ASU study finds two bacteria incorporate inorganic carbon in lab tests

Arizona State University researchers reported on September 28, 2026, that two bacteria grown in laboratory cultures incorporated carbon from bicarbonate while still relying on organic matter. The finding matters for scientists tracing how carbon enters microbial biomass in soils and seas, though tests on two isolates cannot show how widespread the process is in nature.

The team studied Pseudomonas putida, a soil bacterium, and Pseudoalteromonas carrageenovora, a marine bacterium. Both are heterotrophs: they depend on organic material for growth. Their uptake of some inorganic carbon does not mean they can grow without that food, or that they perform the same role as plants and other organisms that build biomass chiefly from inorganic carbon.

How the bacteria were tested

The researchers grew the bacteria with different supplies of organic carbon and at warmer and colder temperatures. They added bicarbonate marked with carbon-13 to each treatment, then measured whether that labeled carbon appeared in bacterial biomass. Because the label identifies carbon supplied in inorganic form, its presence in the cells showed that the bacteria had incorporated some of it.

Both species incorporated labeled inorganic carbon in every tested treatment. The reported contribution to biomass ranged from 0.006% to 0.029% for P. putida and from 0.18% to 1.1% for P. carrageenovora. Those figures describe the measured cultures, not the share of carbon fixed by all bacteria in a soil or ocean ecosystem.

The team also examined selected samples of the marine bacterium with a single-cell isotope technique. Individual cells differed in how much labeled carbon they incorporated, a distinction that measurements of a whole culture could obscure. The researchers caution that their reported fractions might be underestimates: respiration during the experiment could have diluted the label in the dissolved inorganic-carbon pool.

Why the team looked beyond organic carbon

Lead author Emma Brown told ASU News that the researchers wanted to understand why heterotrophic bacteria use inorganic carbon, how much they use and what controls the process. Co-author Elizabeth Trembath-Reichert said earlier observations of microbial communities taking up inorganic carbon in fluids from rocks beneath the Mid-Atlantic Ridge helped motivate the laboratory work.

Those observations raised a question that the controlled cultures could address more directly: whether bacteria normally treated as consumers of organic carbon could also put inorganic carbon into their cells. Trembath-Reichert described bacterial carbon use as a spectrum rather than two separate groups of makers and consumers. The new measurements support that distinction for the two species tested.

The authors suggest that this form of carbon incorporation may matter where dissolved inorganic carbon and nitrogen are available but readily used organic carbon is scarce or arrives intermittently. That is an environmental implication of the laboratory findings. The experiment did not measure uptake by these two species in natural soil, seawater or subsurface communities.

What the result means for ocean carbon research

A separate study published in Nature Geoscience in June examined inorganic-carbon incorporation by Alteromonas, another marine heterotrophic bacterial group. Its authors combined ocean datasets, laboratory incubations and measurements of sampled communities. They reported evidence that heterotrophic metabolism can contribute to inorganic-carbon fixation in the dark ocean, and presented a global flux estimate as an extrapolation.

That ocean work concerns a different bacterium and different settings. It provides context for why researchers care about the pathway, but its field observations do not verify the uptake rates measured in the ASU team's two cultures. Likewise, the new laboratory result cannot be used to assign a global carbon-cycle contribution to P. putida or P. carrageenovora.

One practical question is how to interpret future isotope-tracing studies. The new study's authors say researchers may need to account for heterotrophic incorporation when deciding which microbes fixed inorganic carbon. Establishing how common the process is in natural communities, and how much carbon it moves there, will require evidence beyond these two cultured isolates.