Dinoflagellates are a major component of marine phytoplankton and many species are recognized for their ability to produce harmful algal blooms (HABs). Scrippsiella trochoidea is a non-toxic, marine dinoflagellate that can be found in both cold and tropic waters where it is known to produce “red tide” events. Little is known about the genomic makeup of S. trochoidea and a transcriptome study was conducted to shed light on the biochemical and physiological adaptations related to nutrient depletion. Cultures were grown under N and P limiting conditions and transcriptomes were generated via RNAseq technology. De novo assembly reconstructed 107,415 putative transcripts of which only 41% could be annotated. No significant transcriptomic response was observed in response to initial P depletion, however, a strong transcriptional response to N depletion was detected. Among the down-regulated pathways were those for glutamine/glutamate metabolism as well as urea and nitrate/nitrite transporters. Transcripts for ammonia transporters displayed both up- and down-regulation, perhaps related to a shift to higher affinity transporters. Genes for the utilization of DON compounds were up-regulated. These included transcripts for amino acids transporters, polyamine oxidase, and extracellular proteinase and peptidases. N depletion also triggered down regulation of transcripts related to the production of Photosystems I & II and related proteins. These data are consistent with a metabolic strategy that conserves N while maximizing sustained metabolism by emphasizing the relative contribution of organic N sources. Surprisingly, the transcriptome also contained transcripts potentially related to secondary metabolite production, including a homolog to the Short Isoform Saxitoxin gene (sxtA) from Alexandrium fundyense, which was significantly up-regulated under N-depletion. A total of 113 unique hits to Sxt genes, covering 17 of the 34 genes found in C. raciborskii were detected, indicating that S. trochoidea has previously unrecognized potential for the production of secondary metabolites with potential toxicity.

) and nitrate-replete conditions (~20 μM NO 3 -) 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. KEY WORDS: Karenia brevis · Dinoflagellate · Physiology · Nocturnal uptake · Vertical migrationResale or republication not permitted without written consent of the publisher

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Karenia brevis may optimize growth by alternately maximizing exposure to light, migrating up into an oligotrophic water column during the day, and to nutrients (nitrate), by migrating down to the sediment-water interface at night. Understanding how cell behavior contributes to the acquisition of light and nutrients that are separated in space is critical to understanding how K. brevis populations persist in oligotrophic environments. In response to previous modeling efforts that parameterized cell physiology and behavior in nitrate-replete conditions, we examined similar cellular characteristics in a stratified 1.5 m deep mesocosm. The upper 2/3 of the mesocosm, encompassing the surface and middle samples, was nitrate depleted (< 0.5 µM NO 3 -) and simulated an oligotrophic water column. The lower 1/3 of the mesocosm contained 10 µM NO 3 -corresponding to elevated nutrient levels near the sediment -water interface. We sampled uptake rates at 3 depths during the day at light levels of 350, 125 and 60 µmol quanta m -2 s -1 and again at night in the dark. Nocturnal uptake of nitrate in the mesocosm was significantly less than diurnal uptake. Nocturnal uptake rates in the mesocom were intermediate between cells exposed to prolonged nitrate-depleted and nitrate-replete conditions. Both migration, as indicated by diel aggregation patterns, and cell physiology indicate that descent to regions of higher nutrient concentrations were sufficient to maintain average growth rates of 0.3 div d