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 μmol N g dw⁻¹ h⁻¹ with a half saturation constant (K_s) of 1.02–1.08 μM. 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 μmol 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.

中文翻译：

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巨型海藻（Macrocystis pyrifera，Phaeophyceae）对尿素使用的影响因素

尿素是巨型海带（Macrocystis pyrifera）的一种容易获得的氮源，但对其维持增长的潜在重要性知之甚少。的动力学实验结果的平均最大速率指示尿素摄取饱和（V_最大）的0.73-0.92  μ摩尔的N克干重^-1 ħ ^-1用半饱和常数（ķ_小号1.02-1.08的）  μM.海带对尿素的亲和力比其他海藻的亲和力高。但是，对天然存在的浮游植物共同体进行的动力学实验结果表明，浮游植物对尿素的吸收速率比巨型海带高10倍以上。在黑暗中（相对于光照），巨型海带吸收的尿素减少了3–12％，而浮游植物的尿素吸收减少了66–85％。铵盐和硝酸盐也观察到相似的差异，这表明光强度和光循环会影响巨型海藻和浮游植物之间的氮竞争结果。在海带组织脲酶的月度措施在该分别为100倍摄取尿素的速率（较高水平的显示持续的活动0.13-0.35  μ摩尔的N克FW ^-1分钟^-1）。这一发现，加上通过底物的添加来诱导额外的脲酶活性的努力未成功，表明脲酶除了处理从环境中吸收的脲外，还在巨大的海藻生理中起作用。总体而言，我们的结果表明，巨型海带使用多种形式的N（包括尿素）来维持全年的增长。它在白天和晚上都具有连续获取N的能力，这可能有助于抵消其相对于浮游植物的低吸收率，并提高其在低N利用率期间竞争N的能力。