Organic sulfur compounds formation pathways

Figure 1. The most important pathways of assimilatoiy sulfate reduction are shown. The APS pathway is utilized by oxygen-producing, phototrophic eukaryotes and some cyanobacteria. In this pathway a bifurcation occurs where, for APS, sulfur can either be reduced to the oxidation level of cysteine or converted to sulfate-ester compounds, in the PAPS pathway, utilized by some cyanobacteria, but mostly by anoxygenic microorganisms, APS formation leads, through sulfite, to the production of reduced organic sulfur compounds.

The forms of selenium in plants are better known than their biochemical formation. Most of the organic forms are sulfur analogs and the pathways may be similar to sulfur compound formation [47]. Nisson and Benson [37] suggested the following pathway for reduction of selenate to selenite, which is the form which reacts to form amino acids containing selenium. 

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