Note: OK, I got way carried away with this post, going completely overboard with details, graphics, photos and tangents. What can I say except that Lichens are way, way cool.
I’ve come to believe that there are 2 kinds of really common things. The first is things that you notice over and over again, like Starbucks. You see them all over the valley, in every mall, in every shopping plaza. When you got to another city, you leave your city via the airport, and the last thing you see in the airport before you get on your plane is- often as not- Starbucks.
When your plane lands in the other city, and you get off, one of the first things you see in that other airport is Starbucks. And as you leave the airport and travel around that other city in your rental car, you see Starbucks in every shopping plaza all over again.
Tangent: I have Starbucks on my mind because I had the weirdest revelation this weekend. I was out and about with Bird Whisperer and picked up a latte between errands. And as I drank, I realized that I wasn’t enjoying it, and that sometime in the last year or so, I stopped liking espresso-drinks. I still like coffee well enough, but lattes, cappuccinos, macchi-whatevers, I just don’t like them anymore, which is weird, because I’ve been drinking- and enjoying- espresso-drinks for close to 20 years. I don’t know when it happened, it just did. I’m pretty sure Saturday’s latte was my last latte, ever. How weird is that?
Nested Tangent: This is a good place for my latest half-baked “theory”, which is targeted specifically toward single people who are seeking a spouse, and which I call the Theory of Prospective Spousal Difficulty (TPSD.) The TPSD works like this: Single people, when evaluating prospective spouses, consider all kinds of factors- attractiveness, intelligence, sense of humor, kindness, honesty, etc.-and completely ignore the single most important factor to evaluate in someone with whom you’re considering spending the next several decades, which is: How problematic will this person be to live with for the next half-century? The TPSD states that the fastest, most efficient way to evaluate Prospective Spousal Difficulty is to observe their drink order at Starbucks. If the drink order involves more than 3 qualifiers (extra Splenda, non-fat milk, extra-hot/cool, soy/chai/spirit of the Buddha/whatever in the cup) or an exchange of more than 4 sentences, you should not pursue marriage or a long-term relationship with this individual.
Starbucks may seem an extreme example, but there are lots of others- tract homes, golf courses, Geese, Pigeons- they’re super-common and you notice them everywhere you go.
But the other kind of Really Common Thing is the kind that’s all around you, all over the place, but somehow you pretty much never notice it, until one day, you do. And then all of a sudden you can’t go anywhere without noticing it, and you wonder how, for years, for decades, you walked the world and somehow never really saw it.
Lichen is one of that 2nd type of really common things. Once you start looking for it and at it, and start seeing it, it’s everywhere, and you notice it everywhere you go, and scratch your head wondering how you didn’t notice it for so long.
If I had a “wake-up” moment with lichen, it was 4 weeks ago in the Sonoran, climbing Castle Dome Peak with Arizona Steve. We were about 100 yards from the top, and scrambled over a small lip to see this on the North-facing wall in front of us: splotches of soylent-green lichen all over the place. The color was so bright, so powerful, it almost seemed as though the lichen was backlit somehow. For days I couldn’t get that color out of my head, and as I thought about it I started noticing other lichens all over the place- in the Sonoran, back home in the Wasatch, and down in St. George.
Sally at Foothills Fancies was good enough to ID the Castle Dome Peak lichen for me; it appears to be a species of genus Acarospora, possibly or A. staphiana or A. chlorophanum or maybe even Pleopsidium flavum. I lean towards chlorophanum because it’s known for its garish color, visible from miles away, and only seems to grow only on vertical surfaces, for reasons unknown.
I posted last week about Moss, and tried to convey why I think it’s so interesting and cool. But the other thing growing on rocks all over the place, Lichens, are if anything even more fascinating and complex.
Lichens are neither plant, nor fungus, but both. Each “species” of lichen- and there are something like 17,000 world-wide- is actually 2- or sometimes more- species. One of those 2+ species is always a Fungus, and the- or one of the- other(s) is- most of the time- a species of Green Algae. Those last 2 sentences had a lot of qualifiers, and I’ll clear them up in a moment, but first let’s talk about the role of each partner, and then the structure of lichens.
