Effects of light intensity and quality on the relative N and P requirement (the optimum N:P ratio) of marine planktonic algae

Abstract

The relative requirement of N and P (the optimum N:P ratio) by Dunaliella tertiolecta , Phaeodactylum tricornutum , Prymnesium parvum and Thalassiosira pseudonana was studied under various light intensities and spectra. The ratio was determined as the ratio of the minimum cell N and P concentrations (q 0 N and q 0 p when either nutrient was limiting. The ratio varied widely among species; under light-saturation for growth (116 μEin m−2 s −1 it ranged from 11.8 in D . tertiolecta to 36.6 in P. tricornutum . The ratio appeared to be higher at a sub-saturating intensity (24 μEin m −2 s −1 in all except P. tricornutum , mainly because of higher q oN with little change in q oP . In T. pseudonana Q oP also increased, resulting in an insignificant change in the ratio. The ratio varied little within the range of saturation intensity. Light quality affected q oN and q oP as well as the ratio, and the pattern of change varied from species to species. The optimum ratio of individual species was linearly correlated to their q oN except in P. tricornutum . q oN for all species showed a linear correlation with cell protein concentrations irrespective of light conditions. The change of optimum N:P ratios in the three species thus appears to be related to changes in cell protein contents. The ratio of carbohydrates to protein remained constant regardless of light intensity or quality and was higher in P-limited cultures. We conclude that changes in light regime can strongly influence algal nutrient requirements and species interrelationships by altering the optimum cellular N:P ratio.