Why some plants need CO₂ and others don’t

This guide is the deep version of a point made in the pillar on growing aquarium plants: carbon is the resource submerged plants most often run short of. Here we look at why that is, and at the striking range of ways different plants have evolved to get around it — because that range is exactly what separates the plants that limp along without CO₂ from the ones that thrive.

Why underwater plants starve for carbon

A land plant lives in a well-stirred ocean of air where CO₂ is plentiful and moves fast. A submerged plant lives in almost the opposite conditions, and two facts of physics make carbon its chronic bottleneck.

First, CO₂ diffuses roughly ten thousand times more slowly in water than in air (Pedersen, Colmer & Sand-Jensen, 2013, citing Armstrong, 1979). Second, every submerged leaf is wrapped in a thin, near-motionless film of water called the diffusive boundary layer, and carbon has to inch across it by diffusion alone. Measured across four submerged species, that boundary layer accounted for up to about 90% of the total resistance to carbon fixation (Black, Maberly & Spence, 1981). Between the slow diffusion and the sluggish boundary layer, carbon simply cannot arrive fast enough to keep a bright, warm leaf fed.

How much does this actually hold plants back? A lot. Averaged across many submerged species, underwater photosynthesis roughly trebled when dissolved inorganic carbon was raised from ordinary levels to saturating (Nielsen & Sand-Jensen, 1989, reported in Pedersen et al., 2013), and in some conditions the boost was severalfold larger again. Obligate CO₂-users run out of usable carbon at only a few micromoles per litre — around 6 µmol L^-1 at their typical operating pH (Sand-Jensen et al., 1992, via Pedersen et al., 2013). Carbon, not light, is frequently the first thing to run out. (The delivery side of this — how flow thins that boundary layer — is covered in CO₂ at the leaf.)

Two forms of carbon: one scarce, one abundant

Here is the key that unlocks everything. Water holds inorganic carbon in more than one chemical form, and which forms dominate depends on pH. Below about pH 6 it is nearly all dissolved CO₂; from roughly pH 7 to 9 the dominant form is bicarbonate (HCO₃^-); only in very alkaline water does carbonate take over. In the hard, near-neutral-to-alkaline water most tap-water tanks run, there is very little free CO₂ but a large reservoir of bicarbonate. (The carbonate chemistry itself is laid out in KH, GH and the CO₂–pH triangle.)

Plants would love to use that big bicarbonate reservoir — but bicarbonate is a charged ion and cannot simply diffuse into a leaf the way CO₂ does. Using it takes special machinery. And this is the divide: some plants have that machinery and some do not.

The plants that can tap bicarbonate use a carbon-concentrating mechanism: membrane pumps, an enzyme called carbonic anhydrase bound to the leaf surface, and in some species a striking “polar leaf” trick — taking up bicarbonate and pumping out acid on the lower surface while venting hydroxide from the upper surface, so the leaf effectively manufactures its own CO₂ at the site of fixation (Maberly & Madsen, 2002). Documented bicarbonate users read like a list of familiar “easy” aquarium plants: Vallisneria, Elodea, Egeria, Hydrilla, many Potamogeton, and the charophytes. There is also a trade-off worth knowing: plants with strong bicarbonate ability tend to have lower affinity for free CO₂, and vice versa, so obligate CO₂-users are typically the ones adapted to soft, CO₂-rich water (Maberly, 1998).

On the other side sit the plants that cannot use bicarbonate at all. Aquatic mosses are the best-documented example — Fontinalis depends on free CO₂ and cannot make up the difference from bicarbonate (Maberly, 1985) — and the same generally holds for aquatic ferns and liverworts such as Riccia by virtue of their lineage, though the evidence base there is thinner than for mosses. These are the plants for which injected CO₂ is not a luxury but the thing standing between them and carbon starvation.

The overachievers: built-in carbon pumps

A small elite of submerged plants goes further still, running biochemical carbon pumps that land plants took millions of years to evolve.

C4-like photosynthesis. On land, C4 plants (maize, sugarcane) concentrate carbon using two specialised cell types. A handful of aquatic plants pull off the same trick inside a single cell, with no special anatomy, switching it on under carbon stress. Hydrilla verticillata is the textbook case: under low CO₂, high temperature and long days it induces the C4-like pathway, fixing carbon first into four-carbon acids and slashing the wasteful losses of photorespiration (Salvucci & Bowes, 1983). Egeria densa does something similar (Casati, Lara & Andreo, 2000).

