Direct quantification of unicellular algae sinking velocities reveals cell size, light, and nutrient-dependence

Abstract

Eukaryotic phytoplankton, also known as algae, form the basis of marine food webs and drive marine carbon sequestration when their biomass sinks to the ocean floor. Algae must regulate their vertical movement, as determined by motility and gravitational sinking, to balance access to light at the surface and nutrients in deeper layers. However, the regulation of gravitational sinking velocities remains largely unknown, especially in motile species. Here, we directly quantify single-cell masses and volumes to calculate sinking velocities according to Stokes law in diverse clades of unicellular marine microalgae. Our results reveal the cell size, light, and nutrient-dependency of sinking velocities. We identify motile dinoflagellate and green algal species that increase their sinking velocity in response to starvation. Mechanistically, this increased cell sinking is achieved by photosynthesis-driven accumulation of carbohydrates, which increases cell mass and density. Moreover, cell sinking velocities correlate inversely with proliferation rates, and the mechanism regulating cell sinking velocities integrates signals from multiple nutrients. Our findings suggest that the regulation of cell composition according to environmental conditions contributes to the vertical movement of motile cells in the oceans. More broadly, our approach for sinking velocity measurements expands the study of gravitational sinking to motile cells and supports the modeling of marine carbon pump and nutrient cycles.