green algae is not well resolved” (p.3) But despite this lack of knowledge, there is considerable research indicating that most lichen are highly specific in which photobiont is chosen – Nash cites Beck et al 1998 and Rambold et al 1998.
And now the mycobiont
Although the mycobiont gets most of the attention within lichen taxonomy, they too are given the shaft (my opinion, not necessarily Nash’s). In most literature, the lichen symbiosis is described as a controlled parasitism, whereby the mycobiont is receiving more than its fair share from the photobiont. But in defense of the fungi, the evidence indicates that this claim is simply unfair.
Lets list them off: 1)The mycobiont creates over 95% of the lichen thallus, i.e. builds and maintains and is the living house for the algae. 2) The mycobiont creates the chemicals that make the lichen unpalatable and/or poisonous to herbivores and decomposers. 3) The fungus also may be responsible for enhancing water uptake by lowering the water potential of the thallus, essentially assisting the lichen to pull in additional water from the surrounding air – Nash makes an interesting comment that this is the opposite of a vascular plant system. 4) The fungus creates a protective layer that shields the algae and cyanobacteria from the destructive forces of UV radiation. And 5) as far as nutrition goes, there is some evidence that the fungus may help with the recycling of nitrogen and phosphorus (see Hyvarinen and Criftenden 2000; Ellis et al. 2005). Nash proposes that perhaps the haustoria (the little fingers of the fungi that probe into the cells of plants) are not only taking carbohydrates from the photobiont, but is also giving it inorganic nutrients in a similar manner as the mycorrhizal fungi give nutrients to a plant root in exchange for sugars.
And just to help close the lid on the idea that the mycobiont is a mere parasite, the green algae Trebouxia spp., which is found in 20% of lichen, is rarely found free living. This introduces very interesting questions about the reproduction and genetic diversity of Trevouxia that we will discuss later.
Lichens as an Ecosystem
Need I say more than what has been said above? Lets just add in that fungi breath in oxygen and breathe out carbon dioxide, and algae/cyanobacteria breathe in carbon dioxide and breathe out oxygen. They are their own little Earthships, now how cool is that?!