In last week's post, we talked about kleptoplasty, or the ability of nudibranch sea slugs to “steal” photosynthetic powers from the algae that they eat. In that case, they digest the algae and get the nutrients the algae provides for food, and they separate out the chloroplasts. The chloroplasts are the organelles in plants, algae, and some bacteria where photosynthesis occurs.
This is a perfect example of the two types of energy acquisition in living things. All living species are categorized as either heterotrophic- they make energy from the food they eat- or autotrophic – they make their own energy from the environment.
The autotrophic algae use energy from sunlight to power their systems and store any extra energy as carbohydrates. The nudibranchs eat the stored carbohydrates and instead of digesting the chloroplasts along with the rest of the algae, they can hold on to the chloroplast for use in their own tissues. In this case, the process doesn’t end well for the algae, but there is another aquatic example of taking advantage of algae’s autotrophic power that has a happier ending for all parties. Many cnidarians (pronounced nye-dare-ians) partner up with certain photosynthetic algae, called zooxanthellae (pronounced zoo-zanth-elle-ay).
Even though they look a bit like plants, cnidarians (corals, anemones, hydroids, and jellyfish) are animals, and they also eat food as you would expect animals to do, but most corals and anemones, as well as some types of jellyfish, also photosynthesize! Like the nudibranchs, these animals are both heterotrophs and autotrophs.
If we focus in on one branch of the cnidaria, Class Anthozoa, we are looking exclusively at the anemones and corals. Corals are sessile (stationary) and most are filter feeders. Unfortunately, there is not a lot of food floating around in the coral reef environment for them to filter even if they sweep the water with their tentacles 24/7. To make up for this, they take advantage of the energy source that shines upon their tropical waters 12 hours a day- the sun. As much as 90% of the energy a coral needs to survive comes from sunlight through photosynthesis (1)
This is a mutualistic form of symbiosis, and both the algae and the coral are better for the association.
The classic coral that makes up a stony structure of a reef might seem like a single animal, but each one of made up of thousands of individual coral polyps that live together as a colony. They are connected to each other and act as a single animal, but each polyp has its own mouth to eat and also photosynthesizes on its own. Each works collectively to generate energy for the whole colony structure. The polyp acts with the large coral like an individual bee does to its hive; they need each other to survive! The coral skeletons are white, and the polyps themselves are actually clear so light can get to the zooxanthellae. The colorful pigments in the polyp come from the algae that live within them.
The algae are the ones doing the active photosynthesis, so how exactly are they benefitting from the situation? It turns out warm, tropical waters are crystal clear and blue for a good reason- warm water holds less oxygen and nutrients. So even though the sun is shining brightly and the conditions seems perfect for algal growth, without the coral, the tropics would be something of a desert. There is not enough nutrients for photosynthetic organisms to flourish. The nitrates and phosphates that the algae need are provided by the coral animals. They truly need each other to survive!
In addition, the coral need to protect their algae partners just as much as they do their own bodies, so the algae live inside the coral’s body tissue and are protected by the same defenses that the animal uses for itself. As we read in last week’s blog, this means the coral’s stinging ability protects the coral and all of the algae it is home to. Polyps also contain zooxanthellae (algae) that photosynthesize to provide energy for them (2)
Types of Coral
Not all corals build their own hard structure to protect the soft polyps inside, but those that do are called hermatypic, or reef building corals. Another name you might see is scleractinian- which means “hard ray” in Greek. All of these are just names that mean stony corals that make the beautiful calcium carbonate skeleton that makes a coral reef such a striking and singular ecosystem.
I mentioned that not all corals are stony. Those in the Orders Alcyonacea, Corallimorpharia or Zoantharia are referred to as soft corals because they have no stony skeleton. The soft corals are incredibly diverse and not always appreciated as some of the most colorful additions to the reef. They tend to have fewer but larger polyps with big mouths at the center of the polyp, and they can sometimes look more like an anemone than a true coral. Many are capable of eating larger prey items, so rely more on their heterotrophic food sources than their autotrophic algae for energy. They can live in deeper parts of the reef and don’t always require direct sunlight.
An example of Order Alcyonacea is the gorgonions (also called sea fans):
An example of the Corallimorpharia is the mushrooms:
An example of Order Zoantharia is the zoanthids (or zoas):
Decline in coral Reefs
All of these corals are affected by warming and changing oceans. The difference is that when the soft corals die, they don’t leave behind the evidence in the form of a white, bleached stone structure, but the reef-builders do. Bleaching, however, occurs long before the corals have actually died. Recall that the algae are what provide color to the coral, and when corals are stressed by increasing temperatures, they expel the algae that live in their tissues. This is because high temperatures energize the algae and overwork them. As a result, a higher rate of photosynthesis leads to a higher production of oxygen free radicals as a byproduct, which harm coral tissue.
Because 90% of the coral’s energy comes from their photosynthetic partnership, this greatly weakens them. Their immune system is suppressed and they stop growing. These weakened corals are more prone to diseases, predation, and damage.
Increasing ocean acidity breaks down the calcium carbonate skeleton and can make their protective shell more brittle. Even worse, the increase in frequency and severity of tropical storms breaks them apart, and they do not have the energy to rebuild.
The issue affecting coral reefs is not just warming oceans that causes them to bleach;
it is not just the CO2 in the atmosphere causing ocean acidification;
it is not just runoff and pollution leading to coral disease;
it is not just development and erosion leading to clouded water that reduces photosynthesis;
it is not just the chemicals in sunscreen or microplastics building up in their tissue;
it is not just being kicked by tourists, dragged by boat anchors, or overfished;
it is not just the increase in storms knocking them about...
The problem with coral reefs is that ALL of these things are happening at the same time.
Coral reefs are not the fragile perfectionists that we often assume them to be. They are resilient and have persevered through a great number of issues and recovered slowly but surely. They have built an entire ecosystem that is both animal and habitat at once. However, if we don’t reduce the severe pressure that we have placed disproportionately on these miraculous species, we will be the cause of their complete demise.
Approximately 75% of coral reefs worldwide are currently threatened
If we continue without action, this could reach 90% by 2030 and close to 100% by 2050 (3).
What Can We Do?
Do what you can to reduce your own impact- be aware!