Plankton Share the Spectrum

Dividing the light. Red and green phytoplankton coexist (top) by using different wavelengths of light for photosynthesis.

A new discovery helps explain the unexpectedly high biodiversity of plankton communities. The research shows that different species of marine plankton coexist by using different wavelengths of light to photosynthesize.

The "paradox of the plankton," as described by G. Evelyn Hutchinson in 1961, is that there are so many species occupying an apparently homogeneous environment and competing for the same scant resources. Recent discoveries of new microorganisms have bumped up the species count, making the paradox even more puzzling, says aquatic microbiologist Jef Huisman of the University of Amsterdam.

Scientists have traditionally supposed that competition for light (as well as for nutrients like iron) among phytoplankton--diatoms, blue-green algae, and other tiny aquatic organisms--will result in one species shading out the others. But past theories and experiments treated light as a single resource, explains Huisman's graduate student, plankton ecologist Maayke Stomp of the University of Amsterdam. However, pigments absorb and reflect light of different wavelengths, giving the plankton a characteristic color. Along with colleagues at the Netherlands Institute of Ecology, Huisman and Stomp suspected that a diversity of pigments might be the secret to the peaceful coexistence of plankton.

The scientists tested this idea by exposing two closely related strains of picocyanobacteria--one red and one green--to different wavelengths of light in the lab. Under red light, only the green strain survived, presumably because it could absorb the red light for photosynthesis. Conversely, the red strain won out under green light. When exposed to white light, which has the full spectrum of wavelengths, both strains persisted for at least 60 days, the team reports online 10 October in Nature. Experiments with a third type of phytoplankton showed that it could coexist with either of the first two strains by making more of the pigment its competitors didn't have.

The study helps explain the "unexpected plethora" of phytoplankton in apparently homogenous water, says marine biologist Martin Edwards of the Sir Alister Hardy Foundation for Ocean Science in Plymouth, U.K. Huisman thinks the research may also prove useful for estimating the primary productivity of the ocean. Current estimates are based on remote sensing of the pigment chlorophyll a. But the new work shows that other pigments are also important, which means that scientists may have underestimated the productivity of plankton.

Related Sites
Jef Huisman's lab page
Information from NASA about phytoplankton and their role in global climate
A phytoplankton tutorial from the Smithsonian Institution

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