Let's continue with our look at dissolved iron in the oceans, shall we?
You might be asking, “Wait a minute, lady. If phytoplankton are so great and iron in the ocean is so scarce, why don’t those phytoplankton evolve already and use something else that is more abundant? Like, Magnesium, or whatever.” Good point. Iron availability in the oceans is down around the part per billion concentrations, so why do organisms still use it? The answer is that iron is such a great electron acceptor, phytoplankton make due with the little that is around. The whole subject of electron donors and acceptors and how biology makes use them gets a little complicated. It deals with things like enzymes and biochemical pathways and other biological topics I don’t really understand. Biologists understand these things, so touché Biologists. You’ve bested me at understanding chemosynthesis, but I’ll smoke you when it comes to Eulerian and Lagrangian water transport.
Iron comes in two flavors: Ferrous and Ferric. Ferrous iron (Fe+2) is soluble, meaning that it will hang out in the ocean until some little critter or phytoplankton snatches it up. Ferric iron (Fe+3) is insoluble, meaning that it will form a molecule with something else (usually oxygen) and “precipitate” out of solution. Ferric iron is pretty much useless to phytoplankton. They are beggars AND choosers in this game.
Up until about 2 billion years ago, the Earth’s oceans were anoxic (lacking oxygen). Ferrous iron was super abundant in the Earth’s early ocean because there was no oxygen around that would oxidize it. All of the little algae and cyanobacteria and whatever else that was evolving prior to 2 billion years ago loved having all this Ferrous iron around, they were in hog heaven! That is, until photosynthesis showed up. Photosynthesis ruined the Ferrous iron party by pumping the atmosphere full of free oxygen. Atmospheric oxygen ended up in the oceans by way of air-sea gas exchange and all that lovely, useful Ferrous iron was oxidized to Ferric iron. It precipitated out of the Earth’s oceans and created something that geologists know as “Banded Iron Formations”These formations can be found in places like Australia. The bands are layers of iron oxides (rust!) that sank all the way down to the seafloor as photosynthesizers oxidized the Earth’s atmosphere all those billions of years ago. So on the one hand we can thank those prehistoric photosynthesizers for filling our atmosphere with oxygen, but on the other hand they kinda shot themselves in the proverbial foot by creating a world where Ferrous iron is in short supply.
But, like I said, iron is so great at what is does that biology makes due with what little is around. Biologists even have a term for the overindulgence of iron by phytoplankton – it’s called “luxury” uptake. Luxury, not in the sense of a Diatom relaxing on the tiniest chez lounge you can imagine, but in the respect that it will take up more iron than it needs and store it for later use. Whenever I hear the term "luxury uptake" I can only think about obese single celled organisms wearing monocles and driving Rolls Royces. Luxury, ha ha.