Seasonal variation of enzyme activities in Laminaria hyperborea

The patterns of seasonal variation of enzyme levels in the brown alga Laminaria hyperborea (Gunn.) Fosl. have been investigated for the following enzymes: Ribulosebisphosphate-carboxylase (EC 4.1.1.39), phosphoenolpyruvate-carboxykinase (EC 4.1.1.32), glyceraldehyde-3-phosphate-dehydrogenase (NADP dep., EC 1.2.1.12), malate-dehydrogenase (NAD dep., EC 1.1.1.37), L-aspartate-2-oxoglutarate aminotransferase (EC 2.6.1.1), and mannitol-l-phosphate-dehydrogenase (EC 1.1.1.17). The first four enzymes exhibit a circannual periodicity, characterized by a pronounced spring-maximum of enzyme activity in April and May. As a consequence, the phylloid can maintain high metabolic rates from early spring on, although water temperature has then only slightly risen above the annual minimum. This findings is discussed in relationship to the growth- and developmental cycle of L. hyperborea and to the seasonal variation of photosynthesis and light-independent CO₂-fixation. The seasonal pattern, outlined above, correlates well with the circannual fluctuations of the nitrogen content of the sea and with the variation of the internal nitrogen- and nitrate-content of the alga. This coincidence may indicate that nitrogen levels play an important role in the regulation of enzyme activities and, hence, the metabolic capacities of L. hyperborea.Abbreviations PEPCK phosphoenolpyruvate carboxykinase (EC 4.1.1.32) - RUBPC ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - GAPDH (NADP dep.) glyceraldehyde-3-phosphate dehydrogenase (NADP dependent) (EC 1.2.1.12) - MDH (NAD dep.) malate dehydrogenase (NAD dependent) (EC 1.1.1.37) - AAT L-aspartate-2-oxoglutarate aminotransferase (EC 2.6.1.1) - Mannitol-1-P DH mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) - LIF lightindependent CO₂-fixation - DHAP dihydroacetone phosphate - PEP phosphoenolpyruvate - 3-PGA 3-phosphoglycerate - OAA oxaloacetate