15: Fixing Carbon Fixation

Biological Carbon Fixation

Nature has produced many enzymes that can fix atmospheric CO₂, and, of course, everyone knows that photosynthesis is the source of most fixed CO₂ in the biosphere.  Plankton, cyanobacteria, algae, and plants are key in removing atmospheric CO₂ through photosynthesis.  Yet, we can't plant enough trees to reduce CO₂ in the atmosphere in the next decade to avoid some of the worst consequences of anthropogenic climate change.  Old-growth trees (mostly gone or under significant stress) are best at removing CO₂.  New trees would take decades of growth before their effect on carbon drawdown would be consequential.  We must also fix carbon dioxide to decrease atmospheric CO₂ and make more food!

Carbon (about 100 petagrams) is sequestered yearly in net primary production (carbon fixation into biomolecules). This is split almost equally between land and ocean organisms.  The key enzyme in this process is Rubisco in the C3 (Chapter 20.4) and C4/CAM (Chapter 20.5) pathways.  The enzyme is large, containing many large subunits and an equivalent number of small ones.  A chaperonin is required for folding.  It is also a very slow enzyme with a k_cat of around 2-10 CO₂/sec. In addition, it can bind another substrate, O₂, and engage in a competing reaction, photorespiration, as described in Chapter 20.4.   

Figure \(\PageIndex{1}\) shows an interactive iCn3D modelof ribulose 1,5-bisphosphate carboxylase/oxygenase from Synechococcus PCC6301 (1RBL)

This structure is a hetero 16-mer consisting of 8 small and 8 large chains. The small chains are gray, and the large ones are different colors. Each large subunit contains a bound reaction intermediate analog, 2-carboxyarabinitol 1,5-bisphosphate.

This chapter will focus on improving and designing new ways to capture carbon dioxide and reduce its atmospheric concentration. Remember that the best way to address anthropogenic climate change is to drastically reduce the burning of fossil fuels.

Naturally occurring pathways to fix carbon

(Much of this immediate section derives from the following reference: Natural carbon fixation. Sulamita Santos et al.,  Natural carbon fixation and advances in synthetic engineering for redesigning and creating new fixation pathways, Journal of Advanced Research, Volume 47, 2023, Pages 75-92, ISSN 2090-1232, https://doi.org/10.1016/j.jare.  Creative Commons license

Six naturally occurring pathways fix carbon. These are illustrated in Figure \(\PageIndex{2}\) below.  

Figure \(\PageIndex{2}\):  Natural carbon fixation. Sulamita Santos et al.,ibid.

Panel (A) shows the CBB cycle, which we discussed in great detail in Chapter 20.4. The enzymes involved are ribulose-1,5-bisphosphate carboxylase/oxygenase, 3-phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, and ribulose-phosphate epimerase.

Panel (B) shows the reverse (reductive) TCA cycle, which we discussed in Chapter 16.4 in the section on the α-ketoacid pathway—a primordial, prebiotic anabolic "TCA-like" pathway. The enzymes include ATP-citrate lyase, malate dehydrogenase, succinyl-CoA synthetase, ferredoxin (Fd)- dependent 2-oxoglutarate synthase, isocitrate dehydrogenase, and PEP carboxylase. 

Panel (C) shows the Wood–Ljungdahl (or reductive Acetyl-CoA) cycle, which we discussed in Chapter 30.1.  At the top of the pathway are the acetogens Archaea, and at the bottom are the methanogens Archaea.  The enzymes include MPT-methylene tetrahydromethopterin, MFR-methanofuran, THF, tetrahydrofolate. 

Panel (D) shows the 3-hydroxypropionate (3HP) cycle.  The enzymes include acetyl-CoA carboxylase, propionyl-CoA carboxylase, methylmalonyl-CoA epimerase, succinyl-CoA:(S)-malate-CoA transferase, trifunctional (S)-malyl-CoA, -methylmalyl-CoA, mesaconyl-CoA transferase, mesaconyl-C4-CoA hydratase. 

