Role of dissolved nitrate and phosphate in isolates of and toxin-producing .

, a producer of toxins associated with diarrhetic shellfish poisoning (DSP) and/or pectenotoxins (PTXs), is a mixotrophic species that requires both ciliate prey and light for growth. Linkages have been described in the literature between natural abundances of the predator and its prey, , and culture experiments have demonstrated that prey, in addition to light, is required for toxin production by ; together these suggest is a critical component for growth and toxicity. However, little is known about the role of dissolved inorganic nutrients on growth or that of toxin-producing . Accordingly, a series of experiments were conducted to investigate the possible uptake of dissolved nitrate and phosphate by 1) starved of prey, 2) feeding on , and 3) grown in nutritionally-modified media. All single-clone or mixed cultures were monitored for dissolved and particulate nutrient levels over the growth cycle, as well as growth rate, biomass, and toxin production when appropriate. did not utilize dissolved nitrate or phosphate in the medium under any nutrient regime tested, i.e., nutrient-enriched and nutrient-reduced, in the absence or presence of prey, or during any growth phase monitored, i.e., exponential and plateau phases. Changes in particulate phosphorus and nitrogen in , were instead, strongly influenced by the consumption of prey, and these levels quickly stabilized once prey were no longer available. , on the other hand, rapidly assimilated dissolved nitrate and phosphate into its particulate nutrient fraction, with maximum uptake rates of 1.38 pmol N/cell/day and 1.63 pmol P/cell/day. While did not benefit directly from the dissolved nitrate and phosphate, its growth (0.37±0.01 day) and toxin production rates for okadaic acid (OA), dinophysistoxin-1 (DTX1) or pectenotoxin-2 (PTX2), 0.1, 0.9 and 2.6 pg /cell/day, respectively, were directly coupled to prey availability. These results suggest that while dissolved nitrate and phosphate do not have a direct effect on toxin production or retention by , these nutrient pools contribute to prey growth and biomass, thereby indirectly influencing blooms and overall toxin in the system.