Diurnal cycle and kinetics of ammonium assimilation in the green alga Ulva pertusa
- 156 Downloads
The kinetics of ammonium assimilation was investigated in Ulva pertusa (Chlorophyceae, Ulvales) from northeastern New Zealand. Ammonium assimilation exhibited Michaelis–Menten kinetics with a maximum rate of assimilation (Vmax) of 54 ± 5 μmol g−1 dry weight h−1 and half-saturation constant (Km) of 23 ± 8 μM. In contrast, values for ammonium uptake were considerably higher with a Vmax of 316 ± 59 μmol g−1 dry weight h−1 and Km of 135 ± 46 μM. At environmentally relevant ammonium concentrations (5 μM), assimilation accounted for most (70%) of the ammonium taken up. Darkness decreased the maximum rate of ammonium assimilation by 83%. We investigated the hypothesis that rates of biosynthetic processes are greater in the early part of the day in Ulva. Consistent with this hypothesis, the maximum rate of ammonium assimilation in U. pertusa peaked in the morning and coincided with low levels of the photosynthetic product sucrose, which peaked in the afternoon. There was a diurnal cycle in the rate of ammonium uptake and assimilation in light and dark, but the amplitude was much greater for assimilation than uptake. Moreover, our data suggest that net ammonium assimilation only occurs during the day in U. pertusa. We suggest that two major roles for diurnal cycles are minimisation of interspecific competition for resources and metabolic costs.
KeywordsAssimilation Ammonium Concentration Diurnal Cycle Ulva Nitrate Reductase Activity
Financial support was provided to T.A.V.R. from the University of Auckland Research Committee. F.G. thank UMR for financial support.
- Flores-Moya A, Gómez I, Viñegla B, Altamirano M, Pérez-Rodríguez E, Maestre C, Caballero RM, Figueroa FL (1998) Effects of solar radiation on the endemic Mediterranean red alga Rissoella verruculosa: photosynthetic performance, pigment content and the activities of enzymes related to nutrient uptake. New Phytol 139:673–683CrossRefGoogle Scholar
- Koroleff F (1983) Determination of ammonia. In: Grasshoff K, Ehrhardt M, Kremling K (eds) Methods of seawater analysis. Verlag Chemie, Weinheim, pp 150–157Google Scholar
- Levavasseur G, Giraud G (1982) Modification de la photosynthèse nette d’une Ulve de Roscoff en function de la durée d’éclairement. Physiol Vég 20:143–154Google Scholar
- Syrett PJ (1981) Nitrogen metabolism of microalgae. In: Platt T (ed) Physiological bases of phytoplankton ecology. Can Bull Fish Aquat Sci 210:189–209Google Scholar