Panel (E) shows the hydroxypropionate/4-hydroxybutyrate (HP/HB) and the dicarboxylate/4-hydroxybutyrate (DC/HB) cycles.  The enzymes include pyruvate synthase, PEP-carboxylase, malate dehydrogenase, fumarate hydratase/reductase, acetyl-CoA/propionyl-CoA carboxylase, 3-hydroxypropionate-CoA ligase/dehydratase, methylmalonyl-CoA mutase, succinyl-CoA reductase, 4-hydroxybutyrate-CoA ligase, crotonyl-CoA hydratase, and acetoacetyl-CoA-ketothiolase.

Table \(\PageIndex{1}\) below shows a comparative description of the natural and synthetic carbon fixation pathways.

Sulamita Santos et al.,  Natural carbon fixation and advances in synthetic engineering for redesigning and creating new fixation pathways, Journal of Advanced Research,
Volume 47, 2023, Pages 75-92, ISSN 2090-1232, https://doi.org/10.1016/j.jare.  Creative Commons license

Plants and photoautotrophic microorganisms fix CO₂ by the Calvin cycle using NADPH and ATP, producing O₂.  Some anaerobic photosynthetic bacteria don't produce O₂.  In chemolithoautotrophic microorganisms, energy sources (i.e., electron donors like H₂, H₂S, sulfur, Fe²⁺, nitrite, and NH₃ found in bedrock - the lithosphere) other than NADPH, ATP, and light can be used to drive CO₂ uptake. For chemoorganotrophic microbes, reduced organic molecules such as sugars and amino acids serve as electron (donor) sources.  

Genetic engineering and synthetic biology are now employed to improve preexisting enzymes and create new pathways for carbon capture.

As a simple example, people are trying to engineer carbonic anhydrase, which converts CO₂ to HCO₃^-, for transport to the lungs, where it is converted back to CO₂ and released.  It is a diffusion-controlled enzyme with a k_cat/K_M reported as high as 8 x 10⁷ M^-1s^-1, so how can it be made better?  One way is to engineer thermal stability into the enzyme. A critical problem for the forward reaction is that the enzyme is readily reversible; thus, bicarbonate, HCO₃^-, is a competitive inhibitor of the forward reaction. In addition, the enzyme can be engineered to be more stable at higher pH to allow product (HCO₃^-) removal by the addition of OH^- in the process of mineralization, as shown in the equation below,

HCO₃^- + OH^- →  CO₃^2- + H₂O

where the carbonate anion can precipitate in the presence of divalent cations like  Ca²⁺, Mg²⁺, and Fe²⁺.  Here is a link to a Literature-based Guided Assessment on thermoengineering of carbonic anhydrase.  

Now, let's explore using new pathways created by synthetic biology to capture carbon.  Some of these pathways are engineered to produce reactants (feedstocks) for biofuels, which we discussed in depth in Chapter 32, the section on chemical synthesis.  We'll focus on the CETCH pathway, the reductive glyoxylate and pyruvate synthesis (rGPS) cycle, and the malyl-CoA-glycerate (MCG) pathway.

CETCH (Crotonyl-CoA-EThylmalonyl-CoA-4Hydorxybutyl-CoA) pathway

To engineer a new pathway, Swander et al identified efficient carboxylases from known ones (acetyl-CoA carboxylase, Rubisco, propionyl-CoA carboxylase, PEP carboxykinase, 2-oxoglutarate carboxylase, and pyruvate carboxylase), all of which we have discussed in previous chapters.  They created new pathways, calculated free energy and ATP/NADPH requirements, and optimized the pathways.  They chose CoASH–dependent carboxylases and enoyl-CoA carboxylases/reductases.  Figure \(\PageIndex{3}\) below shows the reaction of one key carboxylase.  

Figure \(\PageIndex{3}\): Carboxylase used in the CETCH pathways to fix CO₂  

The pathway was named CETCH (Crotonyl-CoA-EThylmalonyl-CoA-4Hydorxybutyl-CoA) which catalyzes this next reaction in cell lysates (in vitro):

2CO₂ + 3NAD(P)H + 2ATP + FAD → glycolate + 3NAD(P) + 2ADP +2P_i + FADH₂

The rate of CO_2­ fixation by the CETCH pathways was similar to the rate of the Calvin cycle rates in cell lysates. 

