Sulphur assimilation

Any change in enviromnental conditions such as light, temperature, water and nutrient availability may modulate the photochemical reactions of photosynthesis to a different extent than the biochemical reactions involved in carbon reduction cycle, photorespiration, and nitrogen and sulphur assimilation. Consequently, these environmental changes will modulate excitation pressure. Excitation pressure reflects an imbalance between energy absorbed through photochemistry and energy utilized... 

The metabolism of sulphur plays an important role in metal response, since most chelators, including metallothioneins and phytochelatins, contain this element. APSl encodes an ATP sulfurylase and Brassica juncea has been engineered to constitutively express this gene (Pilon-Smits et al., 1999). The plants showed increased S assimilation, higher glutathione (GSH) levels and were more tolerant to Se, accumulating 2-fold higher Se levels in the shoots. 

ZuS + < Cls = 2FeCl + ZnCl+S that is, for every equivalent of sulphide of zinc one of the sesquichloride of iron is reduced to the stats of protochloride, while sulphur is separated from the former, and chlorine. from the iron salt assimilated, The liquid, as soon as it becomes cleaT by the deposition of the sulphur, is filtered off, and the quantity of iron in the state of protoxide estimated voiumetrically by means of standard solutions of per raanganate or of biehrotnato of potassa, and the amount of zinc deduced therefrom, since 56 parte of iron wili represent 32 0 of zinc,... 

The chemical half-life of DMSP in seawater is >8 years (Dacey and Blough 1987), which results in high abiotic stability under natural conditions (moderate temperatures and pH). Therefore, most of the DMSP removal is through enzymatic processes. In the microbial food web, dissolved DMSP has many fates and several recent reviews on the microbial pathways and involved mechanisms have been published (Bentley and Chasteen 2004 Kiene et al. 2000 Lomans et al. 2002 Yoch 2002). They all show that DMSP can be readily used in a complex network of enzymatic conversions. This versatility indicates that this single compound is of major importance for the nutrition of the bacterial community. Indeed, several studies have shown that DMSP alone can contribute 1 to 15% of the total bacterial carbon demand in surface waters. Moreover, DMSP assimilation can satisfy most, if not all the, sulphur demand of marine bacteria (Kiene and Linn 2000 Simo et al. 2002 Zubkov et al. 2001). Since the focal point of this section is the quantification of DMSP removal, only the overall effects of the main pathways originating from DMSP (Fig. 1) will be discussed here. 

Fig. 3.16 Generalized scheme of the role of bacteria in the carbon cycle and its coupling to the nitrogen and sulphur cycles (after Fenchel Jorgensen 1977 Jorgensen 1983a, b Parkes 1987 Fenchel Finlay 1995 Werne et al. 2002). For clarity, the forms of N, S and P liberated at each stage of mineralization are summarized on the left side of the diagram, where they contribute to the general mineral pools from which assimilation occurs. The nitrate reduction zone refers to the dissimilatory processes involved in denitrification.

Most soil sulphur is combined in organic compounds. Higher plants normally use sulphate ions although they can also assimilate sulphur-containing... 

Higher plants and many microorganisms utilise sulphate as their source of sulphur. The initial step in sulphate assimilation involves the action of ATP on sulphate ions to give adenosine-5 -phospho-sulphate (APS). Further phosphorylation by ATP and sulphurylase then occurs and 3 -phospho-adenosine-5 -phosphosulphate (PAPS) is formed. 

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