Biosynthetic controls an the C-13 contents of organic components in the photoautotrophic bacterium Chloroflexus aurantiacus

To assess the effects related to known and proposed biosynthetic pathways o n the C-13 content of lipids and storage products of the photoautotrophic b acterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell m aterial, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The bulk cell materi al was 13 parts per thousand depleted in C-13 relative to the dissolved ino rganic carbon. Evidently, inorganic carbon fixation by the main carboxylati ng enzymes used by C. aurantiacus, which are assumed to use bicarbonate rat her than CO2, results in a relatively small carbon isotopic fractionation c ompared with CO2 fixation by the Calvin cycle. Even carbon numbered fatty a cids, odd carbon numbered fatty acids, and isoprenoid lipids were 14, 15, a nd 17-18 parts per thousand depleted in C-13 relative to the carbon source, respectively. Based on the C-13 contents of alkyl and isoprenoid lipids, a 40 parts per thousand difference in C-13 content between the carboxyl and methyl carbon from acetyl-coenzyme A has been calculated. Both sugars and p olyhydroxyalkanoic acid were enriched in C-13 relative to the alkyl and iso prenoid lipids. To the best of our knowledge this is the first report in wh ich the stable carbon isotopic composition of a large range of biosynthetic products in a photoautotrophic organism has been investigated and interpre ted based on previously proposed inorganic carbon fixation and biosynthetic pathways. Our results indicate that compound-specific stable carbon isotop e analysis may provide a rapid screening tool for carbon fixation pathways.