A field study of nitrogen storage and nitrate reductase activity in the estuarine macroalgaeEnteromorpha lingulata (Chlorophyceae) andGelidium pusillum (Rhodophyceae)

Abstract

The purpose of this field study was to determine the relationship between environmental conditions, particularly high nitrate (NO 3 − ), low salinity events, and both nitrogen (N) storage (NO 3 − , ammonium [NH 4 + ], free amino acids [FAA], protein, and total N) and nitrate reductase (NR) activity in the macroalgaeEnteromorpha lingulata andGelidium 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 μM NO 3 − , 0.9–12.5 μM NH 4 + , 3–28°C, 61–2,375 μmol PAR m−2 s−1). 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 NO 3 − , NH 4 + , 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 NO 3 − reduction. BothE. lingulata andG. pusillum are capable of taking up and storing NO 3 − when it becomes available. Greater NO 3 − availability produced larger NH 4 + and FAA pools along with higher rates of NR activity inE. lingulata, but notG. pusillum, suggesting thatE. lingulata is able to metabolize NO 3 − more rapidly during high NO 3 − , low salinity events. Differences in the susceptibility ofE. lingulata andG. pusillum to NH 4 + inhibition and salinity stress combined with their different growth strategies help to explain the seasonal trends in total N. Total N inE. lingulata ranged from 2.57% to 6.39% dw, while the slower growingG. pusillum showed no significant variation in total N content (3.8–4.1% dw). These results led to the conclusion thatE. lingulata responds more quickly thanG. pusillum to high NO 3 − , low salinity events and that these events have a larger effect on the overall N content ofE. lingulata.