Low temperature causes discoloration by repressing growth and nitrogen transporter gene expression in the edible red alga Pyropia yezoensis

The reduced availability of nitrogen sources in seawater leads to discoloration of the edible red seaweed Pyropia yezoensis and induces the expression of genes encoding ammonium, nitrate and urea transporters. In the present study, we demonstrate that low temperatures can also cause discoloration of this economically important seaweed. Thus, we addressed regulatory mechanisms of cold-inducible discoloration. When P. yezoensis thalli were incubated at 0, 5 and 10°C, the thalli exhibited retarded growth and discoloration, along with reduced phycoerythrin contents. Fertilization with nitrogen sources did not recover this discoloration at 0°C, suggesting that defects in nitrogen absorption cause low temperature-induced discoloration. The expression levels of many nitrogen transporter genes were reduced at low temperature. We propose that low temperature-mediated repression of ammonium, nitrate and urea transporter gene expression promotes the reduced absorption of nitrogen sources in P. yezoensis, thereby leading to discoloration. This process is different from the well-known mechanism underlying discoloration in P. yezoensis under nitrogen-deficient conditions at normal culture temperature.