Carbon fixation via C4 photosynthesis allows plants to tolerate drought, high temperatures, and nitrogen and carbon dioxide limitations. In plants, the C4 pathway has evolved from the ancestral C3 pathway multiple times. To identify developmental changes that accompanied the transition between photosynthetic types, Lundgren et al. investigated leaf anatomy in Alloteropsis semialata. The ratio of mesophyll to bundle sheath tissue, as determined by the number of minor veins, correlates with the photosynthetic route used by a plant. Growth under controlled conditions indicated that the presence of minor veins is not a plastic response to the environment.
Organisms live in an interconnected dynamic web in which they make, degrade, and interconvert compounds containing carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The carbon cycle involves the oxidation of organic compounds to produce CO2 by heterotrophic organisms and the incorporation ("fixation") of CO2 from the environment into living tissue by autotrophic organisms. Heterotrophic organisms (most animals) obtain the energy for life by conserving the energy obtained by oxidizing organic molecules to CO2 in the form of reducing equivalents (electrons) and adenosine triphosphate (ATP). Autotrophic plants, bacteria, and archaea fix CO2 by a process in which the energy of electrons and ATP is used to produce biomolecules, such as sugars, amino acids, and lipids, thereby replenishing these essential organic molecules in the ecosystem. Cumulatively, autotrophy occurs on the huge scale of 7 1016 g of carbon fixed annually (1).
Rising atmospheric carbon dioxide (CO2) concentration as a result of extensive use of fossil fuel resources is one of the main causes of global warming. Natural photosynthesis converts 100 billion tons of CO2 into biomass annually (1). Although natural photosynthesis plays a vital role in absorbing CO2 emitted from fossil fuel use, it cannot prevent the net increase of atmospheric CO2 concentration since the Industrial Revolution. Natural CO2 fixation is mainly achieved by a CO2 fixation pathway called the Calvin cycle, in which ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is the key enzyme. To date, six CO2 fixation pathways, including the Calvin cycle, have been discovered (2). On page 900 of this issue, Schwander et al. (3) report a synthetic CO2 fixation pathway that is more energy efficient than the Calvin cycle, expanding the capabilities for recapturing atmospheric CO2 for use as a carbon feedstock.
Atmospheric carbon dioxide levels are rising, triggering global climate change, scientists agree. Researchers have been searching for ways to scrub some of this damaging gas from the atmosphere, and the answer may have been right in front of them. "We actually have taken our inspiration from nature itself," says Tobias Erb, a biochemist at the Max Planck Institute for Terrestrial Microbiology in Germany, in a phone interview with The Christian Science Monitor. Plants and other photosynthesizing organisms can turn carbon dioxide into biomass. And now Dr. Erb and his team have built a synthetic pathway to do that more efficiently – at least in a test tube, and perhaps someday in plants or other organisms.
Most of the ocean is dark. Yet it is in this darkness, away from photosynthesizing sunlight, that most planetary carbon cycling occurs. Pachiadaki et al. show that nitrite-oxidizing bacteria in one phylum are the predominant fixers of dissolved inorganic carbon in the mesopelagic ocean. The authors sequenced thousands of single amplified genomes of marine prokaryotes. They identified more than 30 nitrite-oxidizing obligate chemoautotrophic bacteria that were unable to transport carbohydrate and that expressed nitrite oxidoreductase. This enzyme provides electrons to drive a reverse tricarboxylic acid cycle that fixes the carbon. Many of the genomes also suggest organisms that have the capacity to produce ammonium and other substrates, possibly to feed nitrite-producing metabolic partners.