Uptake kinetics and assimilation of phosphorus by Catenella nipae and Ulva lactuca can be used to indicate ambient phosphate availability

Uptake, assimilation and compartmentation of phosphate were studied in the opportunist green macroalga Ulva lactuca and the estuarine red algal epiphyte Catenella nipae. The Michaelis-Menten model was used to describe uptake rates of inorganic phosphate (P sub(i)) at different concentrations. Maximum uptake rates (V sub(max)) of P-starved material exceeded V sub(max) of P-enriched material; this difference was greater for C. nipae. Uptake and allocation of phosphorus (P) to internal pools was measured using trichloroacetic acid (TCA) extracts and super(32)P. Both species demonstrated similar assimilation paths: when P-enriched, most super(32)P accumulated as free phosphate. When unenriched, super(32)P was rapidly assimilated into the TCA-insoluble pool. C. nipae consistently assimilated more super(32)P into this pool than U. lactuca, indicating C. nipae has a greater P-storage capacity. In both species, super(32)P release data showed two internal compartments with very different biological half-lives. The rapidly exchanging compartment had a short half-life of approximately 2 to 12 min, while the slowly exchanging compartment had a much longer half-life of 12 days in P-starved C. nipae or 4 days in P-starved U. lactuca. In both species, the slowly exchanging compartment accounted for more than 90% of total tissue. U. lactuca and C. nipae responded differently to high external P sub(i). U. lactuca rapidly took up P sub(i), transferring this P sub(i) into tissue phosphate and TCA-soluble P in a few hours ( approximately 90% of total P). C. nipae took up P sub(i) at lower rates and stored much of this P in less mobile TCA-insoluble forms. Long-term storage of refractory forms of P makes C. nipae a useful bioindicator of the prevailing conditions of P sub(i) availability over at least the previous 7 days, whereas the P-status of U. lactuca may reflect conditions over no more than the previous few hours or days. C. nipae is a more useful bioindicator for P status of estuarine and marine waters than U. lactuca.