Atom 15n Excess

Growth (days to weeks) and uptake rates (hours to days) were assessed. In the growth experiments, K. brevis, previously depleted in nitrogen (N), was grown at 2 light intensities in the presence of varying concentrations of Synechococcus. Under high irradiance (300 µmol photons m–2 s–1; 14 h light: 10 h dark cycle), expo¬nential growth rates approximated those of phototrophic growth without Synechococcus (0.26 to 0.35 d–1). At this irradiance, K. brevis cells in all treatments grew for about 10 d. Under lower irradi¬ance (43 µmol photons m–2 s–1), exponential growth rates of K. brevis cells varied with the enrichment level of Synechococcus, with rates under the highest Synechococcus enrichment level being nearly twice (0.58 d–1) those observed for the high irradiance treatments. Short-term uptake experiments (3 experiments, 2 clones) were done to examine the N-specific rates of grazing of 15N-labeled Syne¬chococcus. N-specific rates of grazing ranged from 9.28 × 10–4 h–1 to 1.22 × 10–2 h–1 and varied with the relative proportion of Synechococcus:K. brevis. These rates represent a range of 0.026 to 2.15 pmol-N K. brevis–1 d–1, or 0.96 to 83.8 Synechococcus K. brevis–1 h–1. Evidence of Synechococcus inside K. brevis was provided by confocal microscopy. Grazing by K. brevis thus enhances the range of nutritional substrates available to meet its growth requirements, and may play a substantial role in sustaining natural populations in inorganic N-poor waters.KEY WORDS: Synechococcus Karenia brevis mixotrophic • • • • grazing rates Sunshine nitrogen source • • • Harmful Algal Blooms Red Tide

INTRODUCTION Karenia brevis is a potentially toxic dinoflagellate that causes red tides in the Gulf of Mexico, and is most prevalent along the plateau southwest of Florida (eg, Vargo et al. 2001, 2004).