Effects of cell size and shape on plankton diversity

Shapes of plankton cells have a huge variety. We analyse the shapes of more than 5700 samples of marine unicellular plankton from six locations around the globe. We show that there are regular patterns both in the effects of shape elongation on species diversity and in the mechanical constraints on cell dimensions. The cell surface area scales with volume approximately as S~V^2/3, implying no tendency for elongation with increasing volume. However, cell size has a strong impact on the potential cell elongation with intermediate-volume cells including the largest variety to shapes ranging from oblate to extremely elongated forms. Almost for all phylogenetic groups in our study with the exception of diatoms, the elongation and volume of cells are the most crucial drivers of phytoplankton diversity. The most diverse plankton groups have the minimal S:V ratio, implying that minimization of cell wall area is likely a more beneficial strategy than its increase. Our results have important implications for understanding and classifying the morphological diversity of plankton communities.