Background and aims Arbuscular mycorrhizal (AM) hyphae represent an important route for input of plantderived C to soil, but impacts of these inputs on microbial communities and processes are poorly understood. In this study we characterised pathways of C-flow through microbial communities associated with AM hyphae and quantified impacts on mineralisation of native SOM. Methods Continuous, steady-state 13 CO 2 labelling was applied throughout the growth period (60 d) of Lolium perenne. Exclusion meshes were used to control access of roots and AM hyphae to soil, and plant-derived C was quantified within microbial PLFA and NLFA, and soil CO 2 efflux was partitioned into plant-and soil organic matter (SOM) derived components.Results Pathways of C-flow through hyphosphere and mycorrhizosphere communities were distinct, as was the fate of plant-derived C from AM hyphae accessing soil through 37 and 1 μm meshes. Mineralisation of native SOM was increased in all treatments, relative to unplanted controls, and this priming effect was largest for AM hyphae accessing soil through the 1 μm mesh size. Conclusions We demonstrated that AM hyphae can strongly increase mineralisation of native SOM and identified distinct pathways of C-flow through hyphosphere communities. Our results suggest that, in addition to affecting rates of litter decomposition, AM hyphae may have a significant influence on turnover of native SOM.

Conversion of sn-glycero-3-phosphate-2-3H to cardiolipin in mitochondria isolated from guinea pig liver has been established. Cardiolipin thus formed was isolated and characterized. Under similar conditions, microsomes were not able to catalyze the biosynthesis of cardiolipin in a detectable amount. The mechanism for the biosynthesis of cardiolipin in mitochondria, similar to that described in bacteria, is suggested. These results have established mitochondrial capability to synthesize polyglycerophosphatides.

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Nuclei-free homogenate, prepared from guinea pig livers, was fractionated into subcellular particles which were then examined for the activities of two microsomal marker enzymes, glucose-6-phosphatase and NADPH: cytochrome c reductase. In an incubation system containing sn-glycero-3-phosphate, fatty acid, and various cofactors the intracellular distribution of acyl-CoA: sn-glycero-3-phosphate acyltransferase(s) was studied and compared with the distribution of the two microsomal marker enzymes.Results obtained showed that the highest specific activity for the acylation of sn-glycero-3-phosphate was associated with the microsomal fraction and the activity in each subcellular fraction paralleled activities of the two microsomal marker enzymes. Furthermore, the amount of acyl-CoA: sn-glycero-3-phosphate acyltransferase activity observed in the mitochondrial and submitochondrial fractions could be accounted for by the content of endoplasmic reticulum as determined by the marker enzymes. This observation was also true for brain, heart, and kidney, as well as for rat liver.These results are interpreted as evidence that isolated mitochondria are unable to synthesize phosphatidic acid by direct acylation of sn-glycero-3-phosphate.

The formation of labelled phosphatidylglycerophosphate and phosphatidylglycerol from L-glycero-3-phosphate-2-3H and cytidine diphosphate (CDP)-D-diglyceride in subcellular particles of sheep brain has been studied, establishing the predominant location of this enzyme system in mitochondria. The general characteristics and the optimal conditions for the biosynthesis of these lipids have been determined and the chromatographic separation and isolation of phosphatidylglycerophosphate and phosphatidylglycerol are described. In addition, the chemical and enzymatic degradations supporting the structure and configuration for each biosynthesized lipid are presented. The enzymatic dephosphorylation of phosphatiadylglycerophosphate to phosphatidylglycerol with mitochondria isolated from sheep brain has been found. No biosynthesized cardiolipin was, however, detected.The mechanism of the enzymatic reactions for the formation of these lipids has been studied by using L-glycero-3-phosphate[33P]-2-3H and CDP-D-diglyceride; the pathway found by Kennedy et al. (1) for the biosynthesis of phosphatidylglycerol in liver is applicable in the biosynthesis of this compound in sheep brain mitochondria.

The enzymatic activities of marker enzymes (NADPH – cytochrome c reductase and glucose-6-phosphatase) and synthetic enzymes (acyl-CoA:sn-glycero-3-phosphate acyltransferase, CTP:sn-3-phosphatidic acid cytidyltransferase, and CDP-diglyceride:sn-glycero-3-phosphate phosphatidyltransferase) were measured in both isolated mitochondria and microsomes from liver of guinea pig and rat. Results thus obtained show a significant difference in activities of these enzymes between subcellular particles within species and between two examined species. The activity of acyl-CoA:glycero-3-phosphate acyltransferase in guinea-pig mitochondria parallels the activity of microsomal marker enzymes in this fraction, while in rat liver mitochondria the activity is relatively higher and cannot be accounted for by the microsomal content as determined by marker enzymes. Implications of these results regarding mitochondrial autonomy in the biosynthesis of polyglycero-phosphatides and their precursors are discussed.

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