Uptake and Reduction of Nitrate: Algae and Fungi

Abstract

Algae have been used for nitrate and nitrite assimilation studies since the beginning of this century (Warburg and Negelein 1920). In the meantime a large body of knowledge has been accumulated and requires separate treatment in this chapter of the Volume, although many processes in nitrate assimilation are common in algae and higher plants. Some differences are due to the different evolutionary backgrounds of the groups or to different structural properties of the enzymes, but some are merely due to the relatively large surface of contact that algal cells have with the external medium. For reasons of evolution, the structural characteristics and the enzymes of blue--green algae (cyanobacteria) are very different from those of eucaryotes and require special paragraphs. However, the differences between e. g. green algae or diatoms and higher plantsFig. 1Scheme of nitrate assimilation and related alkalinization under steady state conditions. Upper part represents cells of microalgae, the whole scheme vacuolated cells. R' nitrate reductase; R`` nitrite reductase; in brackets transport related to ammonium accumulation and release; A counter-transport (antiport); c co-transport; P H+ extrusion pump (ATPase). Protein synthesis in cytoplasm omitted. Stoichiometry of alkalinization in microalgae in steady state: H+ net uptake (or net OH- release)=NO3-net uptake − NO2-release+NH4+releaseseem to be much less clearly defined with respesct to nitrate assimilation, and many of them can be explained with regard to morphological organizaion.