Is Carbon Dioxide Necessary for Photosynthesis?

Carbon dioxide is not just helpful for photosynthesis; it is the essential raw material that plants convert into sugar. Without CO₂, a plant can still capture light energy and split water molecules, but it has nowhere to send that energy. The carbon in carbon dioxide literally becomes the carbon in glucose, cellulose, and every other organic molecule a plant builds. That said, photosynthesis is not a single reaction but a two-stage process, and the two stages have a surprisingly different relationship with CO₂.

Two Stages, One Bottleneck

Photosynthesis happens in two linked phases. The first, often called the light reactions, takes place in the thylakoid membranes of chloroplasts. Here, light energy splits water molecules, releasing oxygen and producing energy carriers that the cell can use. The second phase, the Calvin cycle, takes place in the surrounding fluid of the chloroplast. This is where CO₂ enters the picture: an enzyme called Rubisco grabs a molecule of CO₂ and attaches it to a five-carbon sugar, kicking off a chain of reactions that ultimately produces a three-carbon sugar the plant uses for growth and energy storage.

If you remove CO₂ from the equation, the light reactions still run. Chloroplasts exposed to light will keep splitting water and releasing oxygen even with no CO₂ present, as long as they have something to accept the electrons they are generating. This was demonstrated decades ago in what became known as the Hill reaction, where isolated chloroplasts produced oxygen in the presence of an artificial electron acceptor and light, with no carbon dioxide involved at all. The discovery was a landmark because it proved that oxygen production comes from water splitting, not from CO₂, and that the two halves of photosynthesis are genuinely separable.

In a living plant, though, running the light reactions without the Calvin cycle is not sustainable. The energy carriers produced by the light reactions pile up with no place to go, and reactive oxygen species build up and damage the cell. So while CO₂ is technically not needed for the light-dependent half, it is indispensable for the whole process to function as a productive cycle.

The Minimum CO₂ a Plant Needs

There is a threshold below which a plant cannot gain any net carbon from the air. Researchers call this the CO₂ compensation point: the atmospheric CO₂ concentration at which the amount of carbon a plant fixes through photosynthesis exactly equals the amount it loses through respiration and a process called photorespiration. Below this point, the plant is losing carbon faster than it can capture it.

For most common plants, the compensation point sits around 50 parts per million of CO₂ in normal air with about 21% oxygen. That number rises sharply if you increase the oxygen concentration around the plant, climbing to roughly 220 ppm in a pure-oxygen atmosphere, because high oxygen ramps up photorespiration and forces the plant to compensate with more CO₂ fixation just to break even.¹ Current outdoor air contains about 420 ppm of CO₂, well above the compensation point but far below the level that would saturate Rubisco’s capacity.

Photorespiration itself is worth understanding here because it reveals a quirk of Rubisco that makes CO₂ even more critical. Rubisco does not exclusively grab CO₂; it also reacts with oxygen, producing a molecule called 2-phosphoglycolate that the plant then has to salvage through an energy-expensive pathway that releases both carbon and nitrogen.² The higher the ratio of CO₂ to oxygen around Rubisco, the less photorespiration occurs. So CO₂ is not just a substrate; its concentration relative to oxygen determines how efficiently the plant uses its own enzyme.

How C4 Plants Concentrate CO₂ Internally

Some plants have evolved an elegant workaround for Rubisco’s split loyalties. In C4 plants like maize, sugarcane, and many tropical grasses, CO₂ is first captured in the outer mesophyll cells by a different enzyme that does not react with oxygen. The resulting four-carbon compound is then shuttled into specialized inner cells called bundle sheath cells, where it releases its CO₂ directly to Rubisco. This two-step shuttle effectively concentrates CO₂ around Rubisco to levels far above what the outside air provides.

Measurements in maize show that under normal conditions of bright light, warm temperatures, and atmospheric CO₂ of about 340 ppm, the CO₂ concentration around Rubisco in the bundle sheath cells reaches roughly 900 ppm, about three times higher than what Rubisco in a comparable non-C4 plant would experience.³ This internal concentration mechanism nearly eliminates photorespiration and makes C4 plants highly efficient in hot, bright, dry environments where stomata tend to close and restrict CO₂ entry.

