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

Abstract

Uptake, assimilation and compartmentation of phosphate were studied in the opportunist green macroalgaUlva lactucaand the estuarine red algal epiphyteCatenella nipae. The Michaelis–Menten model was used to describe uptake rates of inorganic phosphate (Pi) at different concentrations. Maximum uptake rates (Vmax) of P-starved material exceededVmaxof P-enriched material; this difference was greater forC. nipae. Uptake and allocation of phosphorus (P) to internal pools was measured using trichloroacetic acid (TCA) extracts and32P. Both species demonstrated similar assimilation paths: when P-enriched, most32P accumulated as free phosphate. When unenriched,32P was rapidly assimilated into the TCA-insoluble pool.C. nipaeconsistently assimilated more32P into this pool thanU. lactuca, indicatingC. nipaehas a greater P-storage capacity. In both species,32P release data showed two internal compartments with very different biological half-lives. The rapidly exchanging compartment had a short half-life of ≈2 to 12 min, while the slowly exchanging compartment had a much longer half-life of 12 days in P-starvedC. nipaeor 4 days in P-starvedU. lactuca. In both species, the slowly exchanging compartment accounted for more than 90% of total tissue.U. lactucaandC. nipaeresponded differently to high external Pi.U. lactucarapidly took up Pi, transferring this Piinto tissue phosphate and TCA-soluble P in a few hours (≈90% of total P).C. nipaetook up Piat lower rates and stored much of this P in less mobile TCA-insoluble forms. Long-term storage of refractory forms of P makesC. nipaea useful bioindicator of the prevailing conditions of Piavailability over at least the previous 7 days, whereas the P-status ofU.lactucamay reflect conditions over no more than the previous few hours or days.C. nipaeis a more useful bioindicator for P status of estuarine and marine waters thanU. lactuca.