Inorganic nitrogen metabolism in blue light was studied for the green alga
Ulva rigida C. Agardh collected in the south of Spain (Punta Carnero, Algeciras) in the winter of 1987. NH
4
+ has been reported to inhibit NO
3
- uptake; however,
U. rigida showed a net NO
3
- uptake even when the NH
4
+ concentration of the external medium was three or four times greater than the concentration of NO
3
-. NO
3
- uptake rates were similar in both darkness and in blue light of various photon fluence rates (PFR) ranging from 17 to 160 mol m
-2 s
-1. Since NO
3
- uptake is an active mechanism involving the consumption of ATP, respiratory metabolism can provide enough ATP to maintain the energetic requirement of NO
3
- transport even in darkness. In contrast, NO
3
- reduction in
U. rigida was highly dependent on the net photosynthetic rate. After 7 h in blue light, intracellular NO
3
- concentrations ([NO
3
-]
i
) were higher in specimens exposed to intensities below the light compensation point (LCP) than in those incubated at a PFR above the LCP. When PFR is below the light compensation point, NO
3
- reduction is low, probably because all the NADH produced by the cells is oxidized in the respiratory chain in order to produce ATP to maintain a steady NO
3
- transport rate. The total nitrogen (TN) and carbon (TC) contents decreased from darkness to 33 mol m
-2 s
-1 in blue light. In this range, catabolic processes prevailed over anabolic ones. In contrast, increases in TN and TC contents were observed above the light compensation point. The C : N ratio increased with light intensity, reaching a stable value of 17 at 78 mol m
-2 s
-1 in blue light. Intracellular NO
3
- concentration and NO
3
- reduction appear to be directly controlled by light intensity. This external control of [NO
3
-]
i and the small capacity of
U. rigida to retain incorporated NO
3
-, NO
2
- and NH
4
+ ions may explain its nitrophilic character.
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