We are what we eat, they say. It might be more accurate to say that we are what phytoplankton at the bottom of the food chain serve up. Now, as the planet warms, in the oceans by the poles the phytoplankton are starting to dish up junk food, a new study warns.

Climate change is racing apace at the poles and the tiny plants are like a canary in a coal mine. Their discomfiture, or at least the change already observed in their internal composition, suggests that the oceanic food chain is changing.

What are phytoplankton and why should you care? They are tiny algae that like (almost) all plants, make their own energy by photosynthesis. They don’t eat per se. They drift near the surface of the water because they need sunlight to perform photosynthesis. They also need to absorb minerals like nitrogen from the environment.

Then they die and/or get eaten by the next stage in the food chain: zooplankton, which are unicellular predators; jellyfish and krill, which are tiny shrimp; and in turn the zooplankton, jellies and arthropods get eaten by bigger animals, which get eaten by bigger animals, et cetera.

So phytoplankton are the primary source of food for life in the ocean from unicellular predators to blue whales. Ultimately this food chain ends with us because we also eat marine animals.

Why do we do that? Unlike phytoplankton, we non-plant slobs need to eat for energy, which is sad. Non-plants need to care about phytoplankton because new research at the Massachusetts Institute of Technology suggests, based on a model but crucially, also on actual observations over 40 years – that as sea surface temperatures rise, which they are, and the sea ice is melting, which it is, the phytoplankton in the polar seas are adapting.

“Adapting” ostensibly sounds like a positive…but, adaptation how exactly?

Algae in high latitudes are producing less protein and more carbohydrates. That is how they are adapting. This may be okay from their perspective – so they get plump, big deal.

But they are the diet of the ocean, and from the perspective of other life forms that eat them, they are turning into fast food, Shlomit Sharoni and Mick Follows of MIT explain in the journal Nature Climate Change.

This isn’t just theory. A decrease in protein manufacture by phytoplankton in the polar seas has been empirically demonstrated.

An alga producing more carbs and lipids and less protein will deliver more calories to the animal that eats it, but is lower in overall nutrients. It’s like living on candy instead of steak and salad. It’s poor-quality food.

In parallel, according to the model, subtropical algae will do the opposite. They will produce more protein and less carbs as they too adapt. This has not yet been demonstrated in the wild.

What have we? Change all around: algae in the far south and north making less protein as warming proceeds, and algae in the mid-latitudes possibly making more. The general direction is: What was is not what will be.

What will that do to the food chain? Their model doesn’t address that question, though speculation is free.

So, speculatively, it could theoretically affect the entire oceanic food chain in different ways in different places. But as the whole world warms, which it’s doing, the trajectory may be towards fat algae that don’t provide as much nutrition for eaters. This does not augur well.

Popcorn of the sea

Technically, to predict how phytoplankton will handle climate change, Sharoni and Follows’ model begins with what the algae do with the carbon they breathe in.

The question: Do algae change the allocation of carbon to making proteins, or carbs, based on the external conditions?

The answer: Yes. In high-nutrient, low-light, high-latitude environments, they allocate more carbon to making proteins. In bright, nutrient-poor subtropical regions, allocation favors carbohydrates and fats.

Why why why? To photosynthesize, plants use certain proteins to “capture” the light. If the light is dimmer, they need more of the protein. Ice reflects light; water absorbs it. As the ice melts in the extreme north and south, algae are getting more light and don’t need as much protein to capture it. So they allocate less carbon to making protein and more to carb manufacture, Sharoni explains. Ta da!

Meanwhile in the bright but nutrient-poor subtropic waters, plants don’t need so much light-capturing protein but can do with more sugars, which is what carbs are. See?

Now, looking ahead, Sharoni modeled: What will the wee plants do in a world where carbon dioxide reaches double the level it is now, an already terrifying 450 parts per million, and rises to 900ppm? Will the phytoplankton allocate the carbon they breathe in any differently?

We just note that our planet is on the worst-case trajectory: fossil fuel use keeps increasing and warming is accelerating so we are on the path to 900ppm. There are no consensus models for what the planet will be like to live on at that point.

What did Sharoni find? Under warming conditions, algae in the high latitudes getting much more light reduced carbon allocation to protein by about 15 to 30 percent (this has been empirically observed). There is no empiric data on the behavior of the algae in the subtropics but her model predicts they will increase carbon allocation to protein by about 20 percent.

Why? Because as the ocean surface gets hotter, stratification in the oceans increases. The water columns mix less and fewer nutrients reach the surface from the seabed.

Pity the poor phytoplankton who like surface and light but there are no nutrients, so they sink down to find some. The biomass shifts downward, as science puts it.

How much lower? Say 50 meters, and it’s quite dark down there so they need more photosynthesis protein. Such a shift in biomass has been observed, Sharoni adds, at the Station ALOHA monitoring site near the Hawaiian Islands.

A bear and a zooplankton walk into a bar

So what have we? At the poles we are already moving towards a fast-food ocean. With climate change, the ocean’s diet is changing and how that will affect us all, Sharoni and Follows cannot say, but the model says the nutritional composition of the surface ocean will look very different by the end of the century.

A survey of the literature shows that when fish eat poor-quality food, their biomass diminishes, Sharoni adds.

Does all this mean that in a warmer world, the phytoplankton eating, or becoming, fast food will be shrunken and smaller? If they have less protein and more fat?

No, it means they will be lardasses, and if I am a zooplankton who eats fatty but protein-poor phytoplankton, then I will get less nitrogen and other protein stuff for my own biomass, and that does’t sound like it augurs well.

What have we? The first concrete prediction for how warming will reorganize marine ecosystems, altering the entire food web structure and reshaping ocean biogeochemistry itself. Fat phytoplankton deliver a lot more energy than protein-rich cells, Adam Martiny of the University of California points out in an accompanying opinion on the paper; but they deliver less of the minerals animals need to grow and reproduce.

But from the phytoplanktons’ selfish perspective, maybe it’s all good. At the poles, high quantities of fat help the little cells survive the protracted dark winter. And Mick Follows points out that that we know some zooplankton also like to pack on lipids to help over winter, so it’s not a simple story.

Sounds familiar. “Lipids and long winters” is a thing in bears and so is “Fat Bear Week,” following autumnally triggered feasting by the ursines to bulk up before the long sleep and what they’re slavering for is fat fat fat, it turns out.

Experimentation has demonstrated that a pre-hibernation bear when given its druthers will select a diet in which 17 percent of energy comes from protein, and the rest from fat.

So what have we? The microscopic algae seem to be adjusting to climate change; climate change is changing “what they’re made of” and the question is how the rest of the food chain will react. Looks like we’re going to find out.