SINKING RATE VERSUS CELL-VOLUME RELATIONSHIPS ILLUMINATE SINKING RATECONTROL MECHANISMS IN MARINE DIATOMS

It has been shown that for dead marine diatom cells or diatom cells wh ich are severely stressed metabolically, larger cells sink faster than small cells as dictated by Stokes' Law. In these cases, the slope of the sinking rate versus cell volume relationship within a culture reac hes a maximum. Within cultures of rapidly dividing cells, larger cells ' sinking rate is reduced physiologically to that of smaller cells and the slope of this relationship approaches zero. In several marine dia tom species between 5 and 100 mu m in diameter, deviations from the ma ximum slope of the volume versus sinking rate relationship could be us ed to quantify the physiological reduction of sinking rates. This allo wed us to differentiate 2 different components of sinking rate control , the ballasting component (driven by changes in cell composition and volume) which, when dominant, causes sinking sates to be proportional to cell volume and the energy-requiring, protoplast and vacuolar compo nent which, when active, allows sinking rates to become independent of cell volume. Across the 9 species of diatoms examined, including the 3 single-celled species (Ditylum brightwellii, Thalassiosira pseudonan a, and T. weissnogii), 4 chain-forming coastal bloom diatoms (T. aesti valis, Skeletonema costatum, Chaetoceros debilis and C. compressum) an d 2 large floating open ocean species (Ethmodiscus sp. and entire Rhiz osolenia spp. mats), these was a strong correlation between log cell v olume and sinking rate only for cells that were metabolically inactiva ted either through extended dark treatment or through treatment with t he respiratory inhibitor KCN. This was true both within and between cu ltures. However, no correlation between sinking rate and cell volume w as found for rapidly growing cells maintained at saturating irradiance s. This supports the notion that there is no obligate correlation betw een cell volume and sinking rate for metabolically active cells. This potential for cellular modification of the sinking rate versus volume relationship suggests that physiological state may be an important fea ture to include in models where carbon flux is predicted on the basis of particle size spectra. We suggest that the minimum cell Volume nece ssary for active sinking rate control is ca 200 mu m(3), and that this represents a lower limit for Villareal's (1988; Deep Sea Res 35:1037- 1045) theoretical minimum volume necessary for positive buoyancy.