Cell shape: the soul of phytoplankton

Together with Alex Ryabov, I recently published a short post in the 02/2021 HIFMB newsletter, in which we basically outline the ideas and main findings of our study on cell shapes of phytoplankton. Here, the text of the post.

Cell shape: the soul of phytoplankton

Phytoplankton – unicellular photosynthetic microbes which are the major primary producers of the world’s ocean – display a bewildering variety of cell shapes and forms. Surprisingly, not much is known about how its shape diversity relates to its taxonomic richness, evolutionary origin, and ecological function. This opens novel, as yet untapped, approaches to marine biodiversity research. Such research could potentially improve understanding about how accelerating changes in the marine environment might affect planktonic communities or, more fundamentally, lead to a better understanding of the evolutionary principles of life.

To paraphrase Aristotle, we can say that form is the soul of matter, and therefore it should have a significant influence on the fate of that form’s owner. Unicellular life has been formed over billions of years. We do not know how the first unicellular organisms looked, but we do know that the development of complex organisms (the Cambrian explosion) led in the beginning to an amazing variety of body forms. Although the number of species has increased since then, many of the original body shapes were later rejected by competition and evolutionary selection, which favor species with most ergonomic morphology. To understand what makes for a successful body shape, consider a related question: Which body shapes lead to the greatest abundance and species diversity, allowing these species to better adapt to natural conditions?

As the focus of HIFMB is on diversity in marine environments, we turn to algae, and since we are only at the beginning of the journey, limit ourselves to unicellular organisms. Marine phytoplankton make excellent candidates for such a study, being numerous enough to apply statistical laws, and exhibiting great diversity in species and shapes, yet readily amenable for calculating area, volume, and shape characteristics such as aspect ratio and deviation from spherical shape.

We expected to find the greatest species richness in elongated or flattened cells because, unlike, for example, spheres, these forms have a greater potential to build complex structures. But, to our surprise we found the greatest diversity in cells of compact forms with equal linear dimensions such as cubes or spheres. In particular, species at the extreme ends of the size spectrum, both very small and very large, were mostly spherical. In contrast, cells of intermediate size showed the greatest variation, from flat to extremely elongated shapes, even though for them, too, most species had compact cell shapes. Of all shapes, a sphere has the minimum surface area for a given volume and any deviation from the spherical shape “extends” the cell surface. We found the surface extension (socalled sphericity), which compares a cell’s surface to that of a sphere of equal volume, to be a convenient characteristic of cell shape. This measure is independent of cell volume and, besides, has a biological meaning in that it is proportional to the gains and losses caused by increasing surface area. It turned out that for almost all phyla the number of species exponentially decreased with surface extension: 25% of genera had spherical cells, 50% had compact shapes with a surface less than 50% greater than the corresponding sphere surface, and the surface of 75% of all genera exceeded the sphere surface by less than a factor of two. We observed similar laws for species abundance, so that species with compact cells were also the most abundant.

In terms of shape, diatoms stood out from all other phyla. While most algae had compact cells with ellipsoidal or conic shapes, most diatoms were cylindrical or prismatic with elongated or flattened shapes. Perhaps the appearance of silica cell walls in diatoms was an important evolutionary innovation that allowed diatoms to achieve such great shape diversity. Our results paint a phenomenological portrait, but the mechanisms driving the success of various shapes are not yet fully understood. It also raises questions, looking beyond unicellular organisms, regarding effects of body shape in higher life forms. How, for example, do diversity and abundance of flowers depend on the form of leaves and petals, and which plant shapes lead to the greatest taxonomic richness? These and many other questions about the effect of shape on species fitness and diversity remain open.