Abstract

Pyropia yezoensis (Rhodophyta), an economically valuable marine red alga, undergoes changes in nitrogen levels during growth. The increased availability of nitrogen sources in seawater promotes algal growth and increases the photosynthetic pigment content. Results of the experiment for determining nitrate uptake rates showed that the thalli rapidly absorb nitrate in the first two days and accumulate large amounts of nitrate and nitrite on the first and third days, respectively. Interestingly, although the thalli absorbed high levels of nitrate and grew rapidly under nitrate-sufficient conditions, the nitrogen utilization efficiency decreased. In the present study, we cloned nitrogen assimilation-associated enzymes , nitrate reductase , nitrite reductase , and glutamine synthetase . Additionally, results of the reverse transcription-polymerase chain reaction assay indicated that the levels of these enzymes increase at 12 h in response to nitrate sufficiency, while their expression is inhibited at 24 h. We propose that excessive ammonium and amino acids produced through metabolism inhibit the nitrate reduction process. This experiment complements the metabolism and accumulation of nitrate in P. yezoensis under nitrate-sufficient conditions.