What is mixotrophy and why does it matter? – Clamsplaining

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When I was a student in grade school biology, before I had an inkling of my destiny to become a clam man, I remember learning about food webs. We learned that one part of the food web was made up of autotrophs ("self feeders"), such as plants and algae, which make their own food via photosynthesis (note that there are autotrophic organisms also using chemosynthesis to make food, but we’ll leave those for another blog). The other main portion of the food web are the heterotrophs ("different feeders"), which make their living eating plants, other heterotrophs, or all of the above. The heterotrophs release nutrients as waste which are taken up again by the autotrophs to start the cycle again. It was all so clear-cut and satisfying!

The high school bio version of different trophic levels (Source: Wikipedia)

A lot of the tree of life does indeed follow these neat categories, so it made sense for my high school bio class to start with the basics. But at the time, I didn’t understand how many rule-breakers there are, who don’t fit in one neat trophic category. It turns out that between autotrophs and heterotrophs, there is a broad spectrum of weirdos that do a bit of both. We call these weirdos mixotrophs, because they mix different modes of feeding! I have spent a lot of my career studying mixotrophs, not really on purpose, but for two reasons: 1) They’re interesting as heck and 2) Because we don’t know enough about how they work, despite how important they are.

Microscope views of several types of phytoplankton, which ingested fluorescent beads, proving they engage in some amount of grabbing and eating other particles, not just photosynthesis. Source

In the ocean, phytoplankton are by far the biggest autotrophic group in terms of biomass. Phytoplankton are tiny algae that float along with currents, using chloroplasts in their cells to gather sunlight, which they use to make sugars through photosynthesis. Those sugars are their food. Most phytoplankton are happy to stick to this autotrophic lifestyle. But a subset of mixotrophic phytoplankton have evolved to not just use sunlight to gain their energy, and instead gain an advantage from munching bacteria or other phytoplankton, using those nutrients as fertilizer for their photosynthesis. Others only are photosynthetic through thievery, stealing chloroplasts from other cells that they harness for photosynthesis, in a process called kleptoplasty. Mixotrophic phytoplankton are able to subsist at times of year when other fully photosynthetic plankton struggle, such as when nutrients or light levels are low. Many of them are able to pursue more active lifestyles through the additional nutrition they gain via their murderous side hustles, allowing them to swim to stay in the highest, brightest part of the water column. This means, somewhat counterintuitively, they can improve their photosynthesis by picking up a hobby of heterotrophy!

A microscope view of Heterosigma akashiwo, a harmful algal bloom forming plankton, with two zoomed in views showing bacterial cells that the plankton has ingested. Source: Jeong et al., 2011

It turns out that many types of harmful algal bloom forming phytoplankton are mixotrophs, including Heterosigma akashiwo, which formed a red tide that devastated San Francisco Bay in 2022, killing many kinds of fish. Heterosigma is a champion bacteria eater, and my newest paper investigates the seasonal abundance patterns of it and other harmful plankton in SF Bay. We found a group of several harmful mixotrophic plankton tend to thrive in fall, when their ability to harvest alternative sources of nutrients and swim might give them an advantage over their merely autotrophic counterparts.

The common swamp pitcher-plant, Nepenthes mirabilis, seen in Palau.

It’s not just single-celled algae getting in on the mixotrophy game. Some plants have also seen the benefits of this lifestyle. Some of you may know that I am a bit of a fiend for growing carnivorous plants. I have a small "savage garden" of Nepenthes and Sarracenia pitcher plants, Drosera sundews, Pinguicula butterworts, and of course, the famous Venus flytrap (Dionaea), which Darwin (a huge carnivorous plant fan himself) called "one of the most wonderful plants in the world." All of these plants are adapted to inhabit nutrient-poor environments like bogs, or high in trees, using specialized leaf traps to catch and digest bugs as a source of nutrients. Some have evolved to make a cozy home for bats that roost and poop in the plant, supplying fertilizer. Nepenthes lowii has evolved to suspiciously resemble a toilet, with a "lid" that attracts shrews to eat tasty nectar, who then poop in the bowl-shaped leaf below. Carnivorous plants are hardcore adapted for the mixotrophy game, to the extent that most have lost the ability to gather nutrients through their roots like other plants. All carnivorous...