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On this week's journey, we explore the ways things eat in the microcosmos, from Stentors filter feeding to Dileptus hunting down and absorbing its prey.


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Despite huge differences in morphology and biological structures, all living organisms do the same three basic things; they get food, digest it, and excrete waste materials.

Living organisms require energy to live. Some produce their own food, usually through photosynthesis, we call these autotrophs, but many organisms cannot make their own food.

We call these heterotrophs and they...eat. Among heterotrophic single-celled eukaryotes, food is taken into the cell in various methods but once it’s there, it's wrapped by a membrane and forms something called a food vacuole. Then the cell flushes digestive enzymes inside the food vacuole to start the digestion.

Nutrients are taken into the cytoplasm and the waste material left in the vacuole then basically fuses with the outer membrane of the cell and what's left in the vacuole is discharged into the environment. In some cases, like inside this beautiful single-celled Nassula ornata which feeds on filamentous cyanobacteria, content of the food vacuole reacts with the digestive enzymes and changes color. But that process takes time.

Because each vacuole formed at a different time, they are in different stages of digestion which gives this cell it’s colorful polka dots. But how do these heterotrophic organisms get their food? Well, in spite of a remarkable amount of diversity, a lot of microorganisms use one of the same three strategies for getting their food.

Some of this will be familiar to the macro world some of it will not. One of the less familiar is filter feeding which allows larger organisms to consume suspended food particles or much smaller organisms. There are filter-feeders in the macro-world, baleen whales come to mind.

But while a whale must swim through giant clouds of small organisms, in the microcosmos, your food can come to you. Some filter feeders use hair-like structures called cilia to create a vortex that brings other microorganisms or food particles to the cell mouth. These cilia are specialized for this task and their beating creates a current that expediently and beautifully directs every nearby thing into the waiting mouth of the microorganism.

The cilia are often too small for us to see, but you can see their effects. Take a look at these Paramecia. They're consuming tiny, tiny bacteria. and you can see their cilia causing small organisms to tumble across them.

You can also see all of their food vacuoles on the inside, and if you look very carefully, you can see a new food vacuole forming, getting filled up with those tiny, bacterial cells. Some of the best and most obvious filter feeders are rotifers, micro-animals that use cilia to create swirling vortices around their mouth parts. You can see how successful this feeding strategy is by it’s belly full of algae cells.

Every time its mouth fills with more algae, it contracts to swallow the food. Now observe these single-celled organisms called stentors, they are much bigger than most other microorganisms. You can actually see them with the naked eye. and they also use filter feeding to push all of their algal food into their cell mouths. (Ominous music).

Our second feeding mechanism, maybe the most familiar, and the most exciting is called raptorial feeding. Raptorial feeders selectively capture prey and hunt other organisms. In this video you can see Dileptus hunting.

It paralyzes one organism with the touch of its trunk-like proboscis, and then it pulls that organism into itself in a process called phagocytosis. Many of these microorganisms are armed with something called toxicysts. These are little harpoon-like structures filled with toxins and they are located on a particular part of the cell which the microorganism uses for hunting.

These tiny harpoons are then fired when they come in contact with prey organisms which then become immobilized. This is Bursaria, it’s a single-celled organism with a huge mouth, and things have not gone well for the Paramecium that is now inside it. The Paramecium dies immediately because of the toxicysts on the inside of the Bursaria, so at least it was quick.

Now get ready for some truly gorgeous footage of a micro-animal’s day going south. First Paradileptus immobilizes the rotifer with fired toxicysts and the animal is swallowed by the single-celled organism as it swims away. This is a Frontonia which is a close relative of Paramecia, but lacks the filter-feeding habits of its relatives and feeds predaciously on large diatoms and filamentous cyanobacteria such as Oscillatoria here.

Though, in this case, it turns out this Frontonia bit off a little more than he could chew. Another raptorial feeding style is called histophagy. Histophagous organisms such as these single-celled Coleps attack injured but live animals or other single-celled organisms, sucking off hunks of tissue rather than consuming whole organisms.

When they attack an animal, they enter wounds and ingest tissue often attacking in groups because their chemical sensing abilities attract many of them from a distance, like microscopic vultures. When a number of them gather in one place, it’s hard to avoid another macro-world analog...piranhas, devouring everything soft in no time at all. There is a huge variety of raptorial feeding, this is just the beginning, but we wanted to show you one more before we move on.

This is Vampyrella, an amoeba with a suitable name. It specializes in feeding on filamentous algae. First, it bores a hole through the algal cell wall and then slurps out the gooey, nutritious cytoplasm.

Our final feeding mechanism, for today at least, is diffusion feeding, in which the predator just sits in the same place, relying on the prey to accidentally make contact. This is a Heliozoa, it’s a single celled amoeboid and because of its resemblance to the sun due to the rays coming out of its cell, it’s sometimes called the “sun animalcule”. The rays are called axopodia.

These are sometimes used in locomotion and, in this case, for hunting prey. Axopodia, are cytoplasmic extensions, meaning they’re a part of the cell membrane, even though they look like they’re sticking out of it. Each one has a central supporting rod of microtubules that gives it this rigid structure.

The axopodia are coated in organelles that discharge toxins when touched, which impair or even paralyze Heliozoa's prey. After the organism is captured, those microtubules are drawn back into the cell, thus retracting the axopodia and allowing the cell to swallow the unlucky organism, or prey is just engulfed by extrusions from the cell called pseudopodia. In this video, a rotifer has been captured by a Heliozoa and it is slowly getting eaten by it.

Now this is something that happens fairly frequently, but we did capture something unusual here. While it was stuck to Heliozoa's axopodia, this rotifer actually lays its egg. But neither egg nor the rotifer is going to escape this.

Surprisingly heartbreaking. This is a Suctorian, it is a ciliate just like Paramecium and Stentor. These organisms have hair-like cilia during the early stage of their life, but as adults they develop bundles of tentacles.

Just like in Heliozoa these tentacles are supported by an internal cylinder of microtubules. The tip of the tentacles have extrusomes; these are special structures that attach to and immobilize any other ciliates that touch them. The tentacles eventually penetrate the cell membrane of the prey, and then the contents of the prey is sucked out through the tentacle.

In the clip, a suctorian has caught 4 individual Vorticella with its tentacles and is slowly sucking their cytoplasm. It looks a little like the vorticella have the suctorian surrounded, but in fact, they are powerless to escape it. It’s a dangerous world out there.

The complex chemicals created by organisms to sustain their life necessarily are useful to other organisms, as building blocks and as fuel. And so predation evolved. It’s beautiful, it’s constant, and it’s brutal.

Thank you for watching and for coming on this journey with us as we explore the unseen world that surrounds us. If you want to see more from our Master of Microscopes, James, check out Jam's Germs on Instagram. And if you want to see more from us, that, my friends, is what the "subscribe" button is for!