What are Lichens?
Part of being a naturalist is learning to identify and name organisms in the surrounding environment, and the first step to identification is usually being able to classify an organism as a plant, animal, or fungi. But what do we do when the classification isn't immediately clear?
Lichens are a strange group of organisms that don't immediately fall into any of the easily-identifiable categories we're familiar with. That's because they're actually a combination of two organisms from different groups. The structure of a lichen comes from the mycelia (threadlike structures) of fungi, which allows for the wide variety of lichen shapes you can find. While most of us might picture small bits of greenish crust on tree trunks or rocks when we hear the word "lichen" (see below), lichens can also appear powdery, gelatinous (above), leafy, stringy, or even branching. What's most interesting is that the fungi that form half of the lichen partnership are obligated to be there: they usually can't survive alone in the environment without their symbiont.
The other half of a lichen is usually an algae or a cyanobacterium. Though both of these potential partners contain chlorophyll, the pigment(s) required to perform photosynthesis, neither of them are plants! Cyanobacteria (formerly known as blue-green algae, a term that is no longer accurate according to classification rules) are bacteria that can photosynthesize to acquire nutrients like sugar that are necessary for life. Algae are grouped with other protists, and are better thought of as the ancestors of true plants rather than plants themselves. Because of their ability to undergo photosynthesis to create their own food, both cyanobacteria and algae can function independently in their environment. But when they team up with fungi in a lichen structure, something amazing happens.
The algae or cyanobacterium partner in a lichen continues to go through photosynthesis, creating sugar molecules to feed itself and the fungal partner. In return, the fungal partner gathers minerals from the substrate, holds moisture for both partners, and provides the physical structure of the lichen. While this might sound like an incredibly simple trade-off relationship (what we could call "symbiosis"), the sharing that occurs within a lichen allows both partners to survive in a much wider range of environments than they might have access to individually. Lichens can be found in forests and grasslands, and also in more extreme environments like deserts, tundra, and even inside rocks rather than on the surface. The benefits of the lichen partnership allow them to survive dehydration events, extreme temperatures, and other environmental stressors.
You may be familiar with the idea that you can use lichens and mosses to find your way through the woods, because conventional wisdom says that they are much more likely to be growing on the North sides of trees and rocks, where they are shaded from the sun and retain more moisture throughout the day. Unfortunately, there are several things wrong with this theory.
1) The North side of rocks and trees is the shaded side in the Northern hemisphere, but the South
side of rocks and trees is the shaded side in the Southern hemisphere.
2) Other environmental factors might contribute to lichen growth patterns, such as overgrowth
causing shaded areas that aren't on the North-facing surfaces of substrates, or the differing
qualities of the substrate choices in a habitat.
3) Not all habitats will have lichens growing in them, so you can't rely on their existence to point
you toward North.
However, it's still fair to say that I really lichen this symbiotic relationship between fungi and algae. Lichens are environmental colonizers, agents of change, and occasionally food for animals (including humans). The next time you're out for a walk and notice some lichen, take a closer look at this amazing organism partnership.
Lichens are a strange group of organisms that don't immediately fall into any of the easily-identifiable categories we're familiar with. That's because they're actually a combination of two organisms from different groups. The structure of a lichen comes from the mycelia (threadlike structures) of fungi, which allows for the wide variety of lichen shapes you can find. While most of us might picture small bits of greenish crust on tree trunks or rocks when we hear the word "lichen" (see below), lichens can also appear powdery, gelatinous (above), leafy, stringy, or even branching. What's most interesting is that the fungi that form half of the lichen partnership are obligated to be there: they usually can't survive alone in the environment without their symbiont.
The other half of a lichen is usually an algae or a cyanobacterium. Though both of these potential partners contain chlorophyll, the pigment(s) required to perform photosynthesis, neither of them are plants! Cyanobacteria (formerly known as blue-green algae, a term that is no longer accurate according to classification rules) are bacteria that can photosynthesize to acquire nutrients like sugar that are necessary for life. Algae are grouped with other protists, and are better thought of as the ancestors of true plants rather than plants themselves. Because of their ability to undergo photosynthesis to create their own food, both cyanobacteria and algae can function independently in their environment. But when they team up with fungi in a lichen structure, something amazing happens.
The algae or cyanobacterium partner in a lichen continues to go through photosynthesis, creating sugar molecules to feed itself and the fungal partner. In return, the fungal partner gathers minerals from the substrate, holds moisture for both partners, and provides the physical structure of the lichen. While this might sound like an incredibly simple trade-off relationship (what we could call "symbiosis"), the sharing that occurs within a lichen allows both partners to survive in a much wider range of environments than they might have access to individually. Lichens can be found in forests and grasslands, and also in more extreme environments like deserts, tundra, and even inside rocks rather than on the surface. The benefits of the lichen partnership allow them to survive dehydration events, extreme temperatures, and other environmental stressors.
You may be familiar with the idea that you can use lichens and mosses to find your way through the woods, because conventional wisdom says that they are much more likely to be growing on the North sides of trees and rocks, where they are shaded from the sun and retain more moisture throughout the day. Unfortunately, there are several things wrong with this theory.
1) The North side of rocks and trees is the shaded side in the Northern hemisphere, but the South
side of rocks and trees is the shaded side in the Southern hemisphere.
2) Other environmental factors might contribute to lichen growth patterns, such as overgrowth
causing shaded areas that aren't on the North-facing surfaces of substrates, or the differing
qualities of the substrate choices in a habitat.
3) Not all habitats will have lichens growing in them, so you can't rely on their existence to point
you toward North.
However, it's still fair to say that I really lichen this symbiotic relationship between fungi and algae. Lichens are environmental colonizers, agents of change, and occasionally food for animals (including humans). The next time you're out for a walk and notice some lichen, take a closer look at this amazing organism partnership.
Comments
Post a Comment