That is already happening iirc and being sold as another bullshit tech solution to our energy problems. Its like carbon offsets or carbon capture. Like bro just use less fossil fuels and stop destroying the existing natural carbon capture systems ffs.
Also, there's a bunch of ways to make Algae blooms in the ocean. Apparently even just dumping a bunch of iron dust in the ocean would cause lots of algae blooms - but we don't do it.
Fun fact, depending on your definition of "fun". Deniers sometimes argue that plants will just grow to absorb the extra co2. This doesn't work in general, because most plants aren't limited by co2 availability. There are some exceptions, and the algae that causes red tide is one of them. So we have that to look forward to.
Because it has a lot of side effects and the oceans are under a lot of stress because of climate change already. So for the moment we don't fuck with it.
So when we say Climate Change is an immediate and irreversible threat... There's another clause there that's something like: but not to us right now as a species, more to the biodiversity that would be put at risk if we tried producing more Algae? So we're not going to address the "immediate and irreversible threat" in that way (use of algae), because it might upset other things in the ocean.
Would those other things be stuff like... Er... Important stuff. I'm just not sure about this stuff because I don't know that much about Climate Change in relation to Algae and Oxygen production.... And what/when the threat would warrant it.
To work as a carbon capture mechanic, iron fertilization-driven algae blooms would have to die and sink to the bottom of the ocean, thus locking up their carbon in oceanic rock.
The concern is they would die and float, releasing all that carbon back into the atmosphere via decomposition gases. Then we would have all the effort of the fertilization, all the ecosystem disruption of the algae bloom, and maybe negative benefit as far as carbon since the ecosystem disruption could mess up carbon sinks that were actually working.