American Society of Naturalists

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“Why do phytoplankton evolve large size in response to grazing?”

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Pedro Branco, Martijn Egas, Spencer R. Hall, and Jef Huisman (Jan 2020)

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Why do phytoplankton evolve large size in response to grazing? A new explanation based on selection for nutritious food

Evolution of phytoplankton size: a new explanation based on coevolution, selective grazing and nutritious food

Nauplius larva of a copepod (the transparent animal) grazing on freshwater phytoplankton, in this case the green alga <i>Pandorina morum</i> (green cells) and the ‘golden alga’ <i>Synura</i> sp. (yellow cells).<br/>(Photo © Wim van Egmond, provided by Science Photo Library, with permission)
Nauplius larva of a copepod (the transparent animal) grazing on freshwater phytoplankton, in this case the green alga Pandorina morum (green cells) and the ‘golden alga’ Synura sp. (yellow cells).
(Photo © Wim van Egmond, provided by Science Photo Library, with permission)

Phytoplankton are among the tiniest organisms on Earth. Yet they display wide variation in size, from small species less than a micrometer to large species of almost a millimeter. Small phytoplankton species tend to be better competitors for nutrients, whereas large phytoplankton may be more difficult to handle for grazing zooplankton. Hence, the evolution of large phytoplankton is often explained as a physical defense against grazing. A major difficulty with this explanation, however, is that zooplankton may counter this escape from grazing by coevolving to a large size as well. So why do phytoplankton evolve large size in response to grazing?

Here, Pedro Branco, Martijn Egas, Spencer Hall and Jef Huisman propose a new explanation. They argue that a major evolutionary advantage for large phytoplankton is their low nutritional quality. To investigate this hypothesis, the authors develop a mathematical model in which the individual sizes of phytoplankton and zooplankton coevolve. The model makes two key assumptions supported by observational studies. First, the nutrient content of phytoplankton cells varies allometrically with cell size, such that large phytoplankton cells contain less nutrient per unit cell volume than small phytoplankton cells. Second, zooplankton can graze selectively, with a preference for nutritious food. Combining these two assumptions, the model predicts that selective grazing on small but nutritious food will favor evolution of large phytoplankton of low nutritional quality. Hence, a size-dependent change in food quality may explain why phytoplankton of large size often dominate in waters with high grazing pressure.


Abstract

Phytoplankton are among the smallest primary producers on Earth, and yet display a wide range of cell sizes. Typically, small phytoplankton species are stronger nutrient competitors than large phytoplankton species, but they are also more easily grazed. In contrast, evolution of large phytoplankton is often explained as a physical defense against grazing. Conceptually this explanation is problematic, however, because zooplankton can coevolve larger size to counter this size-dependent escape from grazing. Here, we hypothesize that there is another advantage for the evolution of large phytoplankton size not so readily overcome: larger phytoplankton often provide lower nutritional quality for zooplankton. We investigate this hypothesis by analyzing an eco-evolutionary model that combines the ecological stoichiometry of phytoplankton-zooplankton interactions with coevolution of phytoplankton and zooplankton size. In our model, evolution of cell size modifies the nutrient uptake kinetics of phytoplankton according to known allometric relationships, which in turn affect the nutritional quality of phytoplankton. With this size-based mechanism, the model predicts that low grazing pressure or nonselective grazing by zooplankton favors evolution of small phytoplankton cells of high nutritional quality. In contrast, selective grazing for nutritious food favors evolution of large phytoplankton of low nutritional quality, which are preyed upon by medium- to large-sized zooplankton. This size-dependent change in food quality may explain the commonly observed shift from dominance by small picophytoplankton in oligotrophic waters with low grazing pressure to large phytoplankton species in nutrient-rich waters with high grazing pressure.