Mixing it up in the web of life
Many types of marine plankton are either animal-like or plant-like. But a huge number are both, and they are upending ideas about ocean ecology.
Their color gave them away. Ecologist Diane Stoecker was looking at plankton in samples of ocean water from the dock in Woods Hole Harbor in Massachusetts some 40 years ago when she spotted something strange. Under the microscope, she recognized Laboea strobila, shaped like an ice-cream cone — “yellowish green and very beautiful,” she recalls — and the smaller, more spherical Strombidium species — also oddly greenish.
Stoecker knew that these single-celled critters, named ciliates for the hairlike cilia that they bear, got their energy by feeding on other, smaller organisms. So why were the ones she saw so green — a color that generally signifies photosynthesis? Was the pigment leftover food, ingested algae or just the algae’s chloroplasts?
After some groundbreaking experiments, Stoecker was one of the first scientists to describe how these types of plankton not only hunted their prey, but also sequestered the chloroplasts of their food sources and used them to get energy from sunlight. “I was very excited to find that they really were photosynthetic,” she says.
Traditionally, marine microplankton had been divided similarly to species on land. You had plant-like phytoplankton, such as algae, and animal-like zooplankton that ate the phytoplankton. What Stoecker found was that some of these organisms were somewhere in the middle: They could eat like animals when food was present and photosynthesize like plants in the light. “If you think about it, it can be the best of both worlds,” says marine ecologist Dave A. Caron of the University of Southern California.
Today, there’s growing realization that these in-between beasties — dubbed mixotrophs — are not only widespread but also play vital roles in the ecology of the oceans.
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