Seasonal variations in nitrate reductase activity and internal N pools in intertidal brown algae are correlated with ambient nitrate concentrations

Abstract

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 Ϯ 33 nmol NO3 -min -1 g -1 in F. vesiculosus versus 55 Ϯ 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 mmol 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.

Nitrate Assimilation: The Role of In Vitro Nitrate Reductase Assay as Nutritional Predictor

“…Lartigue & Sherman (2005) observed the same trend in Enteromorpha sp. A like inducing response, under the same conditions, has also been observed in other macroalgae (Gao, Smith & Alberte 1995;Lartigue & Sherman, 2005;Young et al, 2007;Martins et al, 2009;Cabello-Pasini et al, 2011). Nitrate reductase activity in Arctic species appears to be directly enhanced by nitrate addition, with relatively little feedback from the N-status of the cell (Gordillo et al, 2006).…”

“…Species with high NR activity during the winter may present a cold acclimation component. This high activity has been reported in Laminaria saccharina (Davison & Davison 1987), Fucus vesiculosus (Collén & Davison 2001), and L. digitata, Fucus serratus, Fucus vesiculosus and Fucus spiralis (Young et al (2007). Macroalgae in tropical and sub-tropical environments presented high temperature tolerance during assaying (Lopes et al, 1997;Chow et al, 2007;Granbom et al, 2004;Martins et al, 2009), the optimum usually being higher than normal.…”

Nitrogen and sulfur assimilation in plants and algae

“…Nitrate vacuolar accumulation has also been proposed for some intertidal macroalgae; in some Laminaria and Fucus species, for instance, a nitrate internal concentration more than 10-fold higher than in the environment can be found in the winter, when growth is slower; this stored nitrate is then depleted in spring/summer (Young et al, 2007).…”

Differences in growth, morphology and tissue carbon and nitrogen of Macrocystis pyrifera within and at the outer edge of a giant kelp forest in California, USA

“…Low levels of this carbohydrate may result from low rates of photosynthesis, generally a result of light limitation, or rapid growth (Chapman & Craigie 1978). Similarly, when external supplies of nitrogen are replete, nitrogen taken up in excess of growth demands are stored in vacuoles as nitrate reserves (Young et al 2007), and growth can be sustained by stored nitrate under conditions of high light but low ambient nitrogen availability. Information on the availability of kelp storage compounds is critically important when assessing the environmental causes for differences in growth, and the degree to which kelp can de-couple growth from ambient conditions through nutrient storage varies.…”