Effect of tissue nitrogen and phosphorus quota on growth of Porphyra yezoensis blades in suspension cultures

The effect of tissue N and P on growth of Porphyra yezoensis (strain U-51) blades in suspension cultures was investigated. Blades had the ability to store N in excess of requirements. The critical (0.40% fresh wt) and subsistence (0.153% fresh wt) levels of N were constant regardless of N source (NO3- or NH4+) or light level. Blades did not have the ability to store excess P over the range of P loads given. The subsistence quota for P was higher when NH4+ was given, suggesting a decreased ability to utilize tissue P for growth. NO3- was a better source of N than NH4+ in terms of growth. Blades became bright green in colour when N limited, suggesting a link between phycoerythrin and tissue N. The optimal molar N: P of 13-15 was constant regardless of N source (NO3- or NH4+) or light level. N: P <13-15 indicated N limitation, while N: P >13-15 indicated P limitation. P limited and light limited blades could store more N when NH4+ was given, than when NO3- was the N source, suggesting physiological mechanisms for taking advantage of this usually ephemeral N source, even when growth was limited. N and P reserves were used up relatively quickly (5 days), a characteristic of opportunistic species. Tissue analysis for N and P was a useful technique for determining nutrient status of P. yezoensis blades in land based tanks. As long as tissue N >0.40% fresh wt. and an N: P supply ratio of 13-15 is maintained, blade growth is not limited by N or P.