Symbiosis: The Hidden Engine of Life
In the beginning...
Natural selection is seen as a way for different animals to compete for a limited amount of resources, killing off the weaker ones, so that the stronger individuals can reproduce and thrive, leading to a stronger general population. Many people think that this is the main way animals evolve and adapt, but honestly, they tend to ignore a very important interaction. Symbiosis, defined as a persistent association between distinct species, spans from mutualism (helping each other out), to parasitism (leaching off of another organism). Across this range of interactions, partners can exchange materials, services, and information, across specific contexts. This can shape genomes (the genetic makeup of an organism), regulate populations, and even impact the earth as a whole.
The dynamics between symbiotic partners often cause physiological changes based around their interactions, for example, specialising tissues, body parts, and behaviours that are involved within their interactions. Many symbionts (partners in a symbiosis) undergo genome reduction, losing genes that they no longer require, as their partner is fulfilling their needs enough for the original genes to become pointless.
Symbiotic parters that are transmitted through parent to offspring tend to align more with the livelihood of the host symbiont as their future often relies on the reproduction of the host. Symbiotic partners that are transmitted between two unrelated species can be more opportunistic, like pathogens, and often, their effects can vary. For example, the protoctist in malaria is considered a symbiont which doesn’t really harm the mosquito in which it is carried, but can seriously harm human beings.
In all of these cases, cooperation persists exclusively when the benefit of the relationship outweighs any disadvantages in a specific condition. Because of this specificity, stability can be a problem for both partners in a symbiosis, and some kind of enforcement is often needed.
Cleaning symbioses in freshwater fish–shrimp systems
In clear streams, fish called cichlids routinely present themselves to cleaner shrimps in the genus Macrobrachium. These fish are considered to be client fish in this case, because they are about to receive a service. The fish - i.e. the“client” -slows its swimming speed, slightly opens its mouth, and spread its fins. The cleaner shrimp would then approach by touching its antennae against the body and head of the fish. If it isn’t chased off, it begins to feast on dead tissue and external parasites on the fish’s body, and both parties can benefits. The fish gets a routine cleaning up; the shrimp gets a predictable meal.
However, the shrimp can often be tempted. Lots of fish have a mucus coating, which nourishes them, and is high in energy and nutrients. The shrimp could easily grab a bite of that mucus, which could be detrimental to the fish. Unfortunately for the shrimp, this act of “cheating”, and taking a nibble often doesn’t work out due to several enforcements.
To begin with, we need to think of this symbiosis as a contract. The touching of the shrimps’ antennae, and the stillness of the client fish initiates a sort of contract, as they are both able to create an expectation. The shrimp will eat, and the fish will be cleaned, which lowers the chance of misunderstanding between the two organisms. Secondly, if the shrimp understands this, and still eats the mucus coating, it can easily be eaten itself, or jut chased off. In most cases, it’s much smaller than the fish, an underdog.
Most interestingly, there is partner choice, and reputation. Client fish prefer to go back to a shrimp which are gentle, and quick. Unreliable shrimp are generally avoided. Over repeated interactions, honesty isn’t partnership, but a strategy that maximises access to future clients.
To frame this interaction less romantically, lets use some theory. A model called tit-for-tat is generally used here, and it refers to the initial offer of cooperation, but defections from the partner punished. In nature, a lot of these kinds of interactions happen, and the supply and demand of different materials and services shape who cooperates with whom. Although interactions like the ones between fish and shrimp may seem insignificant, if its done regularly, and enough of these fish and shrimp work together, it could be massive. For this example, since the cichlid fish has no more parasites, it can be healthier, survive longer, and grow faster. This could influence predator-prey dynamics and community structure, changing everything on a much larger scale. This can show that symbiosis is literally imbedded into the food web, and is a stabilising mechanism that helps keep populations in place.
However, there’s always context. If the amount of parasites declines due to temperature of water quality changes, the amount of food available to the shrimp through symbiosis decreases, and the symbiosis between cichlid and shrimp starts to fail. Lower visibility can also disrupt the visual cues in the initial ritual before the interaction. This specific activity will stay the same while conditions ensure the benefits are positive to BOTH parties, and it begins to fade when it doesn’t.
Azolla–Anabaena: nitrogen, carbon, and long timescales
The floating fern named Azolla hosts a bacteria named Anabaena Azollae (very similar names here, they’re linked!). The Anabaena bacterium can turn atmospheric nitrogen in the water (which the plant cannot absorb and use) into a more available form of nitrogen for the Azolla fern. In return, the fern supplies it with sugars, and a protected environment within its lead cavities. Under favourable light and temperature conditions, the Azolla fern can expand, forming a mat over the surface of the water that doubles in mass within days. For centuries, farming have cultivated rice using this symbiosis to suppress weeds, and improve nitrogen availability to other plants. The partnership is effective because it is closed, and can happen time and time over.
