How Do Submerged Aquatic Plants Get Carbon for Photosynthesis?
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Submerged aquatic plants get carbon from the water around them, and the research shows that bicarbonate is one of the forms they can use. If you are asking how do submerged aquatic plants get carbon for photosynthesis, the answer starts with water chemistry rather than with air. This article explains the mechanism step by step, draws only on two peer-reviewed sources, and flags where the evidence stops short of a full picture.
Quick answer
- Carbon in water exists in several dissolved forms, and their balance depends on pH, temperature and ionic strength.
- Bicarbonate cannot cross membranes freely, so plants convert it near the cell surface using carbonic anhydrase.
- Blocking carbonic anhydrase reduced underwater photosynthesis in the species studied, which points to the enzyme’s role.
- Some plants can change their leaves and carbon use when conditions change.
The carbon supply problem underwater
The first source frames the issue directly: “Angiosperms invading water from air will have faced problems of carbon supply.” That statement is the starting point for the mechanism. Most plants that evolved on land do not grow submerged, and the same study notes that “higher land plants do not grow under submerged conditions.” The move into water is therefore a physiological challenge, and the study treats it as something some species manage and most land plants do not.
Several forms of carbon coexist in water
Water does not hold carbon in only one form. The first source notes that “The relative proportions of dissolved inorganic carbon (DIC) constituents” vary, and it names the factors: they “depend on ionic strength, temperature and pH.” In practical terms, the mix a submerged plant encounters changes with the water it lives in. The second source adds a chemical sequence: atmospheric carbon dioxide “reacts with water molecules to form carbonic acid”, which then dissociates to bicarbonate and carbonate.
The second source explains why bicarbonate matters in seawater. It refers to “the ~120 times higher concentration of bicarbonate”, which makes that form of inorganic carbon more available to marine photosynthesisers. The same review also explains why dissolved carbon dioxide alone is a weak option. It says that the low affinity of Rubisco, the enzyme that fixes carbon, means that most marine photoautotrophs “would be severely restricted were they to rely only on dissolved” carbon dioxide.
Bicarbonate needs help to get into the cell
Carbon dioxide and bicarbonate behave differently at the membrane. The review says carbon dioxide “can enter cells easily by diffusion.” Bicarbonate is different: “the charged bicarbonate ion cannot cross biological membranes without the use of transport proteins.” That contrast explains why bicarbonate use is a distinct strategy rather than a default. A plant that wants to use bicarbonate needs either transport proteins or a way to convert it into a form it can take up.
Carbonic anhydrase turns bicarbonate into usable carbon
The conversion step is where carbonic anhydrase comes in. The review describes the most common marine route, in which bicarbonate is converted within acidic micro-zones of diffusion boundary layers, including the cell walls, “as catalysed by an outwardly acting carbonic anhydrase (CA).” The review introduces this route as “The most common way in marine macrophytes to utilise” bicarbonate, which places it at the centre of the marine picture.
The first source tests the enzyme directly in its species. It reports that “Ethoxyzolamide, an inhibitor of intracellular carbonic anhydrase, significantly inhibited underwater photosynthesis in submerged leaves.” That result links the enzyme to carbon uptake underwater. The two sources describe different locations for the enzyme: the review discusses the outward-acting form near the cell surface, while the first source’s inhibitor targets the intracellular form. Both point to the same chemistry, but they do not describe the same step in the same way.
The first source also reports that “Submerged leaves and submerged terrestrial leaves were able to use bicarbonate.”
How one amphibious plant changes its leaves
The first source’s species, Hygrophila difformis, shows how acclimation works in practice. The study says the plant “develops serrated and dissected leaves when grown in terrestrial and submerged conditions, respectively.” Submerged leaves were thinner: “Compared with the leaves of terrestrially grown plants, leaf thickness of submerged plants was significantly thinner.” The stomatal density also changed: “The stomatal density on the abaxial surface of submerged leaves was also reduced.”
