Grazing by Karenia brevis on Synechococcus enhances its growth rate and may help to sustain blooms

Grazing rates of Karenia brevis Clones CCMP2228 and CCMP2229 were determined in laboratory experiments using Synechococcus sp. Clone CCMP1768 as food. 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 15 N-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.

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