Factors influencing urea use by giant kelp ( Macrocystis pyrifera , Phaeophyceae)

Abstract

Urea is an available and readily used source of nitrogen for giant kelp, Macrocystis pyrifera , but little is known about its potential importance for sustaining growth. Results of kinetic experiments indicate urea uptake saturates at an average maximum rate ( V max ) of 0.73–0.92 μ mol N g dw −1 h −1 with a half saturation constant ( K s ) of 1.02–1.08 μ M. The affinity of giant kelp for urea was high relative to that reported for other seaweeds. However, results of similar kinetics experiments with natural, co‐occurring phytoplankton communities indicate that the rate of urea uptake by phytoplankton was > 10‐fold higher than that of giant kelp. Urea uptake by giant kelp decreased 3–12% in darkness (relative to in light) compared to a 66–85% decline for phytoplankton. Similar differences were observed for ammonium and nitrate, suggesting that light intensity and photocycles influence the outcome of competition for N between giant kelp and phytoplankton. Monthly measures of urease in kelp tissues revealed persistent activity at levels that were 100‐fold higher than rates of urea uptake (0.13–0.35 μ mol N g fw −1 min −1 ). This finding, coupled with unsuccessful efforts to induce additional urease activity through substrate additions, suggests that urease plays a role in giant kelp physiology beyond that of processing urea taken up from the environment. Collectively, our results suggest giant kelp uses multiple forms of N including urea to sustain year‐round growth. Its consistent capacity to acquire N during both day and night may help offset its low uptake rates relative to phytoplankton and increase its ability to compete for N during periods of low N availability.

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