In a more expansive approach, Gleizer used synthetic biology to convert E. coli from a heterotroph to an autotroph, with its biomass (carbon reservoir) derived from CO₂.  Formate (HCO₂^-) was used as a source of reducing power (electrons), as it was oxidized by an added formate dehydrogenase to produce NADH, which was then used to support the autotrophic fixation of CO₂ via the Calvin cycle enzymes.  Using isotopically labeled ¹³CO₂ to track carbon flow, after 10 generations of evolution, the cells were completely autotrophic, fixing CO₂.  To accomplish this, they knocked out genes for phosphofructokinase (glycolysis) and glucose-6-phosphate dehydrogenase (pentose phosphate pathway) to impair these main metabolic pathways. They added carbonic anhydrase (to interconvert CO₂ and HCO₃^-), and Rubisco and phosphoribulokinase.  As formate was ultimately converted to CO₂, the net effect was not carbon neutral, but could be if atmospheric CO₂ was used to make formate (for a feedstock) by electrochemical reduction.

Figure \(\PageIndex{4}\) below shows the next synthetic E. Coli autotroph reactions.

Figure \(\PageIndex{4}\):  Schematic Representation of the Engineered Synthetic Chemo-autotrophic E. coli.  Shmuel Gleizer et al., Conversion of Escherichia coli to Generate All Biomass Carbon from CO2, Cell, 179 (2019). https://doi.org/10.1016/j.cell.2019.11.009.   Creative Commons license.  

CO₂ (green) is the only carbon source for all the generated biomass. CO₂ fixation occurs via an autotrophic carbon assimilation cycle. Formate is oxidized by a recombinant formate dehydrogenase (FDH) to produce CO₂ (brown) and NADH. NADH provides the reducing power to drive carbon fixation and serves as the substrate for ATP generation via oxidative phosphorylation (OXPHOS in black). The formate oxidation arrow is thicker than the CO₂ fixation arrow, indicating a net CO₂ emission even under autotrophic conditions.

Figure \(\PageIndex{5}\) shows that almost 100% of carbon atoms after many generations are labeled with ¹³C (detected by mass spec analysis) derived from ¹³CO₂.   

Figure \(\PageIndex{5}\): Isotopic Labeling Experiments Using ¹³C Show that All Biomass Components Are Generated from CO₂ as the Sole Carbon Source.  Gleizer et al., ibid.

(A) Values are based on LC-MS analysis of stable amino acids and sugar-phosphates. The fractional contribution of ¹³CO₂ to various protein-bound amino acids and sugar phosphates in evolved cells grown on ¹³CO₂ and naturally labeled formate was almost complete, indicating full ¹³C labeling of biosynthesized amino acids. The reported numbers are the ¹³C fractions of each metabolite, calculated as the effective ¹³CO₂ fraction out of the total inorganic carbon (which decreases due to unlabeled formate oxidation to CO₂). The numbers in parentheses are the uncorrected measured values of the ¹³C fraction of the metabolites. 

Synthetic reductive glyoxylate and pyruvate synthesis (rGPS) cycle and the malyl-CoA-glycerate (MCG) pathways

These pathways were created to synthesize acetyl-CoA, pyruvate, and malate from CO₂ in cell-free systems, freeing the system from cell growth and regulatory requirements and making it insensitive to oxygen.  These molecules are also intermediates in the created cycle, which could operate continuously for hours at the same or greater rates of CO₂ fixation compared to photosynthesis. The cycle is shown in Figure \(\PageIndex{6}\) below.

Figure \(\PageIndex{6}\):  The rGPS–MCG cycle with acetyl-CoA as the end product. Luo, S., Lin, P.P., Nieh, LY. et al. A cell-free self-replenishing CO₂-fixing system. Nat Catal 5, 154–162 (2022). https://doi.org/10.1038/s41929-022-00746-x .  Creative Commons Attribution 4.0 International License, http://creativecommons.org/licenses/by/4.0/

The rGPS cycle consists of the reductive glyoxylate synthesis (rGS) pathway (blue) and the reductive pyruvate synthesis (rPS) pathway (green). The malyl-CoA-glycerate (MCG) pathway (orange) consists of the rGS and glycerate pathways. The red arrow indicates the carboxylation reaction. Gcl, glyoxylate carboligase; Tsr, tartronate semialdehyde reductase; Gk, glycerate 2-kinase; Eno, enolase and 2PG, 2-phospho-d-glycerate

