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Nitrogen and sulfur are abundant constituents of plant and algal cells that are assimilated at the lowest oxidation number, as NH₄⁺ and S²⁻, although they can (in the case of sulfur, usually must) be acquired with their highest oxidation number, as NO₃⁻ and SO₄²⁻. Some occasional differences and variants exists for transport and assimilation systems; the greatest differences in the way vascular plants and algae use N and S, however, most probably resides in regulation. For instance, nitrate assimilation in plants is strongly regulated by phosphorylation. In algae, redox regulation appears to be more important. Similarly, sulfate reduction has its main control step at the level of APS reductase in higher plants, whereas in algae a redox regulation has been recently been hypothesized for ATP sulfurylase, the first step in sulfate assimilation. Unfortunately, the information on the regulation of N and S acquisition and …