How many gigatons of CO2 will my "cyanobacteria bioreactor" pull out of the atmosphere per year?

Question

So, as CO₂ levels continue to rocket in the atmosphere, its effects prove to be devastating for the world. Ice caps melt, sea levels rise, hurricane intensity increases, and lots of unpleasant things happen. Scientists are desperate to find a solution to sequester this CO₂. So, scientists come up with a solution, which lies in something unexpectedly small, but which is extremely important for life on Earth- Cyanobacteria.

Cyanobacteria and algae are extremely similar at first glance, but there are lot of subtle differences between the latter, such as nucleoids instead of nucleus.

Cyanobacteria are nothing more than the blue-green scum that tends to envelop your lakes for weeks at a time. Since these cyanobacteria produce a lot of toxins, for small areas like lakes, cyanobacteria are considered extremely toxic, and are destroyed by chemicals.

However, we tend to underestimate the blue-green cyanobacteria. They are efficient producers of oxygen. In fact, the primordial oxygen that we take in with each breathe, was made by cyanobacteria 2.7 billion years ago. Cyanobacteria also are great nitrogen fixers. They fix atmospheric nitrogen into usable nitrates, essentially making free fertilizer.

So, the plan is given below.

Large quantities of cyanobacteria are dispersed in the North Atlantic Ocean, along with some iron for nutrient supplement. As the cyanobacteria disperse inside the ocean, they start to pull out CO₂ out of the atmosphere, producing large amounts of oxygen in the process. This CO₂ is sequestered in the form of even more cyanobacteria, initiating a feedback loop of CO₂ absorption. The goal here is to get the CO₂ around to below pre-industrial levels. During this time, as the CO₂ gets removed from the atmosphere, the Arctic and Antarctic ice caps grow in size, reflecting even more sunlight into space, acting as a catalyst in the "global cooling".

However, cyanobacteria, after dying would decay in the ocean and absorb the O₂, and release CO₂, reversing the process. This however can be easily remedied by a simple solution:

Large quantities of arid sand and rocks are dumped onto the dead-cyanobacteria water, causing them to sink into the ocean. So, instead of rotting at the surface of the sea and releasing CO₂, the dead CB will now be anaerobically decomposed at the bottom of the seabed, along with dead sea organisms and whatnot, to produce petroleum and similar compounds, while the living cyanobacteria is photosynthesizing to remove CO₂ from the atmosphere.

Yes, the gasoline in your car came from rotten fish and dead seaweed

During this time, the usable nitrates produced as a byproduct of the "bioreactor" are collected by the crew, to be used as fertilizers

Factors

Salinity

Since this cyanobacterial bioreactor takes place in the North Atlantic Ocean, I am going to assume the average salinity at about 36%, or 36 grams per litre.

Temperature

This takes somewhere in the North Atlantic Ocean, but near the Equator, so that it won't be frostly. I will assume the mean temperature at 25$^{\circ}$C

Insolation

As it is situated near the equator, I will assume that it would be close to 300 W/m².

Area

The area of the ocean seeded with cyanobacteria will be about the size of Texas, about 700,000 km₂

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Goals

Reducing CO₂ concentrations

So, the primary goals of the question is to reduce the CO₂ levels considerably, so as to erase global warming.

However, here's a catch. CO₂ is also essential for plants and cyanobacteria to photosynthesize. If the CO₂ levels decreased by too much, then plants will die to lack of CO₂ for photosynthesis.

The threshold at which plants die from CO₂ starvation is about 150 PPM. So the goal is to bring the CO₂ down to low, but not too low levels. Practically speaking, this means that we have to bring the concentration down to an optimum ~250-200 PPM, which is low enough to stop global warming and well below even pre-industrial levels (~280 PPM), but high enough for plants to not starve CO₂, and be able to photosynthesize.

Nitrates for Fertilizer (An Unintended byproduct

Here, the difference between cyanobacteria and algae is pretty much prominent.