Microbial electrosynthesis from CO₂

We mentioned above that if the formate used in the CETCH pathway could be synthesized electrochemically from CO₂, the pathway would be truly carbon-neutral. New microbial electrochemical methods are being developed to synthesize various small molecules that could serve as feedstocks for industrial chemical synthesis.  The carbon in CO₂ has an oxidation number of +4, while the carbon in formic acid has an oxidation number of +2.  Hence, two electrons must be added electrochemically to make the CETCH pathway truly carbon neutral.  Bigger electrochemical reductants of CO₂ require more electrons.  Figure \(\PageIndex{7}\) below shows how key feedstocks could be made electrochemically from CO₂ and how they are usually made in industry.  

Figure \(\PageIndex{7}\) :  Overview of the Main Products Formed from Microbial Electrosynthesis (MES) From CO₂, Along With the Main Industrial Methods to Manufacture These Products.  Jourdin et al., Trends in Biotechnology, April 2021, Vol. 39, No. 4 https://doi.org/10.1016/j.tibtech.2020.10.014.  Creative Commons Attribution (CC BY 4.0)

These feedstocks could be produced by reductive electrosynthesis using electrons generated by water oxidation via electrolysis.  Released electrons  (oxidation number of O in water is -2 and 0 in O₂) move to a biocathode to reduce CO₂, as illustrated in Figure \(\PageIndex{8}\) below. 

Figure \(\PageIndex{8}\):  Reactor configurations for MES-based CO₂ conversion: (a) H-type, (b) single chamber, (c) dual chamber, (d) continuous stirred tank, and (e) schematic of the electron transfer mechanism.  G. S. Lekshmi et al., Microbial electrosynthesis: carbonaceous electrode materials for CO₂ conversion.  Mater. Horiz., 2023, 10, 292-312.  DOI: 10.1039/D2MH01178F.  Creative Commons Attribution-Non Commercial 3.0 Unported Licence

The biocathode consists of a biofilm of cells printed onto a carbon support (graphene, graphite, or carbon nanotubes).  

Marine CO₂ Removal Through Geoengineering

As the climate worsens, more climate scientists are considering ways to improve the oceans' ability to capture CO₂. Both inorganic and biological methods have been proposed. These methods will likely have unintended consequences, affecting biodiversity and the natural mechanisms for carbon sequestration. Hence, they are both controversial.

Inorganic:  

Here is a review from Chapter 32.3:  The Carbon Cycle and Carbon Chemistry

The reversible movement of CO₂ from the atmosphere to the oceans, CO_2 atm ↔ CO_2 ocean, depends on the difference in the partial pressures of CO₂ (ΔpCO₂) in the atmosphere and surface waters.  The reaction is also driven to the right by the removal of CO₂ (aq) as it forms carbonic acid (H₂CO₃), which then forms bicarbonate (HCO₃^–) and carbonate (CO₃^2–). These coupled reactions chemically buffer ocean water, thus regulating ocean pCO₂ and pH.

pCO₂ is not homogeneous in ocean surface waters and varies with current and temperature conditions.  CO₂ can be more readily released from upwellings of nutrient-rich and warm waters, especially in the tropics.  In cold Northern waters and the Southern Ocean, where water sinks, it is taken up from the atmosphere (again, CO₂ is more soluble in cold water). 

As we discussed in Chapter 31.1, ocean chemistry of CO₂ largely determines atmospheric CO₂ levels.  The coupled reactions of CO₂ in the oceans are shown below.

\begin{equation}
\mathrm{CO}_2(\mathrm{~g}, \mathrm{~atm}) \leftrightarrow \mathrm{CO}_2(\mathrm{aq}, \text { ocean) }
\end{equation}

\begin{equation}
\mathrm{CO}_2(\mathrm{aq} \text {, ocean })+\mathrm{H}_2 \mathrm{O}(\mathrm{I} \text {, ocean }) \leftrightarrow \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{HCO}_3^{-}(\mathrm{aq})
\end{equation}

\begin{equation}
\mathrm{H}_2 \mathrm{O}(\mathrm{I})+\mathrm{HCO}_3^{-}(\mathrm{aq}) \leftrightarrow \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{CO}_3{ }^{2-}(\mathrm{aq} \text {, sparingly soluble })
\end{equation}

