Natures 7 Ways to fixate CO₂

The CO₂ in our atmosphere becomes an increasing problem. Planting trees won’t cut the cake anymore. But do other mechanisms exist in nature that convert CO₂ into biomass? Into something we can use? There are actually seven different pathways found in nature that do that! And many of them are super energy efficient! You would be surprised to see where plants rank.

So let’s explore some of them! From least energy efficient, to most energy efficient! I sorted them by the amount of adenosine triphosphate they need per CO₂ fixed. Reducing equivalents like electrons from ferredoxins and NAD(P)H is also needed in the conversion of CO₂ to biomass. I indicated them but will not discuss them much.

Calvin–Benson–Bassham cycle

Let’s start with the energetically worst, and also most famous example for carbon fixation in nature. The Calvin–Benson–Bassham cycle (CBB cycle). It is the primary CO₂ fixation pathway in plants, algae, and cyanobacteria. This metabolic pathway actually exists in 6% of microbes! Which is a lot! In photosynthetic organisms, the CBB cycle operates in the chloroplast stroma, using ATP and NADPH from photosynthesis to convert CO₂ into glyceraldehyde-3-phosphate. The cycle consumes three ATP, making it not very efficient in that regard. But, one needs to keep in mind that plants have abundant energy when the sun is shining. So, having a reliable but maybe not very energy-efficient pathway is acceptable.

We as humans however might want to be on the look out for more energy efficient ways. So what else is there…

Reverse Tricarboxylic Acid cycle

The reverse TCA cycle, sometimes called the reductive citric acid cycle, is essentially the citric acid cycle run backwards. Instead of breaking down organic compounds to release CO₂, it uses CO₂ to build them. This pathway is common among anaerobic and microaerophilic bacteria, of which many are found near hydrothermal vents.

It’s considered one of the most ancient carbon fixation pathways, possibly predating oxygenic photosynthesis. The rTCA cycle is more energy-efficient than the Calvin cycle and may represent a glimpse into how the earliest autotrophic life on Earth fixed carbon.

Reductive Glycine Pathway

The reductive glycine pathway is a more recently discovered, ATP-efficient route used by some anaerobic bacteria. It works by sequentially reducing CO₂ to formate, then to methylene-tetrahydrofolate, which combines with ammonia to form glycine.

Although it operates under low-energy, oxygen-free conditions, it’s a clever adaptation for microbes living in environments where other pathways would be too costly. This pathway highlights nature’s ability to innovate even under extreme metabolic constraints.

Wood-Ljungdahl Pathway

The Wood–Ljungdahl pathway, or acetyl-CoA pathway, is another ancient and highly efficient route used by acetogenic bacteria and methanogenic archaea. It requires minimal ATP and directly produces acetyl-CoA, a key molecule for both biosynthesis and energy generation.

Key enzymes in this pathway include formate dehydrogenase (FDH), carbon monoxide dehydrogenase (CODH), and the CODH/acetyl-CoA synthase (CODH/ACS) complex.
CODH can reduce CO₂ to CO with remarkable precision and minimal energy loss — an impressive feat that even modern catalysts struggle to replicate. This efficiency makes the WLP particularly interesting for bioengineering and synthetic carbon capture technologies.

That is why I am a member of a research group that works on one of the key enzymes. CODH! I will introduce our work on this fascinating enzyme in future blog post. So stay tuned if this is something you are interested in!

Other Pathways

There are three more pathways that fixate CO₂ and should be mentioned here as well. To learn in more detail about them I encourage you to check out Garritano et al. (2022)!

Conclusion

Nature has evolved an incredible diversity of strategies to capture and convert CO₂ into useful organic molecules. From the familiar CBB cycle in plants to the hyper-efficient WLP in ancient microbes, these mechanisms reveal just how creative evolution can be when it comes to managing carbon. Some of these pathways thrive in extreme conditions—high heat, no oxygen, or acidic environments— and accomplish CO₂ fixation with remarkable energy efficiency.

Understanding these natural systems isn’t just an academic exercise; it could inspire the next generation of bioengineering solutions for climate change. By learning from the enzymes and cycles that nature has refined over billions of years, we might one day design synthetic systems that capture carbon as efficiently as the best microbes do.

Nature has already written the instruction manual—we just have to learn how to interpret it.