[sacnoradhq]

It doesn't take an unreasonable amount of iron and the sequestering efficiency is 13k:1 C:Fe on a molar basis. https://en.wikipedia.org/wiki/Iron_fertilization

[euroderf]

Do you mean rather: 1:13k Fe:C

From that Wikipedia item,

"Each iron atom converted at least 13,000 carbon atoms into algae."

[sacnoradhq]

Yes, of course.