C4 photosynthesis still absolutely requires atmospheric CO₂. The plant has not eliminated the need for CO₂; it has just gotten much better at hoarding it. The CO₂ still comes from the air, enters through the stomata, and gets fixed. The innovation is in the delivery system, not in the raw material.

CAM Plants and Nighttime Carbon Capture

Crassulacean acid metabolism, or CAM, takes a different approach. Found in cacti, agaves, pineapples, and many succulents, CAM plants open their stomata at night, when the air is cooler and less water is lost to evaporation. They fix CO₂ in the dark using a carbon-capturing enzyme similar to the one in C4 plants, converting it into malic acid that gets stored in cell vacuoles. During the day, stomata close, and the stored malic acid is broken down to release CO₂ internally for the Calvin cycle, which runs using the light energy now available.

The carbon source for this nighttime acid accumulation is partly atmospheric CO₂ and partly the plant’s own stored carbohydrates. Early research assumed starch supplied all the substrate, but studies on species in the Kalanchoë genus showed that starch alone could not account for the amount of malic acid produced overnight. Instead, starch was part of a larger pool of stored sugars that fed the pathway.⁴

Atmospheric CO₂ levels affect how much CAM plants rely on their nighttime strategy. At very low CO₂ concentrations, the kind that prevailed during glacial periods (around 170 to 280 ppm), nighttime CO₂ fixation contributed a larger share of the plant’s total daily carbon gain. When CO₂ is more abundant, daytime C3 photosynthesis takes over a bigger fraction of the workload.⁵ This flexibility hints that CAM may have evolved partly as a response to historically low CO₂, not just to drought.

How Aquatic Organisms Get Their Carbon

Underwater, getting CO₂ to Rubisco poses a different problem. CO₂ diffuses about ten thousand times more slowly in water than in air, so aquatic photosynthesizers face a chronic shortage. Many cyanobacteria, algae, and aquatic plants have responded by evolving carbon concentration mechanisms that actively transport bicarbonate ions from the surrounding water into the cell, then convert them to CO₂ near Rubisco.⁶

Seagrasses, for example, live in water where dissolved CO₂ may be scarce but bicarbonate is plentiful. Their ability to use bicarbonate as a carbon source allows them to photosynthesize in conditions that would starve a land plant dropped into the same water. The enzymes involved, carbonic anhydrases, shuttle carbon between its CO₂ and bicarbonate forms depending on what the cell needs and where. In terrestrial plants, the same CO₂-bicarbonate chemistry plays a signaling role in stomatal guard cells, influencing when pores open and close.

Even with these adaptations, the inorganic carbon ultimately feeding into the Calvin cycle is still CO₂. Bicarbonate is a delivery vehicle, not a replacement. The aquatic carbon concentration mechanisms are solving a transport problem, not eliminating the requirement for CO₂.

Bacteria That Photosynthesize Differently

Not all photosynthesis looks like what happens in a leaf. Long before plants evolved, ancient bacteria developed photosynthetic systems that run on different chemistry. Green sulfur bacteria, for instance, use hydrogen sulfide instead of water as their electron donor. They do not produce oxygen, which is why their form of photosynthesis is called anoxygenic.⁷

What these bacteria share with plants, however, is the need for CO₂ as a carbon source. Green sulfur bacteria fix CO₂ through the reverse tricarboxylic acid cycle rather than the Calvin cycle, but the input is the same: they take inorganic carbon dioxide and build it into organic molecules.⁸ The reverse TCA cycle is actually more energy-efficient for this job, requiring only about two ATP molecules to produce pyruvate from CO₂.⁹

So across the full diversity of photosynthetic life, from cyanobacteria to rainforest trees to sulfur-loving microbes in deep-sea vents, CO₂ fixation is the universal thread. The light-harvesting machinery varies wildly, the electron donors range from water to hydrogen sulfide, but the carbon source is CO₂ in every case.

