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

Cell size correlates with most traits among phytoplankton species. Theory predicts that larger cells should show poorer photosynthetic performance, perhaps due to reduced intracellular self‐shading (i.e. package effect). Yet current theory relies heavily on interspecific correlational approaches and causal relationships between size and photosynthetic machinery have remained untested.
As a more direct test, we applied 250 generations of artificial selection (c. 20 months) to evolve the green microalga Dunaliella teriolecta (Chlorophyta) toward different mean cell sizes, while monitoring all major photosynthetic parameters.
Evolving larger sizes (> 1500% difference in volume) resulted in reduced oxygen production per chlorophyll molecule – as predicted by the package effect. However, large‐evolved cells showed substantially higher rates of oxygen production – a finding unanticipated by current theory. In addition, volume‐specific photosynthetic pigments increased with size (Chla+b), while photo‐protectant pigments decreased (β‐carotene). Finally, larger cells displayed higher growth performances and Fv/Fm, steeper slopes of rapid light curves (α) and smaller light‐harvesting antennae (σPSII) with higher connectivity (ρ).
Overall, evolving a common ancestor into different sizes showed that the photosynthetic characteristics of a species coevolves with cell volume. Moreover, our experiment revealed a trade‐off between chlorophyll‐specific (decreasing with size) and volume‐specific (increasing with size) oxygen production in a cell.

在浮游植物（phytoplankton）物种间，细胞大小与绝大多数性状均存在显著相关性。现有理论预测，体积更大的细胞光合性能更差，这一现象或源于细胞内的自身遮光效应，即包装效应（package effect）。然而当前相关理论高度依赖种间相关性研究方法，细胞大小与光合机制间的因果关系尚未得到实验验证。

为开展更为直接的因果验证，我们对绿色微藻杜氏藻（Dunaliella teriolecta，绿藻门（Chlorophyta））开展了250代人工选择（artificial selection），历时约20个月，使其向不同平均细胞大小的方向演化，并同步监测所有核心光合参数。

向更大细胞体积的选育方向（最终体积差异可达1500%以上使得单位叶绿素分子的产氧量降低——这与包装效应的理论预测相符。但经大体积选育的细胞，其整体产氧速率却显著更高——这一发现完全超出当前理论的预期。此外，单位体积的光合色素（叶绿素a+b，Chla+b）随细胞体积增大而增加，而光保护色素（β-胡萝卜素，β-carotene）则随体积增大而减少。最后，体积更大的细胞展现出更高的生长性能与最大光化学效率（Fv/Fm、更陡峭的快速光曲线（rapid light curves）斜率（α）、更小的光系统II（PSII）捕光天线有效截面（σPSII）以及更高的连通性（ρ）。

总体而言，将单一共同祖先演化至不同细胞大小的实验结果表明，物种的光合特性会随细胞体积发生协同演化。此外，本研究还揭示了细胞内两种产氧策略间的权衡关系：以叶绿素特异性计的产氧能力随细胞体积增大而降低，而以单位体积特异性计的产氧能力则随体积增大而提升。