Organisms capable of autotrophic metabolism assimilate inorganic carbon into organic carbon. They form an integral part of ecosystems by making an otherwise unavailable form of carbon available to other organisms, a central component of the global carbon cycle. For many years, the doctrine prevailed that the Calvin-Benson-Bassham (CBB) cycle is the only biochemical autotrophic CO2 fixation pathway of significance in the ocean. However, ecological, biochemical, and genomic studies carried out over the last decade have not only elucidated new pathways but also shown that autotrophic carbon fixation via pathways other than the CBB cycle can be significant. This has ramifications for our understanding of the carbon cycle and energy flow in the ocean. Here, we review the recent discoveries in the field of autotrophic carbon fixation, including the biochemistry and evolution of the different pathways, as well as their ecological relevance in various oceanic ecosystems.

Beyond the Calvin cycle: autotrophic carbon fixation in the ocean.

Efficient storage of carbon in seeds is crucial to plant fitness and to agricultural productivity. Oil is a major reserve material in most seeds1, and these oils provide the largest source of renewable reduced carbon chains available from nature. However, the conversion of carbohydrate to oil through glycolysis results in the loss of one-third of the carbon as CO2. Here we show that, in developing embryos of Brassica napus L. (oilseed rape), Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) acts without the Calvin cycle2 and in a previously undescribed metabolic context to increase the efficiency of carbon use during the formation of oil. In comparison with glycolysis, the metabolic conversion we describe provides 20% more acetyl-CoA for fatty-acid synthesis and results in 40% less loss of carbon as CO2. Our conclusions are based on measurements of mass balance, enzyme activity and stable isotope labelling, as well as an analysis of elementary flux modes.

Rubisco without the Calvin cycle improves the carbon efficiency of developing green seeds

Regulation of CO2 assimilation in oxygenic photosynthesis: the ferredoxin/thioredoxin system. Perspective on its discovery, present status, and future development.

‘Tunnelling’ is the metaphorical name given to the process, possible in quantum mechanics, but not in classical mechanics, whereby a particle can disappear from one side of a potential-energy barrier and appear on the other side without having enough kinetic energy to mount the barrier. One can think of this as a manifestation of the wave-nature of particles. The wavelength is larger if a particle is lighter. In particular electrons, being very light compared to atoms, have wavelengths as large or larger than atoms at energies found in the valence shells of molecules. Thus, they easily ooze through and around atoms and molecules. We are also concerned with the tunnelling of heavy particles: nuclei, atoms, molecules.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S003358350000175X

Quantum mechanical tunnelling in biological systems

A non-oxidative, cyclic pathway—termed non-oxidative glycolysis—is designed and constructed that enables complete carbon conservation in sugar catabolism to acetyl-coenzyme A, and can be used to achieve a 100% carbon yield to fuels and chemicals. Much of the pyruvate produced from sugars by glycolysis, a metabolic pathway found in almost all living organisms, is decarboxylated to produce acetyl-coenzyme A (CoA) for various biosynthetic purposes. Along the way, however, pyruvate decarboxylation loses a carbon equivalent, limiting the theoretical carbon yield to only two moles of two-carbon (C2) metabolites per mole of hexose. James Liao and colleagues have constructed a non-oxidative, cyclic pathway that allows the production of stoichiometric amounts of C2 metabolites from hexose, pentose and triose phosphates without carbon loss. This pathway, termed non-oxidative glycolysis (NOG), enables complete carbon conservation in sugar catabolism to acetyl-CoA The authors demonstrate NOG both in vitro and in engineered Escherichia coli strains. Industrially this new approach could be used to produce bio-alcohols, fatty acids, biodiesel and isoprenoids from sugars. Glycolysis, or its variations, is a fundamental metabolic pathway in life that functions in almost all organisms to decompose external or intracellular sugars. The pathway involves the partial oxidation and splitting of sugars to pyruvate, which in turn is decarboxylated to produce acetyl-coenzyme A (CoA) for various biosynthetic purposes. The decarboxylation of pyruvate loses a carbon equivalent, and limits the theoretical carbon yield to only two moles of two-carbon (C2) metabolites per mole of hexose. This native route is a major source of carbon loss in biorefining and microbial carbon metabolism. Here we design and construct a non-oxidative, cyclic pathway that allows the production of stoichiometric amounts of C2 metabolites from hexose, pentose and triose phosphates without carbon loss. We tested this pathway, termed non-oxidative glycolysis (NOG), in vitro and in vivo in Escherichia coli. NOG enables complete carbon conservation in sugar catabolism to acetyl-CoA, and can be used in conjunction with CO2 fixation1 and other one-carbon (C1) assimilation pathways2 to achieve a 100% carbon yield to desirable fuels and chemicals.

Synthetic non-oxidative glycolysis enables complete carbon conservation

Photosynthesizing plants have been exposed to C{sup 14}O{sub 2} for short periods of time (0.4 to 15 sec.) and the products of carbon dioxide reduction analyzed by paper chromatography and radio autography. Methods have been developed for the degradation of ribulose and sedoheptulose. These sugars, obtained as their phosphate esters from the above C{sup 14}O{sub 2} exposures and from other experiments, have been degraded and their distribution of radiocarbon determined. The distribution of radiocarbon in these sugars, and other data, indicate that sedoheptulose phosphate and ribulose diphosphates are formed during photosynthesis from triose and hexose phosphates, the latter being synthesized, in turn, by the reduction of 3-phosphoglyceric acid.

/pdf/the-path-of-carbon-in-photosynthesis-xxi-the-cyclic-31nfqvzlgp.pdf

The Path of Carbon in Photosynthesis. XXI. The Cyclic Regenerationof Carbon Dioxide Acceptor

IJCEL-2369 Unclassified Chemistry D i s t r i b u t i o n UNIVERSITY O CALIFORML4 F Radiation Laboratory Contract No. M-7405- eng THE P T O CARBON I N PHdGSYNTHESIS .BI. A H F T E CYCLIC REGENERATION O CARBON DIOXIDE ACCEPTOR H F J. A , Bassham, &Ae A. Benson, Lorel D. K r ~ r ~ Anne Z. Harris, A. T. Wilson and M. Calvin October, 1953 Berkeley, California

The Path of Carbon in Photosynthesis. XXI. The Cyclic Regeneration of Carbon Dioxide Acceptor

Studies of carbon dioxide fixation in green plants using the C{sup 14} isotope have shown that in very short times phosphoglyceric acid contains most of the radioactivity. The tracer is present almost entirely in the carboxyl group. The importance of organic phosphates in the subsequent metabolism of phosphoglyceric acid can be seen from the accompanying photographs.

/pdf/the-path-of-carbon-in-photosynthesis-xvii-phosphorus-1wto3gg4rx.pdf

A.T. Wilson

Papers

The Path of Carbon in Photosynthesis. XXI. The Cyclic Regenerationof Carbon Dioxide Acceptor

The Path of Carbon in Photosynthesis. XXI. The Cyclic Regeneration of Carbon Dioxide Acceptor

The Path of Carbon in Photosynthesis. XVII. Phosphorus Compounds as Intermediates in Photosynthesis

The path of carbon in photosynthesis. 21. the cyclic regeneration of carbon dioxide acceptor