Does energy control the sinking rates of marine diatoms?

There is an energy requirement for the maintenance of low sinking rates (<0.1 m d −1 ) in the marine diatom Ditylum brightwellii. The lowest sinking rates occurred when cells were grown under irradiances saturating for growth. The highest sinking rates occurred when cells grown at growth‐limiting irradiances were placed in the dark, or in cells treated with a respiratory inhibitor. Small cells sank more rapidly than large cells under saturating light, but under severe energy limitation sinking rates were directly proportional to cell volume. When cells were grown under saturating irradiance and then placed in the dark, their sinking rate increased as their respiration rate decreased over time. We suggest that when light is unavailable, a diatom’s ability to use energy gained from respiration may be the principal determinant of its sinking rate. We provide the first estimate of the dependence of cell sinking rate on cell respiration and suggest that the sinking‐respiration relationship may be useful in predicting the sinking dynamics of diatoms during a bloom. We speculate that energetic sinking control may have evolved to allow diatoms to colonize the photic zone and that larger diatoms with lower sinking rates may have an ecological advantage over smaller diatoms with higher sinking rates early in a bloom.