The fungal partner, or technically mycobiont, is the dominant partner in any lichen. It provides the outer structure, the, protection from elements- including wind, desiccation, and UV radiation- and (most of the) color. When you look at lichen, pretty much what you’re seeing is the fungus, in the form what is called a thallus.
The usually-algal partner, or technically photobiont, provides the food. It has chlorophyll and uses photosynthesis to produce sugars, which feed both it and the mycobiont. The relationship is generally described as “mutualistic”, meaning of benefit to both partners, but this is still a subject of debate among lichenologists, as we’ll see in a moment.
Lichens come in all different textures and colors, but usually adhere to one of three different forms. The first, and most common, is called crustose.
Crustose lichens are the most “stuck-on”; they adhere closely to the surface on which they grow, and can actually become embedded in that surface. The top layer of a Crustose lichen is called the cortex, and it’s composed of the Fungus.
The layer below is called the medullary layer, and it consists of specialized fungal hyphae, with the (usually) algal cells up top, directly below the cortex. The medullary hyphae adhere directly to the surface on which the lichen is growing. (For a very basic description of the structure of a Fungus, see this post.)
The second type is foliose, which appear more leafy, and are anchored to their growing surface only at specific points. Foliose lichens have a second cortex, underneath the medullary layer. (For a great foliose pic, check out this recent photo on KB’s blog. It’s the lichen on the upper left of the rock.)
Crustose lichens comprise about 75% of all lichen species worldwide, and they’re far and away the most common in deserts. So this time of year, when I’m mostly down low, most of the lichens I see are crustose.
OK, so now that we know the basic structure, let’s back to the whole “algae-maybe” thing. Most of the time, as in 90%, the photobiont partner is a Green Alga (and more than half the time that Green Alga belongs to a single genus, Trebouxia). But some of the time, it’s actually photosynthetic bacteria, called Cyanobacteria. And other times, it’s some of both, in the same lichen.
Read this part if you don’t know the difference, otherwise skip to next orange heading
So to understand the difference, you need to know the basics of what a Green Alga is. They’re basically really, really simple, really, really basic, plants. So simple in fact, that they’re not classified as plants, though it’s thought that plants evolved from them (or something like them.) When we looked at Mosses, I mentioned how much simpler their structure is than that of vascular plants- no roots, xylem, phloem, etc. Green Algae are simpler still, in that they have no real differentiated cells. The may be single-celled, or multi-cellular (various kelps- pic left- are good examples) but there are no real specialization of cell types.
But simple as Green- or any- Algae is, every algal cell is eukaryotic. Living cells are either eukaryotic or prokaryotic. A eukaryotic cell has “structure.” It has a well-defined nucleus enclosed in a membrane, and smaller, similarly-well-defined smaller structures, or organelles, such as chloroplasts and/or mitochondria, within its body.
All plant, animal and fungus cells are eukaryotic. Every cell in you, your dog, the House Finch at your feeder, the Black Widow in your garage and the grass on your lawn all have the same, clearly recognizable general structure.
But many living cells lack this structure. They have no defined nucleus, and (almost always) no organelle-like bodies within. These cells are prokaryotic, and the best example is bacteria.
There are gazillions of different species of bacteria, and some of these are photosynthetic, and these photosynthetic bacteria are called cyanobacteria. (If you took a biology class before the mid- 1980’s or so you would have heard them called Blue-Green Algae, a terrible name. They’re usually neither blue nor green, and they’re never Algae.)
Tangent: The contrast between a eukaryotic cell and a prokaryotic cell is exactly like the difference between my garage and my next-door neighbor’s garage. My neighbor- let’s call him “Chris”- has a clean, well-ordered, totally structured garage. Every item- skis, tools, kayaks, bikes- is in exactly the right place. My garage on the other hand, has random stuff- bike parts, ski boots, goggles, power tools, pine cones- scattered pretty much all over the place, along with various Black Widow nests and who knows what else. For all I know there could be another family living in there, cooking dinner with my campstove… I’d never know.
Green Algae, like plants, conduct photosynthesis in little organelles called chloroplasts, which appear to have been formerly free-living cyanobacteria that at some point in the distant past were engulfed or absorbed by the eukaryotic algal cells and now form an integral part of those cells. Other chloroplasts appear to have originated as complete Green Algal cells engulfed/absorbed by another eukaryotic cell, and yet other chloroplasts appear to have a thrice-engulfed/absorbed evolutionary past. (I covered this in a bit more detail in this post.)
