Marine phytoplankton face eco-evolutionary pressure to regulate their vertical position in the ocean to access light, which is abundant towards the surface, and nutrients, which are found deeper down the water column. All phytoplankton experience gravitational sinking, which can contribute to their vertical migration. However, the biophysical and molecular mechanisms that impact gravitational sinking have not been systematically characterized across taxa and environmental conditions. Here, Wu et al. combine simulations with measurements of cell mass, volume, and composition to investigate the effects of nutrient availability on gravitational sinking in nine representative unicellular pico- and nanoplankton species. They find that gravitational sinking becomes faster in most species when starved, but the biophysical changes responsible for this vary across species and starvation conditions. For example, the faster sinking of Chaetoceros calcitrans is nearly exclusively driven by cell density whereas that of Emiliania huxleyi is due to cell volume. Overall, their work reveals that phytoplankton physiology has evolved multiple mechanisms that impact gravitational sinking in response to starvation, possibly to support the vertical migration of the cell. This scanning electron microscopy image of Tetraselmis sp., a green alga, portrays the sinking of the cell following starvation.

Image Credit: Teemu Miettinen, Margaret Bisher, Abigail Lytton-Jean