Biosynthetic Controls on the 13C Contents of Organic Components in the Photoautotrophic Bacterium Chloroflexus aurantiacus

Marcel T.J Van Der Meer,Stefan Schouten,Bart E Van Dongen,W Irene C Rijpstra,Georg Fuchs,Jaap S Sinninghe Damsté,Jan W De Leeuw,David M Ward

doi:10.1074/jbc.m009701200

Abstract

To assess the effects related to known and proposed biosynthetic pathways on the (13)C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The bulk cell material was 13 per thousand depleted in (13)C relative to the dissolved inorganic carbon. Evidently, inorganic carbon fixation by the main carboxylating enzymes used by C. aurantiacus, which are assumed to use bicarbonate rather than CO(2), results in a relatively small carbon isotopic fractionation compared with CO(2) fixation by the Calvin cycle. Even carbon numbered fatty acids, odd carbon numbered fatty acids, and isoprenoid lipids were 14, 15, and 17-18 per thousand depleted in (13)C relative to the carbon source, respectively. Based on the (13)C contents of alkyl and isoprenoid lipids, a 40 per thousand difference in (13)C content between the carboxyl and methyl carbon from acetyl-coenzyme A has been calculated. Both sugars and polyhydroxyalkanoic acid were enriched in (13)C relative to the alkyl and isoprenoid lipids. To the best of our knowledge this is the first report in which the stable carbon isotopic composition of a large range of biosynthetic products in a photoautotrophic organism has been investigated and interpreted based on previously proposed inorganic carbon fixation and biosynthetic pathways. Our results indicate that compound-specific stable carbon isotope analysis may provide a rapid screening tool for carbon fixation pathways.

Highlights

To assess the effects related to known and proposed biosynthetic pathways on the 13C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated
Inorganic carbon fixation by the main carboxylating enzymes used by C. aurantiacus, which are assumed to use bicarbonate rather than CO2, results in a relatively small carbon isotopic fractionation compared with CO2 fixation by the Calvin cycle
Our results indicate that compound-specific stable carbon isotope analysis may provide a rapid screening tool for carbon fixation pathways

Summary

Introduction

To assess the effects related to known and proposed biosynthetic pathways on the 13C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The Calvin cycle is used for carbon assimilation by all green plants, algae, and many autotrophic bacteria Another well known carbon fixation mechanism using phosphoenolpyruvate (PEP) carboxylase is found in, for instance, C4 plants and many other organisms in anaplerotic reactions compensating any loss of intermediary components from the tricarboxylic acid cycle. The 3-hydroxypropionate pathway (Fig. 1) was proposed to function in Chloroflexus aurantiacus, a green nonsulfur bacterium [15,16,17] This pathway is a cyclic inorganic carbon fixation mechanism in which acetyl-CoA is carboxylated and reductively converted via 3-hydroxypropionate to propionyl-CoA. The stable carbon isotopic fractionation relative to the carbon source of photoautotrophic organisms ⑀p (defined as ⑀ ϭ (Rcarbon source/Rfixed carbon Ϫ1)103; R ϭ 13C/12C) using the Calvin cycle is in the range of 20 –25‰

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Conclusion