ROLE OF INORGANIC IONS IN CONTROLLING SEDIMENTATION RATE OF A MARINE CENTRIC DIATOMDITYLUM BRIGHTWELLI1,2

Publisher Website
Google Scholar
Abstract
The settling rates and intracellular levels of K+, Na+, Cl‐, Mg2+and Ca2+were measured inDitylum bright‐welli(West) Grunow, grown axenically in an enriched seawater medium at 20 C at 4,000 lx on an 8:16 LD schedule. Cells at the end of the dark period have high Na+(118 mM), low K+(64 mM) and low Cl‐(117 mM) relative to levels at the end of the light period when K+(126 mM) and Cl‐(154 mM) are high and Na+(101 mM) is low. There is no significant change in Mg2+(16–18 mM) or Ca2+(3–4 mM) with time. The net result of the ion changes during the light period is to increase cell density by about 3.4 mg ml‐1. This change can account for the increase in settling rate of ca. 0.3 day‐1during the same interval. The density of the cell contents, calculated from observed ion concentrations, is 15–18 mg.ml‐1less than that of the seawater medium. The ion and settling rate changes are light‐dependent and do not persist in the dark or under constant light (ca. 850 lx), but cells do exhibit a free‐running circadian rhythm in cell division under continuous dim illumination. The cell vacuole expands during the light period and contracts during the dark, apparently in response to the net ion fluxes. D. brightwelliappears to regulate its density by active ion selectivity accompanied by trans‐vacuolar water movement.