PHOSPHORUS UPTAKE BY RESTING CYSTS OF THE MARINE DINOFLAGELLATE SCRIPPSIELLA-TROCHOIDEA

Resting cysts of the marine dinoflagellate Scrippsiella trochoidea wer e produced under phosphorus (P)-deficient conditions, separated from v egatative cells, and incubated for 28 days in darkness at 4 and 20 deg rees C in P-enriched and P-deplete medium. The P content of cysts incu bated in the P-replete medium was significantly higher than that of cy sts in P-deplete medium. As the P content of the cysts increased throu gh time, dissolved inorganic phosphate was depleted in the medium. Thi s decrease cannot be attributed to free-living bacterial uptake, since there was no corresponding increase in bacterial particulate P. Disap pearance of P from the medium can, therefore, only be explained by upt ake associated with the cysts. This could be either direct cyst uptake , uptake by bacteria closely associated with the cysts, or adsorption of P on the cyst wall. Evidence is strongest that the cysts incorporat ed phosphate during the resting stages of dormancy and quiescence, des pite the fact that these are periods of significantly reduced metaboli sm. Accumulation of P during these benthic resting stages would increa se the survival of newly excysted vegetative cells as they re-enter th e water column after germination, providing a competitive advantage ov er other phytoplankton. Freshwater and marine sediments provide a P-ri ch environment which may serve as a potential nutrient pool for dinofl agellate resting cysts. Mobilization of nutrients to and from the sedi ments via cysts must now be evaluated to ascertain whether this could be a significant term in nutrient budgets.