Algae, while a great tool for CO₂ sequestration, are extremely terrible, as they tend to suck out nitrates from the water, and grow uncontrollably due to the nitrate-to-protein conversion, creating toxic blooms of algae.

Cyanobacteria, on the other hand, atleast have a commercial value as they fix nitrogen from the atmosphere to turn it to nitrates, functioning as Rhizobium analogues. These nitrates can be separated out from the water easily by filtering/boiling it out, after which, they can be used as fertilizer.

Algae destroy nitrates, whereas cyanobacteria create nitrates.

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The final question however boils down to:

How many gigatons of CO2 will my "Cyanobacteria-Bioreactor" pull out of the atmosphere every year?

Answer 1

Cyanobacteria, just like any other phytoplankton, eat the carbon dioxide dissolved in the water in which they swim and not the carbon dioxide in the air.

Dissolved carbon dioxide in sea water is about 2.5 millimol/kilogram, or about 100 milligrams/kilogram, or about 100 grams/cubic meter. Of these 100 grams, about 28 grams are the carbon, and about 62 grams are the oxygen.

Let's say that the North Atlantic is 50 million square kilometers; then the top 25 meters, where there is enough light for the cyanobacteria to grow, have a volume of about 1.25E15 cubic meters. If the cyanobacteria eat all the carbon dioxide in this volume, you will have about 130 gigatonnes of organic carbon in the cyanobacteria, about 3 or 4 years worth of global emissions, releasing about 340 gigatonnes of oxygen.

Assuming that the geoengineers are growing a particularly sugary strain of cyanobacteria, the total mass of the little criters will be about 1,600 billion tonnes. For comparison, the total yearly production of wheat in the world is about 800 million tonnes, about 2,000 times times smaller than the imaginary mass of the imaginary cyanobacteria.

As a side effect, in order to grow sugary cyanobacteria in the ocean, the geoengineers will have used about a thousand years supply of phosphate and nitrate fertilizer in the world, prompting mass famine, riots with fire and pitchforks, and thousands of megadeaths.

Since riots with pitchforks are not nice, and some retrograde bourgeois philistines will object to thousands of megadeaths, the geoengineers will be constrained by management to reduce their annual crop of cyanobacteria to about the same size as the annual crop of cereals, or about 3 gigatonnes. The chemical industry will be most happy to accomodate a the increase in the demand of phospate and nitrate fertilizer.

Of those 3 gigatonnes of bacteria, about 0.2 gigatonnes will be carbon, corresponding to about 0.75 gigatonnes of carbon dioxide.

Overall:

• If the geoengineers arrange for the cyanobacteria to eat all the carbon dioxide in the top 25 meters of the North Atlantic, that's about 130 gigatonnes, about 3 or 4 years of global emissions. Unfortunately, using a thousand years supply of phosphate and nitrate fertilizer might not be allowed by the powers that be.

• If the geoengineers limit the cyanobacteria to about the same mass as the global yearly production of cereals, that's about 0.75 gigatonnes of carbon dioxide, about 1/40 of the global yearly emissions. This is clearly insufficient.

• Somewhere in between there is a good compromise.

Answer 2

Tonnes of C02 removal per year? Not many.

Wikipedia article about what you are asking says:

But research in the early 2020s suggested that it could only permanently sequester a small amount of carbon.

There have been experiments that have fertilized the ocean with various rate limiting nutrients. Mostly to see if it is a way of increasing fish catch.

One of the significant findings is that the boost to growth does not last particularly long, on the order of weeks certainly not years. That is to see continued algae growth noticeably beyond baseline, repeated applications of nutrients is required.

This is because over time a non zero percentage of creatures at whichever stage of the food change fall to the ocean floor, taking the rate-limiting nutrients, with them. So within a year the levels of activity are what they were before the application of whichever nutrients. While the falling to the ocean floor does sequester carbon it is a limited amount.