Nitrate uptake, nitrogen storage, and nitrate reductase activity in estuarine Enteromorpha sp. (Chlorophyceae)

Estuaries are dynamic bodies of water where freshwater, often with substantial amounts of nitrate, mixes with seawater from the coastal shelf. As a result, the availability of nitrate to estuarine macroalgae is often episodic, occurring in pulses whose duration and magnitude are unpredictable. This dissertation investigates how Enteromorpha sp., a common estuarine macroalga, from the Mobile Bay estuary (AL, USA) responds to changes in nitrate availability. In particular, this dissertation focuses on how nitrate uptake, internal inorganic nitrogen storage, and nitrate reductase activity vary in response to changes in nitrate availability. Nitrate reductase activity assays were developed that are capable of measuring activity in field as well as the laboratory. A series of laboratory experiments then investigated how nitrate uptake, internal inorganic nitrogen storage, and the rate of nitrate reduction changed during and following a nitrate pulse. The results suggest that during a nitrate pulse the tissue nitrate pool quickly fills and uptake becomes limited by the rate at which the tissue nitrate pool is metabolized. Surveys of the environmental conditions and the nitrogen content of populations of macroalgae over one year demonstrated that environmental factors other than the availability of nitrate were involved in determining the size of internal inorganic nitrogen pools and the rate of nitrate reduction. The effect of water velocity on nitrate uptake rate and the availability of inorganic nitrogen within an Enteromorpha sp. patch community was also explored. In the laboratory, increases in water velocity led to increases in the rate of nitrate uptake for patches of Enteromorpha sp. In addition, dense patches of Enteromorpha sp., developed nutrient gradients, with greater nutrient concentrations and greater uptake on the edges of the patch relative to the center, particularly when water flow was low. Finally, the reliability of nitrate reductase activity as an index for the rate of nitrate incorporation was tested. The relationship between nitrate reductase activity and nitrate incorporation rate was marked by high variability and lacked predictive power. Overall, this body of work should prompt other researchers to consider the dynamic aspect of metabolic responses in the field and the complex interactions between environmental conditions and nitrate metabolism.