How Microalgae Take Up Carbon Dioxide

6 min read · Updated 15 October 2026

Microalgae take up carbon dioxide through photosynthesis: they use light energy to combine CO2 and water into sugars, which they then build into proteins, fats and new cells. In water, the carbon is available mostly as dissolved CO2 and bicarbonate, and many algae can use both. As they grow fast, they remove carbon from the water quickly, which is why a dense culture needs a steady supply and why its pH climbs as it feeds.

How the carbon gets in

CO2 from the air dissolves into water and reacts to form carbonic acid, bicarbonate and carbonate. The balance between these depends on pH. In acidic or neutral water, more of the carbon is dissolved CO2 gas. In alkaline water, most of it is bicarbonate or carbonate.

Inside the cell, the enzyme RuBisCO fixes CO2 into organic molecules in the Calvin cycle. RuBisCO is slow and works best when CO2 is concentrated around it, so many microalgae and cyanobacteria run carbon-concentrating mechanisms. These pump bicarbonate into the cell and convert it to CO2 next to the enzyme. This is the reason that Spirulina, which lives in alkaline lakes full of bicarbonate, grows well in a medium rich in sodium bicarbonate.

Why the pH changes in a culture

When algae remove CO2 and bicarbonate, they shift the chemistry of the water. In poorly buffered water, pH can rise by several units in a day. For Chlorella, which prefers roughly pH 6.5 to 7.5, a drift upward can slow growth. For Spirulina, which prefers pH 9 to 11, the same drift is mostly harmless.

At night the reverse happens. Cells respire and release CO2, so pH drops a little. The size of the daily swing is a quick indicator of how active your culture is.

Where the carbon story comes in

Microalgae and cyanobacteria are responsible for a large share of photosynthesis on Earth, often estimated at around half of it, mostly in the oceans. Growing algae in tanks or ponds is therefore a way to turn CO2 into biomass quickly, since their growth rate is high compared to land plants.

There is a caveat. The carbon is only held for as long as the biomass exists. If the algae are eaten and respired or decay, the CO2 returns to the atmosphere. Algae only provide a lasting benefit if the biomass replaces something with a larger footprint, such as fishmeal or feed protein, or is stored long term. It is more accurate to see algae culture as a low-impact way to produce food and feed than as a stand-alone carbon fix.

Practical points for hobby cultures

Aeration helps twice. Bubbling air keeps cells in suspension and feeds the culture CO2 from the air.
Do not add CO2 blindly. A slight rise in dissolved CO2 can boost growth, but too much drops pH sharply and can harm both Chlorella and any fish in a connected system.
Watch the pH if it matters. A cheap test kit shows whether the culture is leaving its comfort range. See nutrients and culture media for how the medium affects buffering.

Our article on land use in microalgae farming covers the other half of the sustainability picture, and small habits that make culturing greener lists practical steps.

The numbers behind the story

Photosynthesis follows a simple overall equation: six molecules of carbon dioxide and six of water, driven by light, give one molecule of glucose and six of oxygen. In mass terms, fixing one kilogram of carbon needs a little over three kilograms of CO2, and a kilogram of dry algal biomass is roughly half carbon. That means producing one kilogram of dry algae takes up about 1.8 kilograms of CO2, although real systems vary with composition and losses.

In a tank, the CO2 has to come from somewhere. Air contains only about 0.04% CO2, so a dense culture that is only supplied by air bubbling is often carbon-limited, and its growth is set by how fast CO2 dissolves. This is why industrial systems feed concentrated CO2 from flue gas or tanks, and why hobby cultures grow faster with good aeration or a little bicarbonate.

Why bicarbonate matters

Bicarbonate acts as a store of carbon in the water. In Spirulina media, about 16 to 17 grams per litre of sodium bicarbonate creates a large reserve that cells can draw on and that buffers the pH. Cyanobacteria use bicarbonate transporters to pull it in, then convert it to CO2 inside the cell. Green algae like Chlorella also use bicarbonate but are tuned to near-neutral water, where there is more free CO2. That difference is one of the reasons the two cultures need different media; see nutrients and culture media.

A simple experiment you can run

You can see the carbon effect in a classroom or at home:

Fill two identical clear bottles with the same Chlorella culture and the same nutrient dose.
Give one bottle gentle aeration with an air stone and leave the other still, with a loose cap.
Place them side by side under the same lamp, at the same temperature.
Photograph both against a white card each day for ten days.

The aerated bottle should darken faster. The difference is mostly CO2 supply and mixing together, and it is a good demonstration that carbon, not only light, can limit growth. Record the results in a log and compare.

What the carbon ends up as

The fixed carbon goes into three main pools: proteins (about half of the dry weight in many species), carbohydrates and lipids. Under nutrient stress, many algae shift towards storing lipids or starch, which is what biofuel research exploits. For a feed culture, you usually want the opposite: a balanced, nutrient-sufficient medium that gives protein-rich cells.

When the cells are eaten by rotifers or Daphnia, a portion of this carbon is respired by the animals and the rest becomes animal tissue, which is what fish then eat. This is the carbon pathway behind live food cultures.

Common questions

Do algae absorb more CO2 than trees?

Per unit of area and time, fast-growing microalgae can fix carbon faster than most land plants, but the carbon is stored in a small, short-lived biomass, so the comparison is only part of the picture.

Should I add CO2 to my algae jar?

Normally no. Air bubbling is enough for hobby cultures, and extra CO2 requires pH control to be safe.

Why does the pH go up in my culture?

Because the algae use up the dissolved carbon, which makes the water less acidic and more alkaline.

How much CO2 does a hobby jar absorb?

Very little in absolute terms: a few grams per month for a one-litre dense culture. The value of such a jar is educational and practical, not climate-scale.

Do algae produce oxygen all the time?

Only in the light. In the dark they consume oxygen, like any living cell, which is why aeration matters in dense cultures.

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