An Evaluation of Surge Uptake Capability in Macrocystis pyrifera in Response to Pulses of Three Different Forms of Nitrogen

Nitrogen is often a limiting nutrient in marine environments, including southern California. Macroalgae, such as the foundation species Macrocystis pyrifera, giant kelp, rely on nitrate supplied through variable external processes such as upwelling. However, stratification in summer limits nitrate supply to giant kelp, raising questions of how the M. pyrifera is able to maintain growth throughout the summer. Ammonia and urea, however, are also available as nitrogen sources, raising the possibility of their importance during summer nitrogen droughts. Since these N forms can be pulsed and ephemeral, surge uptake of these compounds could facilitate kelp’s persistence through nitrogen deficit. We assessed the ability of M. pyrifera to rapidly take in pulses of three ¹⁵N-labeled forms of nitrogen, nitrate, ammonium, and urea, using whole blade incubation experiments. The results shed light on possible sources of N to giant kelp and other macroalgae during periods of low nitrate availability, which are likely to increase in the future with warming events.