These reactions should be familiar to all chemistry students and were presented previously in Chapter 31.1 and Chapter 2.  A significant source of oceanic bicarbonate is the weathering of rocks such as limestone and marble, both forms of CaCO₃. The relevant reactions are shown below.  

\begin{equation}
\begin{aligned}
&\mathrm{CaCO}_3(\mathrm{~s})+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{Ca}^{2+}(\mathrm{aq})+\mathrm{CO}_3{ }^{2-}(\mathrm{aq}) \\
&\mathrm{CO}_3{ }^{2-}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{HCO}_3{ }^{-}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})
\end{aligned}
\end{equation}

CO₂ is nonpolar and not very soluble in water.  Either is CO₃^2- in the presence of divalent cations like Ca²⁺. However, HCO₃^- is and can be considered a "soluble" form of carbon.  This soluble form from terrestrial weathering enters rivers and eventually reaches the ocean.  It is also the form of carbonate that is transferred into cells by anion transporters for eventual shell formation. HCO₃^- is also a chief regulator of both blood and ocean pH.  Weathering is slow compared to anthropogenic CO₂ emissions from fossil fuel use, but it is nevertheless a key player in the carbon cycle and in regulating ocean pH. 

The same weathering reactions on silicate rocks release silicate ions into rivers and the ocean, where they can be used by diatoms to form CaSiO₄ shells. As the oceans take up more CO₂, they become more acidic, mimicking "weathering" of the shells of living organisms and potentially leading to their death. Silicon is directly underneath carbon in the periodic table, so this simplified reaction is analogous to those we see with CO2 and its inorganic ions.

\begin{equation}
\mathrm{H}_4 \mathrm{SiO}_4=\mathrm{SiO}_2+2 \mathrm{H}_2 \mathrm{O}
\end{equation}

H₄SiO₄ is silicic acid.

With this background in mind, you can understand the proposed method of adding crushed limestone (CaCO₃) to the rivers (which drain into the oceans) to remove atmospheric CO₂. As the solid crushed limestone dilutes, it becomes more soluble. As CO₃^2- is a reasonable base, the water would become more alkaline.  It would mostly get converted to HCO₃^- which can be transported into river shell-forming organisms.  The rest enters the oceans, where it can be transported into organisms and used to make CaCO₃ shells, eventually sinking to the depths where the carbon would be sequestered for 1000s of years.   This would also reduce river acidification.  Whether this process could be run at the scale needed to reduce atmospheric CO2 significantly is unclear.

Biological

This method involves adding Fe²⁺, usually iron (II) sulfate (FeSO₄), to the oceans to promote phytoplankton growth. This process has been called ocean iron fertilization (OIF). One problem with FeSO₄ is that it rapidly oxidizes to insoluble iron(III) species at elevated temperatures. It must be transported to sufficient depths so it doesn't return to the surface. Another problem is the adverse effects of nutrient removal on other marine organisms.  

Summary

(Summary written by Claude, Anthropic)

Carbon fixation — converting atmospheric CO₂ into organic biomolecules — removes approximately 100 petagrams of carbon per year through net primary production, split roughly equally between land and ocean organisms. The dominant enzyme is RuBisCO, which despite its global importance is remarkably slow (kcat ≈ 2–10 CO₂/sec), requires a chaperonin for assembly, and is compromised by a competing oxygenase reaction that drives wasteful photorespiration. While planting trees is widely proposed as a climate solution, old-growth forests are far more effective carbon sinks than new plantings, which require decades before contributing meaningfully to drawdown. The chapter therefore focuses on engineering improved and entirely new carbon fixation strategies.

Natural carbon fixation pathways. Six naturally occurring pathways fix CO₂ beyond the Calvin-Benson-Bassham (CBB) cycle. The reverse TCA cycle fixes CO₂ into pyruvate in green sulfur bacteria using ferredoxin and NAD(P)H. The Wood–Ljungdahl pathway, used by acetogenic and methanogenic Archaea, is one of the most energy-efficient routes, producing acetyl-CoA from two CO₂ molecules using hydrogen as the energy source. The 3-hydroxypropionate cycle (Chloroflexaceae) uses acetyl-CoA and propionyl-CoA carboxylases to fix bicarbonate. The HP/HB and DC/HB cycles (thermophilic Archaea) use hydrogen and sulfur to produce acetyl-CoA under anaerobic conditions. These pathways collectively demonstrate the metabolic diversity available for CO₂ capture and provide a toolkit for synthetic biology.

