EXPERIMENTAL OBSERVATIONS ON THE FLOTATION OF MARINE DIATOMS. II. SKELETONEMA COSTATUM AND RHIZOSOLENIA SETIGERA

The influence of ccl1 age, cell size, cell shape, and mode of colony formation on diatom sinking rates was investigated in 26 experiments with Skeletonema costatum (Grev. ) Cleve and R~~ZOXJ~Z the absolute minimum and maximum rates observed were 0.15 and 7.39 m/day. Senescent populations were characterized by higher sinking rates. The number of cells/chain decreased with increasing culture age, resulting in the dominance of the population by solitary cells and two-celled chains after 16 days. The sinking rate of S. costatum was inversely related to the number of cells/chain (colony size) irrespective of culture age. This suggests that the formation of silica rods and chains by S. costatum is an adaptation to flotation. The mean sinking rate of R. setigera ranged from 0.16 to 1.77 m/day; the absolute minimum and maximum rates observed were 0.05 and 22.71 m/day. Post-auxospore cells sank more rapidly, in general, than prc-auxospore cells, possibly reflecting the lower surface area : cell volume ratio of the former type. Spineless prc-auxospore cells sank more rapidly than those having both apical spines. The sinking rate was generally hjgher in senescent cultures, except where post-auxospore cells dominated. The post-auxospore cells frequently exhibited an unique ascending movement off the bottom of the settling chambers that may be related to the light and dark period characteristics of the experiments. This phenomenon became less pronounced, or ceased, with senescence. observed, with certainty, on only one occasion.