Rates of inorganic nitrogen uptake by the estuarine green macroalgae Enteromorpha intestinalis and Ulva expansa

Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, 621 Charles E. Young Drive Los Angeles, CA 90095 ABSTRACT Rates of nitrogen (N) uptake by Enteromorpha intestinalis and Ulva expansa were investigated in two separate experiments. To measure uptake rates over a range of conditions, we varied initial water column NO3 concentrations (low, medium, and high) and initial algal tissue nutrient status (enriched versus depleted). Uptake rates were determined by measuring the disappearance of NO3 from solution over time (1, 2, 4, 8, 12, and 24 h). E. intestinalis and U. expansa exhibited a high affinity for N. In the low water column concentration treatments, E. intestinalis and U. expansa removed all measurable NO3 from the water within 8 h and 12 h, respectively. Nutrient-depleted algae consistently removed more NO3 than enriched algae over each sampling interval. For E. intestinalis, maximum rates of NO3 uptake increased with increasing initial water column nutrient concentrations, indicating a relationship between uptake and external substrate concentration. The same was true for U. expansa in the low and medium water column nutrient treatments. Maximum rates of NO3 uptake exceeded 200 μmoles g dry wt -1 h-1 by E. intestinalis and 125 μmoles g dry wt -1 h-1 by U. expansa. Nutrient uptake rates were highly variable over 24 h, indicating surge, internally controlled, and externally controlled phases of nutrient uptake. Uptake by E. intestinalis varied from the maximum rate measured to a negative rate over the first 2 h, indicating a release of N from algal tissues. Similarly, U. expansa NO3 uptake rates were greatest in the first hour, decreased to 0 in the second hour, and then returned to normal. Depleted algae showed greater increases in tissue N concentration in response to water column nutrient supplies than enriched algae did, and these increases were concentration dependent. Generally, macroalgae did not grow measurably in these 24-h experiments, indicating a temporal decoupling between nutrient uptake and growth. Critical factors in the process of nutrient uptake by macroalgae are: (1) nutrient concentration in the water column, (2) algal nutrient status, and (3) the various phases of nutrient uptake.