Ecological aspects of nitrogen uptake in intertidal macrophytes
Creators: name: Thomas, Terry Ellen
Publisher: name: University of British Columbia
Year: 2010

description: A comprehensive field and laboratory study of nitrogen uptake in intertidal seaweeds was undertaken. Methods for measuring nitrogen uptake rates were evaluated. Short initial periods of rapid ammonium uptake were common in nitrogen deficient plants. The presence of ammonium inhibited nitrate uptake, but a certain degree of nitrogen starvation overcame this suppression. Laboratory studies with Porphyra perforata showed that nitrogen starved cultures maintained rapid initial ammonium uptake rates. The nitrate uptake system did not remain activated. Nitrogen starvation also resulted in a general decrease in soluble nitrogen content and a transient increase in nitrate reductase activity. The effectiveness of in vitro and in vivo nitrate reductase assays was investigated. The rate of nitrite production in the in vivo assay varied with incubation time. Therefore, the in vitro assay was used. Nitrate grown cultures of Porphyra perforata maintained high ammonium uptake rates. It was suggested that the rate of nitrate reduction was limiting the supply of nitrogen for further assimilation which may control ammonium uptake. Ammonium arid ammonium plus nitrate grown cultures had very low nitrogen uptake rates and nitrate reductase activities. Field studies with Gracilaria verrucosa confirmed that growth on ammonium inhibited nitrate uptake, nitrate accumulation and nitrate reductase activity. The presence of ammonium did not inhibit nitrate uptake rates in severely starved populations. All populations maintained high ammonium uptake rates suggesting that they were nitrogen limited at this time (August). Ammonium and nitrate uptake were saturable in the high intertidal G. verrucosa population but not in the low intertidal population. An investigation was made into the effect of nitrogen source and periodic exposure to air on growth, development and nitrogen uptake in Fucus distichus germlings. Gamete release, fertilization, germination and germling growth had no requirement for a specific form of nitrogen. Periodic exposure to air increased secondary rhizoid development. Ammonium and nitrate uptake rates of the germlings were much higher than for the mature thalli, but the affinity for nitrate was similar. The germlings showed saturable uptake kinetics but the mature thalli did not. The presence of ammonium inhibited nitrate uptake by the mature plants but not by the germlings. Mild desiccation enhanced nutrient uptake rates in several intertidal seaweeds. This uptake response occurred when growth was limited by that particular nutrient and when the thallus had been exposed to periodic desiccation for several weeks. The degree of enhancement, the percent desiccation producing maximum uptake rates and the tolerance to higher degrees of desiccation were related to intertidal location. This was shown to be an intraspecific as well as an interspecific adaptation. Transplant experiments with G. verrucosa showed that enhanced nutrient uptake rates after desiccation were related to intertidal height and not geographic location and that this response could be induced in approximately five weeks. It was suggested that this enhanced uptake response was an adaptation to nitrogen procurement and C/N homeostasis following periodic exposure when carbon was assimilated but when other nutrients were not available.