Assimilatory sulfate reduction

Removal of sulfate from the water column can occur by either assimi-latory or dissimilatory reduction. Assimilatory reduction occurs in the water column, whereas uptake by plankton results in the formation of organic S. 

Bottcher, M.E., Sievert, S.M., and Kuever, J. (1999) Fractionation of sulfur isotopes during assimilatory reduction of sulfate thermophilic gram-negative bacterium at 60 degrees C. Arch. Microbiol. 172, 125-128. 

The biochemical pathway of both assimilatory and dissimilatory sulfate reduction is illustrated in Figure 1. The details of the dissimilatory reduction pathway are useful for understanding the origin of bacterial stable isotopic fractionations. The overall pathways require the transfer of eight electrons, and proceed through a number of intermediate steps. The reduction of sulfate requires activation by ATP (adenosine triphosphate) to form adenosine phosphosulfate (APS). The enzyme ATP sulfurylase catalyzes this reaction. In dissimilatory reduction, the sulfate moiety of APS is reduced to sulfite (SO3 ) by the enzyme APS reductase, whereas in assimilatory reduction APS is further phosphorylated to phospho-adenosine phosphosulfate (PAPS) before reduction to the oxidation state of sulfite and sulfide. Although the reduction reactions occur in the cell s cytoplasm (i.e., the sulfate enters the cell), the electron transport chain for dissimilatory sulfate reduction occurs in proteins that are peiiplasmic (within the bacterial cell wall). The enzyme hydrogenase... 

REDUCTION OF SULFUR COMPOUNDS BY ASSIMILATORY ORGANISMS Sulfate reduction... 

ATP sulfurylase, catalyzes the primary step of intracellular sulfate activation, essential for assimilatory reduction of sulfate to sulfide, involved in methionine metabolism... 

DMS in the ocean is produced during assimilatory sulfate reduction (ASR) by phytoplankton (Figure 1) (3.331. TTiis involves the uptake of oceanic sulfate... 

In contrast to the specialized dissimilatory sulfate reducers, many organisms (humans as well) are capable of assimilatory sulfate reduction. This process, which requires chemical energy in the form of ATP and a series of transfer reactions, can occur anaerobically and aerobically. It produces low concentrations of hydrogen sulfide that are immediately incorporated into organic compounds. Many microbes, plants, and animals have such a metabolic ability. 

Two major pathways are known for the reduction of sulfate. One is the assimilatory pathway, which reduces sulfate to the extent necessary for satisfying the nutritional requirements of the organism. In this pathway, which has been extensively studied in yeast by Robbins and Lip-mann (S68) and Bandurski and his colleagues 369, 370), sulfate is first activated in the presence of ATP by the enzyme ATP-sulfurylase to form adenosine 5 -phosphosulfate (APS). Then in a second reaction, APS is phosphorylated in the 3 position by ATP to form 3 -phosphoadenosine 5 -phosphosulfate (PAPS)... 

The most important metabolic reaction is the assimilation of sulfur into organic forms which ultimately require the reduction of oxidized sulfur to the oxidation level of sulfide. This reduction is effected by the majority of microorganisms (bacteria, algae, fungi) and plants and, because of its abundance, sulfate is the dominant precursor of reduced sulfur. Pathways of assimilatory sulfate reduction are discussed briefly in Chapter 6.2 and depicted in Fig. 6.2.1 (p. 317). 

While sulfate reduction has claimed the most attention by earth scientists, assimilatory organisms also reduce less oxidized forms of sulfur and may thus play a wider role in the geochemical cycling of sulfur than at present... 

Sulfate reduction. All plants, animals, and bacteria metabolize sulfur in order to synthesize amino acids such as cysteine and methionine. The sulfur may be assimilated as sulfate or as organic molecules containing sulfate. The reduction of sulfate in biosynthesis is termed assimilatory sulfate reduction and can take place in aerobic or anaerobic environments (cf. Goldhaber and Kaplan 1974 Rheinheimer 1981 Cullimore 1991). 

---