[phtrivier]

Stupid question : do the bacteria manage the create their organic mass entirely from air + sunlight, or does the "container" need a special substrate / soil / etc... ? How fast would that "deplete" relative to the metals in the anode / cathode ?

Also, how much does it "capture" carbon as part of the photosynthesis ?

[elsherbini]

Here are a bunch of different media people use to grow cyanobacteria:

https://www-cyanosite.bio.purdue.edu/media/table/media.html

Mostly they need nitrogen, phosphate, and sulfur to make protein and nucleic acids, and trace metals,ions, and some vitamins to use as cofactors for enzymes. In the wild, the limiting nutrient for cyanobacteria is often iron or nitrogen.

They don't need an added carbon source in their media since they get to eat dissolved CO2 from the air. Every carbon atom in newly synthesized molecules comes from CO2, and there are (very roughly) 10^10 carbon atoms per bacterial cell. http://book.bionumbers.org/what-is-the-elemental-composition... . So if you know the growth rate you can estimate the carbon fixation rate.

[ascar]

> In the wild, the limiting nutrient for cyanobacteria is often iron or nitrogen.

How can nitrogen be a limiting factor if ~78% of air is nitrogen? I'm assuming they also take the CO2 out of the air, which just makes up 0.04%.

[elsherbini]

Some cyanobacteria can take up nitrogen from the air (well, dissolved inorganic nitrogen), but many can only use organic nitrogen, like N03 or NH4. Those can be limiting in the ocean because lots of life wants to grab it up.

Taking up inorganic nitrogen is called nitrogen fixation and takes a bunch of machinery and energy. Also, oxygen inhibits the process, so for photosynthetic bacteria a bunch of extra steps have to be taken to compartmentalize photosynthesis and nitrogen fixation. So not all cyanobacteria can do it, and even the ones who are able to will prefer to take up organic nitrogen if available, and they grow slower when they have to use N2.

[hn_throwaway_99]

Nitrogen is often the limiting factor for ALL plant growth, which is why it's the primary component of fertilizer.

Nitrogen in air is not normally available to organisms. The triple bond between the 2 nitrogen atoms is very strong and takes a good deal of energy to break. Some bacteria can do this, and legumes famously have evolved to house bacteria capable of fixing nitrogen in their roots.

The main way we get nitrogen to agricultural plants today is by the industrial production of ammonia, which takes enormous amounts of energy and lots of fossil fuels as presently implemented.

[couchand]

Not the GP and I do not have relevant scientific expertise, but my memory from beer is that CO2 stays dissolved longer than N2? Are they underwater?

[hbates]

The liquid in the container would contain medium that contains chemicals that the microalgae need. Heres a common medium recipe BG-11 https://utex.org/products/bg-11-medium

Carbon is captured through photosynthesis and is stored as biomass. But when the microalgae dies the carbon is released again. So to truely store carbon the microalgae needs to be cultivated and buried / contained.