Role of dissolved nitrate and phosphate in isolates of Mesodinium rubrum and toxin-producing Dinophysis acuminata
Mesodinium rubrum (as prey) is a critical component, in addition to light, for growth and toxin production by the mixotrophic dinoflagellate Dinophysis acuminata. Little is known, however, about the role that dissolved inorganic nutrients play in this predator-prey relationship and system toxicity. A series of experiments were conducted to investigate the possible uptake of dissolved nitrate and phosphate by (1) D. acuminata starved of prey, (2) D. acuminata feeding on M. rubrum, and (3) M. rubrum grown in nutritionally modified medium. All single-clone or mixed cultures were monitored for dissolved and particulate nutrient levels over the growth cycle, as well as for growth rate, biomass, and toxin production when appropriate. Dinophysis acuminata did not utilize dissolved nitrate or phosphate in the medium under nutrient-enriched or nutrientreduced regimes, in the absence or presence of prey, or during any growth phase monitored. Changes in particulate phosphorus and nitrogen in D. acuminata were instead strongly influenced by the consumption of M. rubrum prey, and these levels quickly stabilized once prey were no longer available. Mesodinium rubrum, in contrast, rapidly assimilated dissolved nitrate and phosphate, with maximum uptake rates of 1.38 pmol N cell(-1) d(-1) and 1.63 pmol P cell(-1) d(-1), respectively. While D. acuminata did not benefit directly from the dissolved nitrate and phosphate, its growth (0.37 +/- 0.01 d(-1)) and toxin production rates for okadaic acid, dinophysistoxin-1 and pectenotoxin-2 (0.1, 0.9 and 2.6 pg cell(-1) d(-1), 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 D. acuminata, these nutrient pools contribute to prey growth and biomass, thereby indirectly influencing D. acuminata blooms and overall toxins in the system.