Diatoms sink primarily because their density is greater than that of the surrounding seawater.
The fundamental reason why small marine diatoms descend through the water column is related to their physical properties compared to the medium they inhabit. As supported by scientific studies, small marine diatoms generally have a greater density than does seawater (Miklasz and Denny, 2010; Villareal et al., 2014; Kemp and Villareal, 2018).
This difference in density is governed by basic principles of buoyancy. When an object, such as a diatom cell, is denser than the fluid it is in (seawater), the force of gravity pulling the object downwards is stronger than the buoyant force of the fluid pushing it upwards.
- Density Defined: Density is a measure of mass per unit volume. A higher density means more mass is packed into the same amount of space.
- Seawater Density: Seawater density varies with temperature, salinity, and pressure, but it is typically around 1025 kg/m³.
- Diatom Density: The material making up a diatom cell, particularly its silica cell wall (called a frustule), contributes significantly to its overall density, making it generally higher than that of seawater.
Because diatoms are denser than seawater, gravity overcomes the buoyant force, causing them to sink downwards through the water column. This sinking process is a critical factor in the marine carbon cycle, transporting organic matter from the surface to deeper waters.