There are several factors that sustain the mutualism. The nitrogen fixation that the Anabaena perform is energy costly, so naturally, they are supplied with a lot of energy by the fern. However, any “cheating” bacteria are cut out, as the fern regulates how much sugar it gives the bacteria based on a couple of things. Anabaena bacteria that underperform receive less sugar, while efficient workers are maintained. On the microbial side, since the bacteria are in a safe, protected environment, they have shed genes they didn’t need; this trades independence for efficiency. However, this shift was irreversible - typical of very long term endosymbioses (a symbiosis where one organism is physically inside the other).
The partnership goes back a long while. During the Eocene (a period in natural history millions of years ago), extensive blooms of Azolla ferns developed across the Arctic Ocean, which had recently been flooded with freshwater, making it available for freshwater plants like these to grow. As the Azolla ferns died and sank, organic carbon accumulated in sediments across the ocean floor and beaches. Over a long amount of time, the accumulation of that carbon trapped carbon dioxide from the atmosphere and contributed to the earth moving towards global cooling, also known as an ice age. The point here isn’t to assign a singular cause to this transition, but to illustrate that plant-microbe symbioses, under the right conditions, can literally create climate feedback.
Finally, the system echoes a major pattern in eukaryotic evolution. It mimics the origin or mitochondria and chloroplasts. Once upon a time, it is theorised they were free-living bacteria that produced loads of energy. Eventually, they formed an endosymbiosis inside prehistoric cells, coexisting inside of them. The cells provided them with a safe environment, and the bacteria provided the cells with energy, eventually losing genes they would no longer need, and setting the template for ATP generation in respiration and in chloroplasts. This could very well be the reason why all life is the way it is today.
Algal symbiosis in salamander embryos
The spotted salamander (Ambystoma maculatum) deposits its eggs in gelatinous clusters in temporary woodland pool, which don’t last so long. A green alga, Oophila amblystomatis, colonises the egg capsules soon after they are laid. Don’t worry, the salamander babies will be just fine. In fact, there’s a functional exchange. In daylight, algae photosynthesis and elevate the oxygen levels within the egg. Embryos would then develop more rapidly with less of a stress, as they have enough oxygen. Nitrogenous waste from the embryos feeds the algae.
This symbiosis is easily distinguished as unique, as algal cells have not only been found within the egg capsule, but also within the actual cells of the salamander embryo. For a vertebrate, intracellular residency by a photosynthetic algae is very, very unusual.
Now,obviously the Oophila algae is classified as “not self”, so the body of most animals would reject it. However, in the salamander embryo, there is tolerance that omits full endosymbiosis, up until the immune system matures. Then, it is kicked out. Sad, I know.
The phenomenon doesn’t really imply permanent fusion of the two, though. The transmission remains environmental, rather than being passed down from generation to generation, like the Azolla fern and Anabaena bacteria. Even so, the case expands the known space of vertebrate-algal relationships and clarifies how close associations can become under permissive windows in the development of an organism.
In low light or oxygen, the algae may shift their metabolisms, and compete with the embryos for resources, weakening the relationship. Since these two partners meet anew every generation, the composition of the surrounding environment generally has a strong control over how it’s going to go between these two. When the algae are abundant and the conditions match, more embryos survive, and become mature salamanders, affecting the food web out of the water. When conditions shift, naturally this advantage will decline.
So, overall, these symbioses all kind of have a structure. Let’s break down the factors which affect symbiosis:
1. Signalling, coordination, and alignment
2. Partner choice and reputation
3. Sanctions and host control
4. Transmission mode (whether a symbiont is passed down through generations or is introduced anew every generation)
5. Environmental conditions
Obviously, I’ve highlighted symbioses which I find awesome. But there are many, many more, which can all involve all of these factors, different factors, or only some. I’ve only scratched the surface here…
Implications and some finishing thoughts
From all of these case studies, we can see how such a complex relationship can work. Even though salamanders, ferns and fish may not be as intelligent as us (very humble Homo Sapiens moment there), they each have their own niche ways in which they go about. Call it intellect or evolutionary instinct, it’s pretty impressive.
Symbiosis isn’t a decorative exception to the way life works, and people often overlook that. It is one of the main routes by which natural selection creates complexity, and is one of the main reasons that life is the way we we know it.