Why Do Phytoplankton Evolve Large Size in Response to Grazing?

Area of Science:

• Marine ecology
• Evolutionary biology
• Biogeochemistry

Background:

• Phytoplankton exhibit diverse cell sizes, influencing ecological interactions.
• Small phytoplankton are efficient nutrient competitors but vulnerable to grazing.
• Large phytoplankton size is often attributed to physical defense against grazers, but this can be countered by coevolution.

Purpose of the Study:

• To investigate the hypothesis that larger phytoplankton evolve lower nutritional quality as a defense against zooplankton grazing.
• To explore the eco-evolutionary dynamics of phytoplankton and zooplankton size and nutritional quality.

Main Methods:

• Development and analysis of an eco-evolutionary model.
• Integration of ecological stoichiometry with coevolution of phytoplankton and zooplankton size.
• Modeling how cell size affects nutrient uptake kinetics and nutritional quality.

Main Results:

• Low or nonselective grazing favors small, high-quality phytoplankton.
• Selective grazing on nutritious food favors large, low-quality phytoplankton.
• Cell size evolution alters phytoplankton nutritional quality, impacting predator-prey dynamics.

Conclusions:

• Phytoplankton cell size evolution is influenced by grazing pressure and selectivity.
• Nutritional quality is a key factor in phytoplankton size evolution, complementing physical defenses.
• This size-dependent change in food quality may explain phytoplankton distribution patterns in different aquatic ecosystems.