Uptake of dissolved inorganic and organic nitrogen by the benthic toxic dinoflagellate Ostreopsis cf. ovata.

Environmental factors that shape dynamics of benthic toxic blooms are largely unknown. In particular, for the toxic dinoflagellate Ostreopsis cf. ovata, the importance of the availability of nutrients and the contribution of the inorganic and organic pools to growth need to be quantified in marine coastal environments. The present study aimed at characterizing N-uptake of dissolved inorganic and organic sources by O. cf. ovata cells, using the ¹⁵ N-labelling technique. Experiments were conducted taking into account potential interactions between nutrient uptake systems as well as variations with the diel cycle. Uptake abilities of O. cf. ovata were parameterized for ammonium (NH ₄ ⁺ ), nitrate (NO ₃ ^- ) and N-urea, from the estimation of kinetic and inhibition parameters. In the range of 0 to 10μmolNL ^-1 , kinetic curves showed a clear preference pattern following the ranking NH ₄ ⁺ >NO ₃ ^- >N-urea, where the preferential uptake of NH ₄ ⁺ relative to NO ₃ ^- was accentuated by an inhibitory effect of NH ₄ ⁺ concentration on NO ₃ ^- uptake capabilities. Conversely, under high nutrient concentrations, the preference for NH ₄ ⁺ relative to NO ₃ ^- was largely reduced, probably because of the existence of a low-affinity high capacity inducible NO ₃ ^- uptake system. Ability to take up nutrients in darkness could not be defined as a competitive advantage for O. cf. ovata. Species competitiveness can also be defined from nutrient uptake kinetic parameters. A strong affinity for NH ₄ ⁺ was observed for O. cf. ovata cells that may partly explain the success of this toxic species during the summer season in the Bay of Villefranche-sur-mer (France).
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