Environmental and Behavioral Influences on Karenia Brevis' Nitrate Uptake
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Date
2005-04-03
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Abstract
A near-bottom population of Karenia brevis, observed in a 22 m water column on the West Florida Shelf may use sediments as a nutrient source in oligotrophic environments. First, we examined how different prior environmental exposure that cells may encounter migrating away from the sediment influence K. brevis' uptake of nitrate. Second, we examined if K. brevis may use diel vertical migration (DVM) as a nitrate retrieval mechanism.
The first experiment isolated environmental effects on K. brevis' uptake of nitrate. One culture represented cells at the apex of their migration away from the sediments and was grown under high light (350 μmol quanta m-2 sec-1) and reached nitrate depleted conditions (< 0.5 μM NO3) at the time of the experiment. The other culture represented cells that remain near the sediment and was grown under low light (60 μmol quanta m-2 sec-1) 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 enhanced nocturnal uptake relative to cells exposed to 20 μM NO3.
The second experiment examined K. brevis' uptake of nitrate over its DVM. Internal cellular N reserves and uptake rates of nitrate were evaluated at three depths in a stratified mesocosm. The depths, surface, middle and bottom corresponded to light levels 350, 125 and 60 μmol quanta m-2 sec-1. The upper 2/3 of the mesocosm, encompassing the surface and middle samples, contained < 0.5 μM NO3. The lower 1/3 of the mesocosm (bottom sample) contained 10 μM NO3). Cell movement and biochemical state support that vertical migration is a mechanism for nitrate retrieval. 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 couples changes in nocturnal uptake of with its migration behavior to optimize exposure to light during the day and use sediments as a nutrient source at night. Migration of K. brevis may allow it to exist further 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-1 of near-bottom populations. Near-bottom populations of K. brevis may grow undetected in offshore and oligotrophic areas where other phytoplankton may not be able to exploit near-bottom nutrients. These offshore K. brevis populations that are advected onshore by upwelling events may account for population aggregations that suddenly appear at the surface.
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Keywords
behavior, nitrate uptake, ecology, physiology, migration, dinoflagellates
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
