Biochemical Strategies for Growth of Gracilaria Tikvahiae (Rhodophyta) in Relation to Light Intensity and Nitrogen Availability¹

The main effects and interactions between light (I o , full incident sunlight to 0.07 I o ) and NO 3 − loading (0.4 to 4.3 mmol · g dry weight −1 · d −1 ) on growth rate, photosynthesis and biochemical constituents of Gracilaria tikvahiae McLachlan were studied using a factorial design experiment in outdoor, continuous‐flow seawater cultures. Incipient nitrogen limitation in the low NO 3 − loading, I o and 0.57 I o treatments occurred after 2.5 weeks of growth under the experimental conditions and resulted in decreased tissue NO 3 − and R‐phycoerythrin. Tissue NO 3 − and R‐phycoerythrin accounted for up to ca. 15 and 20%, respectively, of the total N in G. tikvahiae suggesting a N reserve role for these N pools. Under light and NO 3 − limitation, growth rate was a parabolic function of the C:N ratio. As light limitation increased, growth rate and the C:N ratio decreased as levels of Chl‐a, R‐phycoerythrin, percent N and percent protein increased. As NO 3 − limitation increased, growth rate and levels of Chl‐a, R‐phycoerythrin, percent N and percent protein all decreased with parallel increases in the C:N ratio. In contrast to the inverse relationship between pigment content and light, ribulose bisphosphate carboxylase (RuBPCase) activity (on both a protein and dry weight basis) varied directly with light. This biochemical acclimation of G. tikvahiae to light and N availability appears to be a process directed towards maximizing photo synthetic capacity and growth.