Nitrate uptake by Karenia brevis.: I.: Influences of prior environmental exposure and biochemical state on diel uptake of nitrate

The ability of a Karenia brevis population to persist in an oligotrophic water column depends on how cell physiology and cell behavior contribute to the acquisition of light and nutrients that often are separated in space. We hypothesized that an aggregation of K. brevis, observed undergoing a diel vertical migration (DVM) in the bottom half of a 22 m water column on the West Florida Shelf, used the sediments as a nutrient source. We tested how the physiology of K. brevis contributed to the acquisition of nitrate by evaluating how nitrate uptake changed with prior environmental exposure. The experimental conditions simulated the extremes that cells might endure during DVM when migrating up into an oligotrophic water column versus cells that remained near the sediment-water interface. The first culture, representing cells that attained the maximum apex of their migration away from the sediments, was grown under relatively high light (350 mu mol quanta m(-2) s(-1)) and reached nitrate-depleted conditions (< 0.5 mu M NO3-) prior to the experiment. The second culture, representing cells that remained near the sediment-water interface, was grown under relatively low light (60 mu mol quanta m(-2) s(-1)) and nitrate-replete conditions (similar to 20 mu M NO3-) prior to the experiment. Cells exposed to nitrate-depleted environments for 12 h prior to the experiment enhanced nocturnal uptake compared to cells continuously exposed to nitrate-replete conditions. Changes in cell physiology may contribute to nitrate acquisition after descent from oligotrophic environments to areas with elevated nitrate concentrations.