Arctic bacteria use acetone pathway to capture carbon dioxide

A bacterium found in Arctic Ocean surface waters appears to use acetone as part of a carbon-assimilation pathway that also incorporates carbon dioxide, according to reporting by nature.com. The finding identifies an unusual form of carboxylation in a photoheterotrophic marine population and suggests that the process could help the organism expand its supply of material for growth.

The pathway was detected in metagenomic samples collected across a north-to-south survey of the Arctic Ocean. Researchers connected the genes involved to one population of Gammaproteobacteria, Porticoccus arcticus. The organism was not simply detected once: its genetic sequences appeared repeatedly in a multiyear survey based on 16S ribosomal RNA, and in some samples they represented as much as 9% of the measured population.

That recurring presence points to an established role for P. arcticus in Arctic marine ecosystems rather than a fleeting appearance in the sampled waters. The study also examined metatranscriptomes, which indicate which genes are active in environmental communities. In those data, the genes for acetone carboxylase and several rhodopsins ranked among the most strongly expressed P. arcticus genes in nutrient-poor Arctic surface waters.

A different use for carboxylation

Carboxylases are enzymes that help attach inorganic carbon, including carbon dioxide, to other molecules. This chemistry is widely associated with autotrophic organisms, which use carbon dioxide to make organic compounds. The new work extends that picture by presenting evidence that a carboxylase can participate in the breakdown and assimilation of an organic compound—in this case, acetone.

The proposed route is therefore not described as ordinary carbon fixation alone. It combines acetone degradation with carbon dioxide incorporation. For a bacterium living in oligotrophic waters, where available nutrients are limited, that combination could provide an additional way to generate the molecular building blocks needed for its cells. The study’s conclusion is that acetone carboxylation may increase the biosynthetic capacity of photoheterotrophic bacteria by supplementing organic-carbon metabolism with inorganic carbon.

The evidence comes from the distribution and activity of genes rather than from a direct laboratory demonstration of the complete pathway. Metagenomes showed that P. arcticus carries the relevant genetic machinery, while metatranscriptomes showed that the acetone-carboxylase genes are highly active in the natural environment examined. Together, those observations associate the pathway with the bacterium’s life in Arctic surface waters.

A streamlined genome with added light-sensing genes

The genomic picture of P. arcticus adds another part to the finding. Compared with related Porticoccus organisms, it has a more compact, streamlined genome. Despite that reduced genomic complement, it retains a complete set of genes for acetone carboxylation.

At the same time, the bacterium has acquired multiple proteorhodopsin genes through lateral gene transfer. Proteorhodopsins are represented in the study as part of the organism’s light-associated genetic toolkit, and the strong expression of the rhodopsin genes in surface waters places them alongside acetone carboxylation among its prominent activities there.

This combination is notable because it joins two features in one population: genes for using light-linked rhodopsins and a pathway that processes acetone while bringing carbon dioxide into metabolism. The findings do not establish that the two systems operate as a single biochemical sequence, but their high expression in the same Arctic population highlights the organism’s distinctive metabolic profile.

Why the Arctic record matters

The repeated detection of P. arcticus in the long-running 16S rRNA record strengthens the ecological significance of the result. A pathway found only in an isolated genomic fragment would offer limited evidence about its importance in the ocean. Here, the organism was consistently present and at times relatively abundant in the sampled Arctic marine communities.

The study therefore places acetone carboxylation within the broader operation of the ocean carbon cycle. Carboxylases already occupy a central position in nature because they mediate carbon dioxide fixation. Evidence that a marine photoheterotroph can use one during acetone degradation adds another possible route by which inorganic carbon enters cellular material.

As reported by nature.com, the work identifies a metabolic module in Arctic bacteria that may help them thrive in oligotrophic surface waters. It also shows how genome surveys, environmental gene-expression data and long-term community records can together reveal an unexpected form of carbon processing in the ocean.