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High in situ rates of phosphate uptake should coincide with high tissue phosphorus content and/or high growth rate and be either supply-driven (largely controlled by the phosphate concentration in the surrounding seawater) or demand-driven (largely dictated by the maximum uptake rate, Vmax, and under the control of the organism). To test this hypothesis, 6 common New Zealand seaweed species (Cystophora torulosa, Melanthalia abscissa, Pterocladia lucida, Ulva intestinalis, Xiphophora chondrophylla and Zonaria turneriana) were used. We calculated in situ rates of phosphate uptake from the kinetic constants of uptake, monthly rates of uptake at a fixed phosphate concentration and seawater phosphate concentration, and compared these rates with monthly tissue phosphorus content. There were no significant differences in the half-saturation constant (Km) values for phosphate uptake by the 6 species. Vmax and affinity (Vmax/Km) were largely a function of the seaweed surface area:volume quotient. In the 5 species where there was a peak in tissue phosphorus levels, it occurred in July or September/October. Peaks in tissue phos- phorus in M. abscissa, P. lucida, U. intestinalis and Z. turneriana coincided with, or occurred soon after, peaks in calculated in situ rates of phosphate uptake. Maximum rates of in situ phosphate uptake were demand-driven in all subtidal species and supply-driven in the only intertidal alga U. intestinalis.

Supply- and demand-driven phosphate uptake and tissue phosphorus in temperate seaweeds