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

Kluwer Academic Publishers
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Journal article
Runcie, J., Larkum, A., and Ritchie, R. 2004 'Uptake kinetics and assimilation of phosphorus by Catenella nipae and Ulva lactuca can be used to indicate ambient phosphate availability', Journal of Applied Phycology, vol. 16, no. 3, pp. 181-194.
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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 (Pi) at different concentrations. Maximum uptake rates (Vrnax) of P-starved material exceeded Vmax 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 32p. Both species demonstrated similar assimilation paths: when P-enriched, most 32p accumulated as free phosphate. When unenriched, 32p was rapidly assimilated into the TCA-insoluble pool. C. nipae consistently assimilated more 32p into this pool than U. lactuca, indicating C. nipae has 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-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 Pi. U. lactuca rapidly took up Pi, transferring this Pi into tissue phosphate and TCA-soluble P in a few hours (~90% of total P). C. nipae took up Pi 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 Pi availability over at least the previous 7 days, whereas the P-status of Ui 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. Abbreviations: ASW, artificial seawater; DW, dry weight; 4>, flux; FW, fresh weight; Krn- half saturation concentration; PAR, photosynthetically active radiation (400-700 nm); P, phosphorus; Pi, inorganic phosphorus; Ni, inorganic nitrogen; Vmax, maximum uptake rate
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