What your poop says about your diet: Iron in the ocean is controlled by zooplankton diet and poop

Reviewing: Richon, Camille, Olivier Aumont, and Alessandro Tagliabue. “Prey stoichiometry drives iron recycling by zooplankton in the global ocean.” Frontiers in Marine Science (2020). https://www.frontiersin.org/articles/10.3389/fmars.2020.00451/full


What you can learn from your poop

These oreos can turn your poop pink! (Source: Wikimedia Commons)


If you ever ate too much beet, kale or Peeps Oreos, you might have found your poop coming out in red, dark green or a vibrant pink (!). The color of your poop can tell you that you snacked on a candy bar last night, or you had a glass of green smoothie for breakfast. Similarly, zooplankton poops reflect what types of food (prey) they were feeding on.


What are zooplankton and why do we care about their poop?

Zooplankton are tiny animals drifting in the ocean that feed on phytoplankton (plants of the sea) and are fed by fish. They are the primary consumers of the marine food web (similar to herbivores like insects and rabbits on land food web). While a healthy zooplankton community is necessary to support smaller and larger fish higher up in the food web, zooplankton poop is also very important to phytoplankton . Zooplankton consume phytoplankton and release fecal pellets (poop) that are rich in nutrients like carbon, nitrogen and iron. Phytoplankton can take up the nutrients released from these fecal pellets to photosynthesize (a process called “recycling”). For photosynthesis, phytoplankton absorb carbon dioxide, an important greenhouse gas that is causing climate change, from the atmosphere. So if iron gets more efficiently recycled, more iron from zooplankton poop is released to phytoplankton, and phytoplankton can use the recycled iron to photosynthesize more actively and remove more carbon dioxide from the atmosphere. Scientists in this study used PISCES, a global ocean biogeochemical model, to find out what things affect the efficiency of iron recycling by zooplankton.

Different types of zooplankton: 1-5 copepods, 6 gastropod larva 7 doliolid 8 fish egg 9 decapod larva (from Wikimedia Commons)
Microscope image of zooplankton fecal pellets (Liszka et al. 2019)

Iron recycling by zooplankton is most affected by food quality

In the last few decades, scientists have come up with different ways to simulate the ocean in their computers with mathematical equations and codes. Ocean models are very useful to complement observations by ships – while sailing to the ocean is quite expensive and the ship can only cover a relatively small area over days to weeks, ocean models are relatively cheaper, and can provide data in the whole global ocean over years.
In this study, the scientists used PISCES, an ocean model that simulates phytoplankton and zooplankton communities, and a number of nutrients that exist in the ocean and are necessary for the planktons to grow. The research team added a term to the model to describe zooplankton iron recycling efficiency to test how different factors would impact iron recycling efficiency. And they found that food quality is the major influencer of zooplankton iron recycling efficiency.
In this ocean model, food quality has been defined as iron to carbon ratio of zooplankton divided by iron to carbon ratio of prey. The iron to carbon ratio in prey reflects the amount of iron in foods (higher ratio means high-iron foods, just as you’d think of high-protein or high-fat foods,), while the iron to carbon ratio in zooplankton varies across species (similar to how some people have more muscles or body fats than others). Researchers found that zooplankton are feeding on prey with a similar iron to carbon ratio to themselves to maximize their iron uptake (and minimize iron recycling, or “loss” of iron for themselves). They also found that the iron to carbon ratio in prey is higher than that in zooplankton in most areas of the ocean. This would mean that zooplankton are consuming iron-rich food and taking up more iron than they need to, so this extra iron gets released back into the ocean, and become available to phytoplankton for photosynthesis.

Figure 3 from the original paper (Richon et al. 2020). The left figure (A) shows iron uptake efficiency by zooplankton, and the right figure (D) shows zooplankton food quality. When food quality is close to 1 (when the iron to carbon ratio of zooplankton is similar to that of prey), zooplankton takes up iron most efficiently (and recycles less).

Why do we care?

Understanding nutrient uptake of zooplankton and its prey can help us how iron recycling may evolve in the future. For instance, we are observing ocean temperature rising over time, and with increasing water temperature, zooplankton food quality or nutrient requirements may change. These changes can affect iron recycling by zooplankton, or even their survival, and also affect other organisms in the marine food web that feed on or fed by zooplankton. In the end, what zooplankton eat and what they poop can tell us a lot about how nutrients in the ocean are being supplied and consumed in the ocean.

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