Engineered carbonic anhydrase. As an entry point to synthetic carbon capture, carbonic anhydrase — which interconverts CO₂ and HCO₃⁻ at near-diffusion-controlled rates (kcat/KM up to 8 × 10⁷ M⁻¹s⁻¹) — is being engineered for greater thermal stability and higher pH tolerance. The key challenge is product inhibition: HCO₃⁻ competitively inhibits the forward reaction. Engineering the enzyme to function at higher pH allows OH⁻ addition to drive HCO₃⁻ to CO₃²⁻, which precipitates with Ca²⁺, Mg²⁺, or Fe²⁺ as insoluble carbonate minerals — permanently sequestering the captured carbon.

Synthetic carbon fixation pathways. The CETCH pathway (Crotonyl-CoA/EThylmalonyl-CoA/4-Hydroxybutyryl-CoA) was designed computationally by selecting efficient CoA-dependent carboxylases and enoyl-CoA carboxylases/reductases from known enzymes and optimizing the pathway for thermodynamic feasibility and cofactor balance. Operating in cell lysates, it fixes 2 CO₂ per cycle to produce glyoxylate: 2CO₂ + 3NAD(P)H + 2ATP + FAD → glycolate + cofactors, at rates comparable to the Calvin cycle in vitro. In a more ambitious approach, Gleizer et al. converted E. coli from a heterotroph into a fully autotrophic organism by knocking out phosphofructokinase and glucose-6-phosphate dehydrogenase (disabling glycolysis and the pentose phosphate pathway), and adding RuBisCO, phosphoribulokinase, and carbonic anhydrase. Formate served as the electron donor, oxidized by formate dehydrogenase to generate NADH for CO₂ fixation. After directed evolution over 10 generations, ¹³CO₂ isotopic labeling confirmed that virtually 100% of cellular carbon derived from CO₂. The rGPS–MCG cell-free pathway (reductive glyoxylate/pyruvate synthesis + malyl-CoA-glycerate) takes this further by operating entirely outside living cells — avoiding regulatory constraints, oxygen sensitivity, and competition with cellular metabolism — while fixing CO₂ into acetyl-CoA, pyruvate, and malate at rates matching or exceeding photosynthesis continuously for hours.

Microbial electrosynthesis. To make pathways like CETCH truly carbon-neutral, formate feedstocks must themselves be produced from atmospheric CO₂. Microbial electrosynthesis (MES) achieves this by delivering electrons from water electrolysis to a biocathode — a biofilm of microorganisms on graphene, graphite, or carbon nanotube supports — where CO₂ is reduced. Converting CO₂ (+4 oxidation state) to formate (+2) requires two electrons; larger products require more. This electrochemical approach, powered by renewable electricity, could close the carbon loop for synthetic fixation pathways.

Marine CO₂ removal by geoengineering. As biological and synthetic approaches develop, ocean-based geoengineering is also being explored, though both proposed approaches are controversial due to potential unintended ecological consequences. The inorganic approach (ocean alkalinity enhancement) involves adding crushed limestone (CaCO₃) to rivers; as it dissolves, it raises water alkalinity, converting CO₂ to HCO₃⁻, which eventually enters marine organisms as shell material (CaCO₃), sinks to the ocean floor, and sequesters carbon for thousands of years — also mitigating river and coastal acidification. Whether this is scalable to climatically meaningful levels remains uncertain. The biological approach — ocean iron fertilization (OIF) — adds FeSO₄ to iron-limited surface waters to stimulate phytoplankton blooms and biological CO₂ uptake. Practical challenges include rapid oxidation of Fe²⁺ to insoluble Fe³⁺ at the surface, the need to deliver iron to depth, and disruption of marine nutrient cycles and biodiversity.

Throughout, the chapter emphasizes that all carbon capture strategies complement but cannot substitute for the primary imperative: drastically reducing fossil fuel combustion.