Seasonal variations in nitrate reductase activity and internal N pools in intertidal brown algae are correlated with ambient nitrate concentrations
2007
YOUNG, ERICA B. | Dring, M. J. (Matthew J) | Savidge, Graham | BIRKETT, DARYL A. | BERGES, JOHN A.
Nitrogen metabolism was examined in the intertidal seaweeds Fucus vesiculosus, Fucus serratus, Fucus spiralis and Laminaria digitata in a temperate Irish sea lough. Internal NO₃⁻ storage, total N content and nitrate reductase activity (NRA) were most affected by ambient NO₃⁻, with highest values in winter, when ambient NO₃⁻ was maximum, and declined with NO₃⁻ during summer. In all species, NRA was six times higher in winter than in summer, and was markedly higher in Fucus species (e.g. 256 ± 33 nmol NO₃⁻ min⁻¹ g⁻¹ in F. vesiculosus versus 55 ± 17 nmol NO₃⁻ min⁻¹ g⁻¹ in L. digitata). Temperature and light were less important factors for N metabolism, but influenced in situ photosynthesis and respiration rates. NO₃⁻ assimilating capacity (calculated from NRA) exceeded N demand (calculated from net photosynthesis rates and C : N ratios) by a factor of 0.7-50.0, yet seaweeds stored significant NO₃⁻ (up to 40-86 μmol g⁻¹). C : N ratio also increased with height in the intertidal zone (lowest in L. digitata and highest in F. spiralis), indicating that tidal emersion also significantly constrained N metabolism. These results suggest that, in contrast to the tight relationship between N and C metabolism in many microalgae, N and C metabolism could be uncoupled in marine macroalgae, which might be an important adaptation to the intertidal environment.
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