Alexandrium catenella was grown in semi-continuous cultures in L1 growth-medium enriched with concentrated dissolved organic matter (DOM) from a diatom bloom. In medium with full nitrate (880 M), the average growth rate was 0.32+0.01 day21 (L1, control without added background (10 DOM). Adding natural marine dissolved organic nitrogen (DON) at levels of 20–30 M above M) led to a higher growth rate (L1þDOM, 0.40+0.00 day21). In medium with lower nitrate level (175 dissolved inorganic nitrogen and DON were used, leading to the highest growth rates (0.43+ M) as well as the uptake kinetics of M) and higher DON (L1/5þDOM treatment), both 0.03 day21). In medium without nitrate [(L1-N)þDOM treatment], the low ammonium con- centrations observed throughout the experiment (,1 A. catenella could not have supported the observed growth rates, leading us to conclude that DON was directly used by this organism, rather than using N remineralized by bacteria (from DON). The decrease of bacteria in DOM enriched bioassays could point to a nutrient limitation and com- petition with A. catenella for organic matter. Viruses likely contributed as an additional factor to keep the bacterial population from becoming dominant.
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