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

Macroalgae or macrophytes are a heterogeneous assemblage of macroscopic eukaryotes belonging to various evolutionary lineages, which live predominantly in aquatic habitats. They have undifferentiated vegetative bodies organized in pseudoparanchymatous and parenchymatous bodies. As with higher plants, marine macroalgae or seaweeds are photosynthetic species that, by harvesting sunlight energy, convert carbon dioxide in oxygen to produce organic compounds, especially carbohydrates. In addition, they require mineral nutrients, essential for growth, development and reproduction, which are incorporated into carbon skeletons. In natural aquatic ecosystems, 95% of the nitrogen which occurs as dissolved dinitrogen gas (N2), is not directly accessible to most photosynthetic-oxygen organisms. Dissolved inorganic nitrogen (DIN) includes the ions, ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-). In seawater, and under natural conditions, about 3,5% is NO3(ca. 0.35 mg NO3-.L-1), which, near to coastal zones, appears in abundance as a product of upwelling or pollution, whence, their importance as the predominant cause of local eutrophication. Thus, nitrate constitutes the prevailing available nitrogen source for macroalgae in the marine environment. The available DIN may be supplemented by dissolved organic nitrogen (DON), this including urea and amino acids. For all eukaryotic photoautotrophs, NH4+, NO2-, and NO3are the only directly assimilated sources. Nitrogen, which is rapidly taken up, is a key element in several compounds present in the cells. It is used to build up amino acids, proteins, nucleoside phosphates, nucleic acids, and other organic N-containing macromolecules. The availability of nutrients, especially nitrogen, in marine habitats is one of the main regulating factors that limit growth, morphology, development, reproduction, distribution, and biochemical composition in seaweeds. The importance of nitrogen for biological life is evident, in that only oxygen, carbon, and hydrogen are more abundant in the cells of photosynthetic organisms. Macroalgae and photoautotrophic organisms have considerable intracellular capacity for storing nitrogen as soluble nitrogen and organic molecules, whereby growth and development can be regulated and limited according to nitrogen uptake. This characteristic for storing and assimilating nutrients, when available and at high concentrations, besides