Nutritional strategy for the preferential uptake of NO3--N by Phaeocystis globosa.

The cosmopolitan alga Phaeocystis globosa forms massive colonies during blooms. In addition to producing haemolytic toxins that affect the ecosystem, centimetre-sized colonies can block nuclear power plant cooling systems. Nitrogen plays a key role in P. globosa blooms; however, the preferred form of inorganic nitrogen taken up by colonies remains unknown. Therefore, we set up four groups with different nitrogen types and levels of enrichment. The culture medium and intracolonial fluid were sampled at different times for the determination of nutrient concentrations to elucidate the nutrient uptake strategy that presumably favours colonies. Although NH 4 + - N and NO 2 - -N were absorbed by the colonies, NO 3 - -N was the favoured nitrogen, and the average uptake rate of NO 3 - -N was significantly higher than that of NH 4 + -N and NO 2 - -N in each group (P < 0.05). In the enriched groups, the replenishment of NO 3 - -N in the intracolonial fluid was faster than that of NH 4 + -N . The symbiotic bacteria and some biochemical processes may explain the differences in the nutrient concentrations inside and outside of the colonies. Considering the high consumption of NO 3 - -N during P. globosa blooms, controlling the concentration of NO 3 - -N is key to preventing P. globosa blooms. [ABSTRACT FROM AUTHOR]