Specific deficit in the synthesis of 6-sulfoquinovsyl diglyceride in Chlorella pyrenoidosa

Qualitative analysis of algal secretions with multiple mass spectrometric platforms

Lipid secretions from algae pose a great opportunity for engineering biofueler feedstocks. The lipid exudates could be interesting from a process engineering perspective because lipids could be collected directly from the medium without harvesting and disrupting cells. We here report on the extracellular secretions of algal metabolites from the strain UTEX 2341 (Chlorella minutissima) into the culture medium. No detailed analysis of these lipid secretions has been performed to date. Using multiple mass spectrometric platforms, we observed around 1000 compounds and were able to annotate 50 lipids by means of liquid chromatography coupled to accurate mass quadrupole time-of-flight mass spectrometry (LC-QTOF), direct infusion with positive and negative electrospray ion trap mass spectrometry and gas chromatography coupled to mass spectrometry (GC-MS). These compounds were annotated by tandem mass spectral (MS/MS) database matching and retention time range filtering. We observed a series of triacylglycerols (TG), sulfoquinovosyldiacylglycerols (SQDG), phosphatidylinositols and phosphatidylglycerols, as well as betaine lipids diacylglyceryl-N,N,N-trimethylhomoserines (DGTS).

The sulfolipid sulfoquinovosyldiacylglycerol is not required for photosynthetic electron transport in Rhodobacter sphaeroides but enhances growth under phosphate limitation

All photosynthetic organisms, with the exception of several species of photosynthetic bacteria, are thought to contain the sulfolipid 6-sulfo-alpha-D-quinovosyldiacylglycerol. The association of this lipid with photosynthetic membranes has led to the assumption that it plays some role in photosynthesis. Stable null mutants of the photosynthetic bacterium Rhodobacter sphaeroides completely lacking sulfolipid were obtained by disruption of the sqdB gene. The ratios of the various components of the photosynthetic electron transport chain, as well as the electron transfer rates during cyclic electron transport, were not altered in the mutants, when grown under optimal conditions. Growth rates of wild type and mutants were identical under a variety of growth conditions, with the exception of phosphate limitation, which resulted in reduced growth of the mutants. Phosphate limitation of the wild type caused a significant reduction in the amount of all phospholipids and an increased amount of sulfolipid. By contrast, the sulfolipid-deficient mutant had reduced levels of phosphatidylcholine and phosphatidylethanolamine but maintained a normal level of phosphatidylglycerol. In addition, two unidentified lipids lacking phosphorus accumulated in the membranes of both wild-type and mutant strains under phosphate limitation. We conclude that sulfolipid plays no significant unique role in photoheterotrophic growth or photosynthetic electron transport in R. sphaeroides but may function as a surrogate for phospholipids, particularly phosphatidylglycerol, under phosphate-limiting conditions.

What role does sulpholipid play within the thylakoid membrane?

Sulphoquinovosyldiacylglycerol is a negatively charged lipid which exists in the thylakoid membrane. It is proposed that a large proportion of this acidic lipid does not form a part of the bulk lipid matrix but is closely associated with protein complexes where it is tightly bound and participates in either optimising catalytic activities, or maintaining the complexes in a functional conformation. Experimental evidence for this proposal is emerging from studies with isolated photosystem 2, and coupling factor complexes.

Biosynthesis of sulfoquinovosyldiacylglycerol in higher plants: the incorporation of 35SO4 by intact chloroplasts

Isolated spinach chloroplasts have been found to incorporate 35SO4 into the plant sulfolipid, sulfoquinovosyldiacylglycerol, at rates of up to 700 pmol mg chlorophyll-1 h-1. The reaction is light-dependent, requires that the chloroplasts be intact, and is slightly stimulated by ATP and UTP. UDP-galactose inhibits the formation of sulfoquinovosyldiacylglycerol, presumably by competing for the diacylglycerol pool. The rates of synthesis observed are sufficient to conclude that the chloroplast is autonomous with respect to the synthesis of sulfoquinovose, the headgroup moiety of sulfoquinovosyldiacylglycerol. No evidence could be obtained to support the concept that the proposed sulfoglycolytic pathway is the biosynthetic route for sulfoquinovose formation.

Sidedness studies of thylakoid phosphatidylglycerol in higher plants

The transmembrane distribution of phosphatidylglycerol was determined in thylakoids from barley (Hordeum vulgare), lettuce (Lactuca sativa) and pea (Pisum sativum) chloroplasts. Phospholipase A2 and phospholipase D digestion and chemical-labelling methods were used. Phosphatidylglycerol was preferentially localized in the outer (stromal) leaflet. The proportion of the phospholipid in this leaflet ranged from about 66% in pea to about 75% for barley and lettuce thylakoids. One of the main fatty acids, trans-delta 3-hexadecenoic acid, was exclusively located in the outer leaflet in all three plant types. The data are discussed in relation to suggested roles for phosphatidylglycerol in thylakoid function.

The lipid composition of a barley mutant lacking chlorophyll b

The acyl-lipid composition of a barley mutant that contained no detectable chlorophyll b was studied. This mutant contained chloroplasts that were much less organized than chloroplasts of normal barley. The mutant contained all the normal acyl lipids, with small increases in the relative concentration of phosphatidylglycerol and diacylsulphoquinoglycerides was unchanged, but most other lipids of the mutant barley contained lower amounts of alpha-linolenic acid compared with normal. There was no difference in the transhexadec-3-enoic acid content of phosphatidylglycerol, which was evidence against this lipid being involved in grana stacking.