Effects of nitrogen and phosphate enrichment on the activity of nitrate reductase of Ulva prolifera in coastal zone

As one of the main species causing “green tides”, Ulva prolifera always inhabits in estuarine areas with changes in salinity and nutrients. Reduced salinity may affect directly or indirectly the processes of uptake and assimilation of nitrate, in which the nitrate reductase (NR) activity play the crucial roles. In this experiment, we investigated the different effects of enriched nitrogen and phosphate on NR activity of Ulva prolifera at salinity 30, 15, and 5 psu. The results showed that when salinity being lowered NR activity decreased under no enrichment (CT) or PO 4 3− enrichment condition. NO 3 − or combination with PO 4 3− could significantly enhance NR activity at three salinities, among which the highest value occurred at 15 psu. Enrichment of NH 4 + significantly decreased NR activity at 30 and 15 psu, but not at 5 psu. The results suggested NR of Ulva prolifera could be triggered by NO 3 − , especially at middle salinity, and keep low when exposed under hyposaline or NH 4 + enrichment for long term to rapidly respond to pulse of NO 3 − in estuarine areas.