Sinking rate versus cell volume relationships illuminate sinking rate control mechanisms in marine diatoms

Abstract

It has been shown that for dead marine diatom cells or diatom cells which 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 reaches a maximum. Within cultures of rapidly dividing cells, larger cells' s~n k i n g rate is reduced physiologically to that of smaller cells and the slope of this relationship approaches zero. In several marine d~a t o m species between 5 and 100 pm in diameter, dev~ations from the maximum slope of the volume versus sinking rate relationship could b e used to quantify the physiological reduction of sinking rates. This allowed 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 rates to be proport~o n a l to cell volume a n d the energy-requiring, protoplast and vacuolar component which, when active, allows si.nklng rates to become independent of cell volume Across the 9 species of diatoms examined, ~ncluding the 3 single-celled species (Ditylun~ brightwellii, Thalassiosira pseudonana, and 7. weissflogii), 4 chain-forming coastal bloom diatoms (T aestivalis, Skeletonema costatum. Chaetoceros debilis and C. cornpressuni) and 2 large floating open ocean species (Ethrnodiscus sp. a n d entire Rhizosolenia spp. mats), there was a strong correlation between log cell volume and sinking rate only for cells that were metabolically inactivated either through extended dark treatment or through treatment with the respiratory inhibitor KCN This was true both within and between cultures. However, no correlation between s~n k i n g rate and cell volume was found for rapidly growlng cells maintamed at saturating irradiances. This supports the notion that there is no obligate correlation between 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 a n important feature to include in models where carbon flux is predicted on the basis of particle size spectra. We suggest that the minimum cell volume necessary for active sinking rate control is ca 200 pm3, and that this represents a lower limit for Villareal's (1988; Deep Sea Kes 35:1037-1045) theoretical minimum volume necessary for posltlve buoyancy.

Diatom sinkings speeds: Improved predictions and insight from a modified Stokes' law

“…Stokes' law predicts how quickly a spherical diatom should sink given a certain overall cell density, r tot . As previously described, diatoms reduce their density through ion exchange, a physiologically costly process (Anderson and Sweeney 1978;Waite et al 1992). In the absence of physiological density reduction, diatoms sink at their maximum speed, which should be governed solely by fluid mechanics (Waite et al 1997).…”

“…Diatoms fit the assumptions of Stokes' law: they are small, roughly spherical, and slow moving. Nonetheless over the last 50 yr, empirical measurements of diatom sinking speeds have shown that the power-law relationship between radius and velocity has a scaling exponent between 1.2 and 1.6 (Smayda 1970;Waite et al 1997). This mismatch between empirical observation and theory has often been noted, (Walsby and Reynolds 1980;Reynolds 2006), but the physical basis for the mismatch is yet to be resolved.…”

“…This physiological manipulation of density explains much of the observed variability in sinking speeds of metabolically active diatoms (Waite et al 1992(Waite et al , 1997Fisher and Harrison 1996). In nutrient-deprived or physiologically inhibited treatments, in which diatoms are metabolically constrained, sinking speed approaches a size-dependant maximum value (Waite et al 1992) and ostensibly should be governed by Stokes' law. Yet even under such treatments, Waite et al (1997) found a scaling exponent of 1.6 between maximum sinking speed and size, contrary to the prediction of Stokes' law.…”

“…In nutrient-deprived or physiologically inhibited treatments, in which diatoms are metabolically constrained, sinking speed approaches a size-dependant maximum value (Waite et al 1992) and ostensibly should be governed by Stokes' law. Yet even under such treatments, Waite et al (1997) found a scaling exponent of 1.6 between maximum sinking speed and size, contrary to the prediction of Stokes' law.…”

The measurement of phytoplankton biomass using flow‐cytometric sorting and elemental analysis of carbon

“…Carbon (pg C cell Bertilsson et al 2003, Heldal et al 2003, and Verity et al 1992 Values taken or calculated from Johnson et al 2009, Durand et al 2002, and Montagnes et al 1994 Values taken or calculated from Waite et al 1997 andMontagnes et al 1994. Table 2 for references.…”