A Field Study of Nitrogen Storage and Nitrate Reductase Activity in the Estuarine Macroalgae Enteromorpha lingulata (Chlorophyceae) and Gelidium pusillum (Rhodophyceae)

The purpose of this field study was to determine the relationship between environmental conditions, particularly high nitrate (NO3 ), low salinity events, and both nitrogen (N) storage (NO3 , ammonium [NH4 ], free amino acids [FAA], protein, and total N) and nitrate reductase (NR) activity in the macroalgae Enteromorpha lingulata and Gelidium pusillum in the lower Mobile Bay estuary (Alabama, USA). The environmental conditions at the collection site varied over the growing season with the most notable changes due to late winter and spring runoff entering the estuary (1–30 psu, 0.3– 25.8 mM NO32, 0.9–12.5 mM NH4, 3–28uC, 61–2,375 mmol PAR m s). Principal component analysis reduced the six environmental variables measured to three principal components. Stepwise, multiple regression analysis was then used to examine the relationship between the principal components and the internal NO3 , NH4 , and FAA pools and NR activity. The results indicate that changes in inorganic N availability and salinity rather than changes in irradiance determine patterns of N storage and NO3 2 reduction. Both E. lingulata and G. pusillum are capable of taking up and storing NO3 2 when it becomes available. Greater NO3 2 availability produced larger NH4 + and FAA pools along with higher rates of NR activity in E. lingulata, but not G. pusillum, suggesting that E. lingulata is able to metabolize NO3 2 more rapidly during high NO3 , low salinity events. Differences in the susceptibility of E. lingulata and G. pusillum to NH4 + inhibition and salinity stress combined with their different growth strategies help to explain the seasonal trends in total N. Total N in E. lingulata ranged from 2.57% to 6.39% dw, while the slower growing G. pusillum showed no significant variation in total N content (3.8–4.1% dw). These results led to the conclusion that E. lingulata responds more quickly than G. pusillum to high NO3 , low salinity events and that these events have a larger effect on the overall N content of E. lingulata.