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Biological oceanography

Hunter-Chiller: Multiple feeding strategies for some of the world’s smallest organisms

The Paper: DÖLGER J., NIELSEN L.T., KIØRBOE T. & ANDERSEN A. 2017. Swimming and feeding of mixotrophic biflagellates. Scientific Reports 7:1-10

Best of Both Worlds

With the surf nipping at her silky calves, Prym walked along the beach in the midday tropical heat. Escaping the trappings of city life and confident that she was now alone, she began to undress. Her clothes carefully placed away from the rising tide, Prym felt a sense of relief as her bare, green skin was exposed to the Sun. She had, after all, forgotten to pack a lunch and this seemed like the most sensible thing to do. As her skin worked to convert carbon dioxide to useable energy, the pangs of hunger began to subside and she was again able to sink into the pool of sensations that paradise would provide.

Figure 1: Haptophytes P. polylepis (right) and P. parvum (left). All three flagella of P. polylepis are longer than that of P. parvum (source: Dölger et al., 2017)

Figure 1: Haptophytes P. polylepis (right) and P. parvum (left). All three flagella of P. polylepis are longer than that of P. parvum (source: Dölger et al., 2017)

Although this is something out of science fiction for humans, some organisms on our planet can both ingest food like an animal and conduct photosynthesis like a plant. They are known as mixotrophs, where –troph comes from a Greek word meaning nourishment, and mixo- just means “in multiple ways”. This ability is mostly found in the ocean where light penetrates the surface waters, and mobility for tiny organisms is far easier than it is on land. Because most of the creatures that perform this enviable feat are microscopic, it isn’t easy to see how they hunt for their food. Although I would certainly watch it, David Attenborough has yet to narrate a documentary on the hunting habits of predators only one cell in size. Because of this, a team of scientists from Denmark combined laboratory observations and mathematical calculations to give insight into this amazing adaptation.

The Science

Led by Ms. Julia Dölger, the team set out to discover just how these single-celled organisms can both capture and eat their prey while avoiding the menu of some larger hunter. They studied two different species of marine algae called haptophytes. Although algae are photosynthetic, these particular creatures are also able to consume prey. They can either capture and ingest smaller organisms or attach to larger fish and begin digesting them from the outside. They do this by moving small, hair-like appendages called flagella. While many single-celled organisms use flagella for movement, haptophytes have a three flagella arrangement where the outer two are used for movement, and the center one (called a haptonema) is sometimes used to capture and consume food.

Figure 2: Mathematical models used to calculate the forces acting on each organism and the fluid surrounding them (source: Dölger et al., 2017)

Figure 2: Mathematical models used to calculate the forces acting on each organism and the fluid surrounding them (source: Dölger et al., 2017)

The two species of algae (Prymnesium polylepis and Prymnesium parvum) have different arrangements and sizes of their flagella which causes them to behave in slightly different ways (Fig. 1). Simply put, all three of P. polylepis’s flagella are longer than those of P. parvum. The researchers wanted to test three distinct attributes of each species; how they swim, how they eat, and how quiet they are (to avoid detection by predators). They first grew cultures of each species and then used a microscope to film them swimming and catching their prey. To aid in visualizing how the fluid flows around them when they move their flagella, microparticles were placed in the water with them. These microparticles acted in the water like pollen does in the air, and the scientists could see all of the tiny currents around the cells of the algae. Once they had enough live footage, the scientists constructed a mathematical model to try and calculate exactly how the forces are acting on both the organism, and the water around them (Fig. 2).

One Size Does Not Fit All

Figure 3: Flow patterns calculated around P. polylepis (top) and P. parvum (bottom) (source: Dölger et al., 2017)

Figure 3: Flow patterns calculated around P. polylepis (top) and P. parvum (bottom) (source: Dölger et al., 2017)

As you may have already suspected, an organism with longer limbs swims differently than an organism with shorter limbs. With its longer flagella, P. polylepis swims at a constant speed while P. parvum has an unsteady beat pattern and consequently swims at varying speeds. The differences in flagellum size and beat pattern also changes the way the fluid flows around their bodies (Fig. 3). When they analyzed their model calculations, the team found that the main reason for the differences between the two species was mostly due to how far away from the cell body the driving force was coming from. In other words, it isn’t so much that the longer flagella operate differently, just that they move that propulsive force further away from their bodies.

What they also found was that although P. polylepis swims more regularly, this does not make it faster than P. parvum. They discovered that despite its erratic movement, P. parvum is the faster of the two because the driving force closer to the body (thanks to its shorter flagella). Additionally, P. parvum is quieter and better able to avoid detection by predators. At his point, you may be thinking that having shorter flagella is an advantage in all aspects of a haptophyte’s tiny little life. Well, it turns out that what P. polylepis lacks in speed and stealth, it makes up for in prey capture. P. polylepis is the only one of the two that can use its haptonema (that middle flagellum) to actively capture small prey and move it to the opening in its body (Fig. 4).

Figure 4: P. polylepis uses its haptonema (center flagellum) to capture prey and move it to the opening on its body for consumption (source: Dölger et al., 2017)

Figure 4: P. polylepis uses its haptonema (center flagellum) to capture prey and move it to the opening on its body for consumption (source: Dölger et al., 2017)

The most significant conclusion the team reached after conducting these observations and calculations, is that neither species can collect enough food to survive without the aid of photosynthesis. Although they have swimming capabilities, these are only useful over a short distance and both algal species are at the whim of the ocean currents. They rely on chance encounters with food, and when that doesn’t happen, they have the unique capability of harnessing the Sun’s energy to fill in the gaps. An enviable trait to think about the next time you forget to pack a lunch!

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