Are Small Diatoms Capable of Positive Buoyancy?

Diatoms are important members of the phytoplankton that contribute significantly primary production and the removal of carbon from the euphotic zone. While passive sinking is well recognized in this group, some very large species are capable of vertical migration. These species can sink into the nutricline, acquire nutrients, and then ascend by regulating their buoyancy. The potential for upward movement by smaller diatoms is much less well understood. Limited observational data on bulk movements as well as models suggest that small diatoms also exhibit positive buoyancy. We studied this phenomenon by measuring the ascent speed of small diatoms using a particle tracking technique. Diatoms of different sizes (Thalassiosira sp., Odontella sp., and Pseudosolenia calcar-avis) were grown in four different light intensities (150, 75, 25, 18 µmol photons m^-² s^-1) and their ascent/descent rates recorded in a salinity-stratified settling column using digital videography and the computer program Image J. Positive buoyancy was not observed in this method with samples sizes of ~50 cells, and the average sinking velocity was greater in large diatoms, as expected. Subsequent experiments combined the SETCOL methodology with particle tracking. Diatom cultures were added to the SETCOL and allowed to sort for 8 hours, separating the positively buoyant cells, if present. At the end of the sorting period, the diatoms that were in the upper chamber of the column were collected and filmed. Average positive buoyancy values of 0.14 and 0.28 meters per day were seen in Thalassiosira sp. grown at low light intensity levels (25 and 18 µmol photons m^-² s^-1) after sorting in the SETCOL (N=40 cells). This work is the first to directly observe ascending behavior in small diatoms. While positive buoyancy was uncommon in this study, it did occur and clearly documents the capacity for smaller diatoms to ascend. If such behavior occurs in the open sea, the unidirectional downward transport by diatoms will need to be reconsidered. It may be particularly important for development of diatom blooms at the base of the euphotic zone.