Have you heard of symbiosis? It's a long-term interaction between different species where at least one of them benefits. We talk about it in another guide, but we just need to know the basics today. Symbioses come in three forms as follows:
We’re not going to talk about parasites in this section; that'll come on the next page. For now we’re going to focus on mutualism and commensalism.
The first example of mutualism is nitrogen-fixing bacteria. This fixing isn’t about repairing nitrogen or sterilizing it. It’s about capturing it.
All life forms need nitrogen. The "amino" part of amino acids comes from the words amine (-NH2) and ammonia (NH3), both of which, you’ll notice, prominently feature nitrogen.
Do you know why people lost at sea can die of thirst, despite being surrounded by an ocean? That’s because the ocean water isn’t drinkable. It’s WAY too salty. Plants are kind of in the same situation. Except instead of water, it’s nitrogen.
Plants are surrounded by nitrogen. It makes up 80% of the atmosphere! However, they can’t use it in that form. They have no way to access that nitrogen directly. Higher organisms depend upon bacteria to fix nitrogen. Bacteria are able to convert nitrogen in the air (N2) into ammonia (NH3) that plants can take up and use.
Of course, nitrogen-fixing bacteria wouldn’t normally make any more ammonia than they need. Hey, why should they? Plants have developed ways to coax the bacteria to share. Certain plants have developed special organs in their roots called root nodules. These organs accommodate gobs of nitrogen-fixing bacteria inside, providing safe, food-filled places in exchange for nitrogen.
Below are two plant roots with nodules.
Image from here.
While several pairs of plant and bacterial species have developed relationships like this, the best-studied relationships are between legume plants (including alfalfa, clover, and soy) and bacteria of the genus Rhizobium.
Not all plants form these relationships, though. Many rely on nitrogen from decaying matter. Remember crop rotation? It’s the process of switching the crops we grow in a particular field from year to year. People have known for a long time that planting legumes every few years would lead to better growth of other crops in ensuing years. Now we know that’s because the bacteria in legume roots are fertilizing the soil with nitrogen-containing compounds.
The word ruminate is now associated with thinking thoughts over. It’s originally associated, however, with the way that cows and other animals, called ruminants, eat. Chewing is a multistep process with ruminants. They chew, swallow, regurgitate, and chew some more.
Grass is tough to digest—humans would starve if they tried to live off it—but cows and other ruminants have developed special stomachs that include an enormous region called the rumen.
The rumen is essentially a giant vat where prokaryotes (bacteria and archaea both) digest grass and other plants. Prokaryotes and ruminants work together to get food. Prokaryotes produce special enzymes called cellulases that digest cellulose, the dominant compound in plant cells. Ruminants chew the food, and then chew it again to help give the prokaryotes better access to the nutrients in it.
Humans have mostly given up on digesting cellulose for energy; we break down most of our food in the stomach and small intestines. We do, however, still have a symbiotic relationship with bacteria in our digestive tracts. These bacteria live in the large intestines where they consume undigested materials and produce several vitamins that we need including vitamin K, vitamin B12, thiamine, and riboflavin.
Indigestible plant mass is important in another way, too. You may have heard about the importance of dietary fiber. A certain amount of indigestible plant mass helps our digestive systems function normally. If you’ve ever seen a dog or cat eating grass, that might be why they’re doing it. To be clear, Shmoop does not suggest trying this at home.
Our skin is covered with hundreds, or even thousands, of different bacterial species. These bacteria crowd out pathogenic bacteria from growing in these environments. Our immune systems ignore these friendly bacteria since they are not doing us any harm. This relationship is considered commensal.
Cyanobacteria were essential for the development of the world as we know. In a way, we have a commensal relationship with the cyanobacteria of the past. That oxygen you’re breathing right now? Yeah, thanks cyanobacteria.
Cyanobacteria were the first organisms on earth known to turn water and carbon dioxide into sugar and oxygen. They considered this oxygen stuff a waste product, but this "trash" made it possible for the rest of us to live here. As cyanobacteria grew and grew, the levels of oxygen on the planet rose, wiping out organisms that couldn’t tolerate it, and setting the stage for organisms, like us, that require it. This event, which occurred about 2.5 billion years ago, is known as the "great oxidation event".
Of course cyanobacteria are no longer the only organisms to perform photosynthesis in this way. Organelles called chloroplasts perform this function in plants everywhere. These chloroplasts, though, are the descendants of ancient cyanobacteria that were taken up by and incorporated into the ancient ancestors of modern plants.
Ruminant livestock, such as cows, are a major atmospheric source of methane, a greenhouse gas that contributes to global warming. The methane that cows—ahem—belch is produced by the rumen archaea while they digest the cow’s food. You can learn more about that here.