Environmental and Behavioral Influences on Karenia Brevis' Nitrate Uptake

SINCLAIR, GEOFFREY ALLAN. Environmental and behavioral influences on Karenia brevis’ nitrate uptake. (Under the direction of Daniel Kamykowski and Thomas Wolcott.) I hypothesized that concentrations of Karenia brevis, observed near the bottom of a 22 m water column on the West Florida Shelf, might be using sediments as a nutrient source in its oligotrophic environment. In order to optimize cell growth, cells would need to alternately maximize exposure to light, by swimming up into an oligotrophic water column during the day, and nutrients (represented by nitrate), by swimming down at night to the sediment-water interface. Understanding how cell physiology and cell behavior contribute to the acquisition of resources (light and nutrients) that are separated in space is critical to understanding how K. brevis populations persist in oligotrophic environments. To understand how K. brevis’ physiology contributes to acquisition of nitrate we tested how K. brevis’ uptake of nitrate changes with different prior environmental exposures. We simulated two conditions that cells might encounter: up in an oligotrophic water column and near the sediment-water interface. The first culture represented cells at the apex of their migration away from the sediments and was grown under high light (350 μmol quanta m sec) and reached nitrate-depleted conditions (< 0.5 μM NO3) at the time of the experiment. The second culture represented cells that remain near the sediment and was grown under low light (60 μmol quanta m sec) and was under nitrate replete conditions (20 μM NO3) at the time of the experiment. Cells exposed to nitrate-depleted environments 12 hours prior to the experiment showed enhanced nocturnal uptake relative to cells exposed to 20 μM NO3. K. brevis may also use its vertical migration to facilitate nutrient acquisition. In order to understand how vertical migration influenced K. brevis’ diel uptake of nitrate we created a 1.5 m deep stratified mesocosm. We examined K. brevis’ internal N and uptake rates at three depths, the surface, middle, and bottom that corresponded to light levels of 350, 125 and 60 μmol quanta m sec. The upper 2/3 of the mesocosm, encompassing the surface and middle samples, was nitrate depleted (< 0.5 μM NO3) and simulated an oligotrophic water column. The lower 1/3 of the mesocosm (bottom sample) contained 10 μM NO3 corresponding to elevated nutrients near the sediment. Cell movement and biochemical state support vertical migration as a mechanism for nitrate acquisition. Nocturnal uptake in the mesocosm was significantly less than diurnal uptake. Nocturnal uptake rates in the mesocom were intermediate between cells exposed to nitrate-depleted conditions and nitrate-replete conditions in the first experiment. K. brevis appeared to couple changes in nocturnal uptake with its migration behavior to optimize exposure to light during the day and access to sediments as a nutrient source at night. Migration of K. brevis may allow it to exist farther offshore in regions where benthic microalgae are limited by light. The nocturnal uptake rates of nitrate are sufficient to maintain average growth rates of 0.3 div day in near-bottom populations. Near-bottom populations of K. brevis may grow undetected in offshore and oligotrophic areas where nonmotile phytoplankton would not be able to exploit near-bottom nutrients. Onshore advection of such offshore K. brevis populations by upwelling events could account for population aggregations that suddenly appear at the surface. ENVIRONMENTAL AND BEHAVIORAL INFLUENCES ON KARENIA BREVIS’ NITRATE UPTAKE By GEOFFREY ALLAN SINCLAIR A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science MARINE EARTH AND ATMOSPHERIC SCIENCES