When Too Much CO₂ Becomes a Problem

If some CO₂ is essential and more CO₂ generally boosts photosynthesis, you might assume that plants near volcanic CO₂ vents would thrive. The reality is more complicated. A recent study of plants growing near extreme CO₂ sources in Yellowstone National Park found that these plants were actually less photosynthetically efficient. They were slower to overcome bottlenecks in their electron transport chains, reached lower peak efficiency values, and needed higher CO₂ levels just to activate the Calvin cycle normally.¹⁰

This suggests that chronic exposure to very high CO₂ can push the photosynthetic machinery into a kind of recalibration where the plant becomes dependent on elevated levels and performs poorly if conditions change. The relationship between CO₂ and photosynthesis is not a straight line that keeps going up. There is a sweet spot, and both too little and too much create problems. For most crops and wild plants today, the limiting factor in photosynthesis is usually CO₂ availability, not excess, but that does not mean more is always better over the long term.

Plants That Gave Up on CO₂ Altogether

If CO₂ fixation is the heart of photosynthesis, what happens when a plant abandons photosynthesis entirely? Parasitic plants offer a striking answer. Species in the broomrape family (Orobanchaceae) range from partial parasites that still photosynthesize to holoparasites that have completely lost the ability. In the most extreme cases, the genes encoding photosynthesis machinery in their chloroplast genomes have been systematically dismantled. Photosystem I genes, photosystem II genes, and the genes for the cytochrome complex are either pseudogenized or deleted outright.¹¹

The gene for Rubisco itself, rbcL, tells an interesting story across these species. In some holoparasites it survives as an intact gene; in others it exists only as a broken pseudogene; and in a few it has been deleted from the genome entirely. This piecemeal loss suggests that abandoning CO₂ fixation is not an all-at-once event but a gradual decay once the selective pressure to maintain it disappears.

Dodder (Cuscuta species) shows a similar pattern. These vine-like parasites wrap around host plants and tap directly into their vascular systems for water and nutrients. Genomic analysis reveals that dodder species have lost multiple photosynthesis-associated genes from both their chloroplast and nuclear genomes, alongside genes for root development, nutrient transport, and disease resistance, reflecting a body plan that has been thoroughly reorganized around parasitism.¹² These plants have effectively opted out of the CO₂ economy, getting all their carbon secondhand from their hosts.

Stomatal Sensing and the CO₂ Feedback Loop

Plants do not passively wait for CO₂ to drift in. Their stomata, the tiny pores on leaf surfaces, actively respond to CO₂ levels. When CO₂ inside the leaf rises, stomata tend to close, conserving water since the plant already has enough carbon coming in. When internal CO₂ drops, stomata open wider to let more in. This feedback loop means that the plant is constantly balancing its need for CO₂ against its need to retain water.

The molecular machinery behind this sensing is increasingly well understood. A protein complex involving a kinase called HT1 and mitogen-activated protein kinases acts as the primary CO₂ and bicarbonate sensor in guard cells, translating changes in CO₂ concentration into signals that open or close the pore.¹³ Rising global CO₂ levels have measurable effects on this system. As atmospheric CO₂ climbs, many plants partially close their stomata, reducing water loss but also potentially limiting the entry of other gases and altering leaf temperature. The ecological consequences of this shift are still being worked out.

Engineering Carbon Fixation Beyond Nature’s Design

Rubisco is famously slow and error-prone, fixing only a few CO₂ molecules per second and frequently grabbing oxygen by mistake. Researchers have long wondered whether synthetic biology could do better. Several groups have now designed artificial carbon fixation pathways that outperform the Calvin cycle on paper and, increasingly, in the lab.

One approach, the CETCH cycle, is an entirely synthetic pathway assembled from enzymes sourced from different organisms. It was designed from scratch to fix CO₂ more efficiently than any natural pathway, avoiding Rubisco entirely.¹⁴ Another recent advance replaced the Calvin cycle in the bacterium Cupriavidus necator with a synthetic reductive glycine pathway. In continuous culture, the engineered strain achieved about 17% higher biomass yield than the wild type and exceeded the yield of any natural organism using the Calvin cycle for the same task.¹⁵

These synthetic pathways still require CO₂ (or a one-carbon compound derived from it, like formate). They are not eliminating the need for carbon dioxide; they are redesigning the machinery that processes it. The bottleneck in natural photosynthesis was never the availability of CO₂ so much as the sluggishness of the enzyme that handles it. If synthetic biology can install a faster, more accurate fixation engine into crop plants or industrial microbes, the same CO₂ in the atmosphere could support substantially more productivity. Whether that leap from bacterial proof-of-concept to a functioning crop plant is five years away or fifty remains one of the open questions in the field.