Effect of marine autotrophic dissolved organic matter (DOM) on Alexandrium catenella in semi-continuous cultures

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 mu M), the average growth rate was 0.32 +/- 0.01 day(-1) (L1, control without added DOM). Adding natural marine dissolved organic nitrogen (DON) at levels of 20-30 mu M above background (10 mu M) led to a higher growth rate (L1+DOM, 0.40 +/- 0.00 day(-1)). In medium with lower nitrate level (175 mu M) and higher DON (L1/5+DOM treatment), both dissolved inorganic nitrogen and DON were used, leading to the highest growth rates (0.43 +/- 0.03 day(-1)). In medium without nitrate [(L1-N)+DOM treatment], the low ammonium concentrations observed throughout the experiment (< 1 mu M) as well as the uptake kinetics of 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 competition with A. catenella for organic matter. Viruses likely contributed as an additional factor to keep the bacterial population from becoming dominant.