Cell size, photosynthesis and the package effect: an artificial selection approach

Area of Science:

• Marine biology
• Photosynthesis research
• Phytoplankton ecology

Background:

• Cell size is a key trait in phytoplankton, influencing many physiological characteristics.
• Existing theories predict larger phytoplankton cells have reduced photosynthetic performance due to self-shading (package effect).
• Causal links between cell size and photosynthetic machinery remain largely untested, relying on interspecific correlations.

Purpose of the Study:

• To experimentally test the causal relationship between cell size and photosynthetic performance in phytoplankton.
• To investigate how artificial selection for cell size impacts photosynthetic parameters.
• To understand the coevolution of cell volume and photosynthetic traits in microalgae.

Main Methods:

• Applied 250 generations of artificial selection to evolve the green microalga Dunaliella teriolecta towards different mean cell sizes.
• Monitored key photosynthetic parameters, including oxygen production, pigment content, and light-harvesting efficiency.
• Quantified changes in chlorophyll, photo-protectant pigments, growth rates, and light response curves.

Main Results:

• Evolved larger cell sizes (>1500% volume increase) led to reduced oxygen production per chlorophyll molecule, supporting the package effect theory.
• Contrary to predictions, larger evolved cells exhibited substantially higher overall rates of oxygen production.
• Volume-specific photosynthetic pigments (Chla+b) increased with size, while photo-protectant pigments (β-carotene) decreased. Larger cells showed higher growth, Fv/Fm, steeper light curve slopes (α), smaller antennae (σPSII), and higher connectivity (ρ).

Conclusions:

• Phytoplankton photosynthetic characteristics coevolve with cell volume, demonstrating a direct causal link.
• A trade-off exists between chlorophyll-specific (decreasing with size) and volume-specific (increasing with size) oxygen production.
• Cellular evolution towards larger sizes yields enhanced growth and photosynthetic rates despite increased self-shading.