Tangent: How do biologists know this? The best clue is the number of membranes surrounding the chloroplast; multiple membranes suggest multiple engulfment/absorption events.
Lichens with a cyanobacterial photobiont are called cyanolichens, and often tend toward dark green or black in color.
Back to Lichen
Lichens have different partners and different structures in part because they’ve evolved their symbiosis multiple times, over and over again. What’s interesting about these partnerships is that in the lab scientists can break out the fungal and algal components separately and feed/grow them independently. But oddly, when they try to put them back together, it’s really, really hard to get them to join up again.
Different mycobionts “treat” their algal/photobiont partner differently. Some absorb such a high proportion of the algal nutrients that they actually kill the algae/cyanobacteria, which maintains its presence within the lichen only by continually reproducing. Others take a more modest proportion, allowing the individual photobiont cells to survive. In all cases it seems to be the Fungus in control, and some lichenologists believe the relationship between the 2 is more accurately described as “controlled parasitism”, as in a sense the Fungi have “domesticated” the algae/cyanobacteria.
But the photobiont benefits by surviving in environments- such as deserts and arctic tundra- where it could never survive alone, due to such challenges as harmful radiation (as we’ll see in a moment) or lack of moisture. Lichens are able to absorb water either as liquid or vapor, and in fact many desert species survive exclusively on fog/dew.
Tangent: I can’t resist an analogy to the “controlled parasitism” aspect of the Lichen symbiosis, which is: Me and My White-Collar Job. My White-Collar Job is clearly the Fungal Partner in our relationship; it determines where I am, what I do, and when I do it. But as the Algal Partner, domesticated though I may be, I do derive some significant benefits from our symbiosis, such as living in a nice house and being able to buy spendy bikes and optics, things I would be largely unable to do if I pursued a path as a free-living Alga (i.e. Homelesss Guy Living Under A Bridge.) Huh. Job = Fungus, Me = Algae. OK, not one of my most inspiring analogies…
How Do They Make Baby Lichens?
One of the strangest and most complex things about lichens is how they reproduce. Like many plants, they can do so sexually or asexually, but the mechanisms are completely different.
Here’s the weird thing about sexual reproduction in lichens: only the fungal partner does it. Fungi are divided up into seven divisions based on reproductive morphology. In 98% of lichens, the fungal partner belongs to the division Ascomycota. Ascomycetes have 2 types of reproductive bodies. The first are called apothecia, and these are basically big cups full of spores (technically ascospores.) Apothecia are easy to see. In the photo below I’ve pointed out several on a clump of Lecanora sp. I photographed on Antelope Island.
Side Note: I decided for this post to skip the bizarre and complicated genetic details of fungal sexual reproduction. I may cover it in a future post, but for now I’ll just mention that it’s at least as complicated a process as that described in this post, and that the number 8 plays an important role.
But these organs only produce fungal spores. When those spores are dispersed and start to grow, they must find an algal/ cyanobacterial partner on their own in order to grow up as lichens.
Lichens also reproduce asexually, but they do so in a way no plant or animal (or fungus) does. We’ve seen that plants can reproduce asexually by several different means. Examples include co-opting the existing reproductive machinery- via apomixis in Dandelions or self-pollination in Columbines- root-cloning- as Gambel Oak and Creosote do- and stem-layering- as Bitterbrush and Periwinkle do. But Lichens have 2 different reproductive organs which have evolved specifically to facilitate asexual reproduction, and both of these produce diaspores, which carry specialized fragments of both the mycobiont and photobiont partners.
The two organ types that produce diaspores are called isidia and soredia. Isidia are basically little knobs on the surface of the lichens that easily break off, while soredia are much like little eruptions of diaspores through the surface. Each diaspore is a little lichen-in-a-spore, and I’m unaware of any such comparably well-developed and specialized asexual reproductive schema in either the plant or animal kingdoms.
Out in the field, the cool thing about lichens is that many different types often occur together, and while species identification can be tough-to-impossible for the amateur in many cases, genus-level identification often isn’t that hard with a good guide. Here’s a common, easily recognizable, and very interesting species in Utah: Xanthoria elegans.