CAM — the night shift. Some plants, most famously the quillworts (Isoetes), borrow the strategy of desert succulents: they fix carbon at night, storing it as acid, when respiration from the sediment and the water column has pushed CO₂ levels up and competition for it has died down. Aquatic CAM is best established in the quillworts (Isoetes), with reports — of varying strength, and some still debated — in genera such as Vallisneria, Sagittaria and Littorella (Keeley, 1998; Klavsen, Madsen & Maberly, 2011). It is a beautiful piece of evolutionary problem-solving aimed squarely at the daytime carbon shortage this article is about.

The white crust: bicarbonate use you can see

Bicarbonate use leaves a visible signature. When a plant strips bicarbonate from hard water, it pushes out hydroxide and drives the pH at the leaf surface right up. Above about pH 8.3, dissolved carbon shifts toward carbonate, and in calcium-rich water that carbonate precipitates as a chalky white crust of calcium carbonate on the leaves — a process limnologists call biogenic decalcification. It has been characterised directly on the leaves of Potamogeton crispus (Zhang et al., 2016), and aquarists see it constantly on Vallisneria, Elodea and Egeria in hard water.

Two things follow. It confirms which of your plants are bicarbonate users. And it is essentially never seen on obligate CO₂-users, nor in a soft, CO₂-injected tank where the leaf surface never gets alkaline enough — which is itself a clue to how differently those two kinds of tank are running.

What this means for your plants

Now the physiology pays off. The following are reasonable inferences from the science above rather than results from aquarium trials, but they line up closely with what aquarists observe.

• CO₂ injection is transformative for CO₂-restricted plants — mosses, liverworts like Riccia, and the demanding, fast-growing carpets and red stems — because those plants cannot reach the bicarbonate pool. Raising dissolved CO₂ lifts exactly the bottleneck the physiology identifies, which is why the effect is so dramatic rather than marginal.
• Many “easy” plants cope without injection because they are competent bicarbonate users, and in hard water that pool is huge. They are not carbon-starved even at low CO₂. Slow, low-demand growers (like Anubias and Java fern) also cope — not necessarily by using bicarbonate, but simply by demanding very little carbon in the first place.
• The soft-vs-hard match is real. Obligate CO₂-users tend to do best in soft, slightly acidic, CO₂-rich water; strong bicarbonate users tolerate hard, alkaline water. That mirrors the affinity trade-off measured in the literature (Maberly, 1998), and it is why some plants that sulk in one person’s tap water flourish in another’s.

The practical takeaway is liberating rather than prescriptive. You do not need to inject CO₂ to keep bicarbonate-using or low-demand plants in hard water; you very likely do need it — or need to choose different plants — if you want the fast, lush, CO₂-hungry species to thrive. Neither is “better.” They are just different points on the spectrum of how plants solve the same underwater carbon problem. For more on keeping that injected carbon steady once you commit to it, see what “stable CO₂” actually means.

References

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2. Black, M.A., Maberly, S.C. & Spence, D.H.N. (1981). Resistances to carbon dioxide fixation in four submerged freshwater macrophytes. New Phytologist 89(4):557–568.
3. Nielsen, S.L. & Sand-Jensen, K. (1989). Regulation of photosynthetic rates of submerged rooted macrophytes. Oecologia 81:364–368. (Reported in Pedersen et al., 2013.)
4. Sand-Jensen, K. & Gordon, D.M. (1984). Differential ability of marine and freshwater macrophytes to utilize HCO₃^- and CO₂. Marine Biology 80:247–253.
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8. Salvucci, M.E. & Bowes, G. (1983). Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms. Plant Physiology 73(2):488–496.
9. Casati, P., Lara, M.V. & Andreo, C.S. (2000). Induction of a C4-like mechanism of CO₂ fixation in Egeria densa, a submersed aquatic species. Plant Physiology 123(4):1611–1621.
10. Keeley, J.E. (1998). CAM photosynthesis in submerged aquatic plants. The Botanical Review 64(2):121–175.
11. Klavsen, S.K., Madsen, T.V. & Maberly, S.C. (2011). Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a review. Photosynthesis Research 109(1–3):269–279. DOI:10.1007/s11120-011-9630-8.
12. Zhang, M. et al. (2016). Occurrence and characterization of CaCO₃ co-precipitation with phosphorus on the leaf surface of Potamogeton crispus. Environmental Science and Pollution Research. DOI:10.1007/s11356-016-7844-1.