PHOSPHATE ACCUMULATION AND METABOLISM BY HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE) DURING DIEL VERTICAL MIGRATION IN A STRATIFIED MICROCOSM1

Diel vertical migration by Heterosigma akashiwo (Hada) Hada (Raphidophyceae) was monitored in a 1.5 in tall microcosm. Vertical stratification, with low salinity and low orthophosphate (Pi) concentration in the upper layer and high salinity and high Pi concentration in the lower layer, was simulated in the tank, analogous to summer stratification in the Seto Inland Sea. The phosphate metabolism of H. akashiwo during this vertical migration was studied using 31P-NMR spectroscopy. At night this species migrated to the lower phosphate-rich layer and took up inorganic phosphate (Pi) which then was accumulated as polyphosphate (PPi) by an increase in the chain length of PPi During the daytime this species migrated to the phosphate-depleted surface water and utilized the accumulated PPi for photophosphorylation by decreasing the chain length of PPi During the first night after the phosphorus was introduced to the previously impoverished waters, the cells took up inorganic phosphate, accumulating the new phosphorus nutrient internally as Pi But the cells did not convert Pi to PPi presumably due to their lack of ATP. After the second day of the experiment, conversion of Pi to PPi at night was much more rapid than on the first day, presumably due to increased ATP availability. Then the cycle continued, with uptake of Pi and conversion to PPi at night at the bottom and its utilization during the day at the surface. These data suggest that the role of PPi in the metabolism of this species appears to be as a phosphate pool which regulates the level of Pi and ATP in the cell. Diel vertical migration allows this red tide species to shuttle between the phosphate-rich lower layer and the photic upper layer in stratified waters. 31P-NMR is shown to be a valuable tool in studying the phosphorus metabolism in migrating organisms.