The study used ethylene to test whether these traits are switchable. It reports that “When the leaves of terrestrially grown plants were treated with ethylene, their leaf morphology and underwater photosynthesis increased to levels comparable to those of submerged leaves.” The result suggests the traits can be turned on in land-grown leaves, which fits the broader statement that “amphibious plants can acclimate to both terrestrial and submerged conditions.”
Other strategies in aquatic plants
The review says that “most marine oxygenic photoautotrophs need a CCM to carry out significant rates of net photosynthesis.” CCM is short for carbon concentrating mechanism, a term the first source also uses in its keywords. The review adds a point about efficiency: whole cells can outperform the enzyme on its own, since most cyanobacteria and microalgae “show much higher affinity for Ci in whole-cell photosynthesis than is exhibited by the isolated Rubisco.”
The first source also names a different route in some species. An alternative carbon method, crassulacean acid metabolism, “can be induced in underwater conditions by some isoetid species.” This shows that submerged plants are not limited to one approach.
For watering habits that affect plants in containers and beds, see how to deep water plants, and for planning around time away, see how to water plants when you are away. Those guides cover routine watering rather than carbon chemistry.
How we chose these sources
We chose two NCBI PMC articles, both opened on 2026-10-28. The first is an experimental study of one amphibious species grown under terrestrial and submerged conditions. The second is a review of marine photoautotrophs, which covers “marine angiosperms or seagrasses” along with microalgae and macroalgae. Both focus on how carbon is acquired, which is the core of the question, and neither is a retail or product page.
What we could not verify
The first study covers one species, so it does not show how every submerged plant behaves. Its experiments used “terrestrial, submerged, terrestrial treated with ethylene and submerged treated with an ethylene inhibitor” as growth conditions, which is a controlled comparison rather than a survey of wild plants. The review focuses on marine organisms, so it is a guide to principles rather than a field guide for freshwater plants.
What the sources do not say
Neither source gives a figure for how much carbon a submerged plant takes from bicarbonate in a given pond, lake or aquarium. The review’s concentration comparison refers to marine conditions, not freshwater ones. The first source gives a general percentage for submerged angiosperms, but this article has not checked how that figure was derived, so it is quoted only as the source’s own statement. This article also does not offer aquarium or pond advice.
Frequently asked questions
Can submerged plants use carbon dioxide directly?
The review says carbon dioxide “can enter cells easily by diffusion,” but it is only one of the dissolved forms of carbon in water. The balance among those forms depends on “ionic strength, temperature and pH” according to the first source.
Do all submerged plants use bicarbonate?
The sources do not show that every species does. The first source says that “About 50 % of submerged angiosperms can use” bicarbonate, so the ability is shared by about half of them according to that statement. It also reports that submerged leaves “were able to use bicarbonate” in the species it studied, and the review says that “most marine oxygenic photoautotrophs need a CCM” to photosynthesise at significant rates.
What does carbonic anhydrase do in this process?
The review describes it as “an outwardly acting carbonic anhydrase (CA)” that helps convert bicarbonate for uptake. In the first source, blocking the intracellular form of the enzyme “significantly inhibited underwater photosynthesis in submerged leaves.”
Can a plant change how it takes up carbon?
The first source says that “amphibious plants can acclimate to both terrestrial and submerged conditions.” It also reports that ethylene treatment shifted land-grown leaves toward submerged traits.
Why is carbon supply harder underwater?
The first source says that “Angiosperms invading water from air will have faced problems of carbon supply.” The review adds that the low affinity of Rubisco makes dissolved carbon dioxide a weak source on its own.
Sources
- Photosynthetic acclimation of terrestrial and submerged leaves in the amphibious plant Hygrophila difformis, NCBI PMC, https://pmc.ncbi.nlm.nih.gov/articles/PMC6426153/ (HTTP 200, opened 2026-10-28)
- Inorganic Carbon Acquisition and Photosynthetic Metabolism in Marine Photoautotrophs: A Summary, NCBI PMC, https://pmc.ncbi.nlm.nih.gov/articles/PMC11944913/ (HTTP 200, opened 2026-10-28)