X. elegans (pic left) is one of the relatively few lichens to have a common name: Elegant Sunburst Lichen (is that a great name or what?) Though there a number of dark orange lichens around here, this is one of the most common in Utah, and easy to spot from a distance. X. elegans has a worldwide distribution, occurring on every continent except… Australia. (You thought I was going to say “Antarctica”, didn’t you? Nope. X. elegans gets along in Antarctica just fine, thank you very much.)
The orange color is produced by a compound called mutatoxanthin, which is a carotenoid produced by the fungal partner. Yes that’s right, a carotenoid- the same class of chemicals we saw that make Autumn leaves yellow and orange, and which provide the color-filtering in the retinal oil droplets in birds’ eyes. You’ve probably noticed that noticed that lichens display many different pigments, and one purpose of these pigments seems to be to filter and optimize the light frequencies that reach the photosynthetic algal layer. Mutatoxanthin appears to play a role in filtering UV light, and X. elegans produces more of this compound- resulting in a deeper, darker orange tone- in areas with high UV exposure.
Tangent: Biologists have documented more than 600 different compounds produced by lichens. For centuries many of these have been used as dyes. More recently a number have been used in developing medicines, specifically antibiotics. And some lichen-produced compounds, including an enzyme in X. elegans, show possible promise in cancer research.
X. elegans was one of the first lichens to be used in lichenometry, a practice used to date the exposure of rock surfaces based on the rates of growth of the lichens growing on them. Patches of lichen spread very slowly- 0.5mm/year in the case of X. elegans, after an initial decade or two to become established. Lichenometry is more or less accurate within about 10% back to 1,000 years or so. It’s most useful for surfaces exposed <500 years, as this is a time period ill-served by radiocarbon dating.
Here’s a picture of a patch of Lecanora sp. with my hand for scale on Antelope Island. I did some kiddie-math and calculated that patch to be around 300 years old, assuming a similar rate of growth to X. elegans. Speaking of dating, Lichens are some of the oldest living things, enduring 10,000 years or more.
So lichens are tough, adaptable, highly evolved, and really old. But the thing that most impresses me about lichen was the thing we started off with- their intimate symbiosis.
Lichens have been around for somewhere between 400 million and 600 million years. But plants and fungi diverged from a common ancestor at least 500 million- maybe over a billion- years earlier. In fact plants are as distantly related to fungi as we are to plants. (Yes that’s right- you’re more closely-related to a yeast infection or Athlete’s Foot than you are to a tree.) And yet after maybe a billion years of separation, they came together to make this seamless, integrated, sophisticated organism. Think about how weird that it is. It’s the equivalent of your cat growing a patch of grass on his back or you having a layer of algae underneath your skin!
Warning: Total Conjecture Ahead
So why is that? Why don’t we have algae growing in or on us to feed us? My best guess is it’s the differing evolutionary “body” paths that animals and fungi followed. Animals of all types have coherent, well-structured bodies, which resist being broken up or re-formed or infiltrated. But Fungi evolved virtual “bodies” of extended hyphae, which spread and grow in all directions, over and through and around soil, wood and rock. This “body flexibility” (the same “dis-incorporated” or “un-embodied” aspect that creeps Jodie out) allows Fungi evolutionary options we lack, which is why we animals never managed to lichenize.
In any case, the whole lichen thing really brings into focus some of the strangest and most marvelous aspects of Fungi, and the radically different approach they’ve followed to multi-cellularity. Fungi have never been something that really interested me in and of themselves. I never set out to learn about or identify them for their own sake. But time and time again over the course this project, I keep bumping into them. Each time I do so, my admiration for them grudgingly grows.
Special Thank You: I’m extremely grateful to Larry St. Clair, Lichenologist and Professor of Botany at Brigham Young University. Larry has assisted me on multiple occasions over the past month with both Lichen and Moss identifications, and I couldn’t have done this post without his help, as well as his excellent guide, A Color Guidebook to Common Rocky Mountain Lichens. I’m also grateful to my friend, mentor and fellow plant-blogger Sally White, who has spent many hours helping me to understand and identify both Lichens and Mosses.