The not-green ocean

That the ocean is blue and not green is evident, yet the reason for this is not as silly as one would think.

In fact planktonic algae are not as abundant as they could could be, if it were not for phosphorous and iron limitation. If life could survive off air and the minerals in sea water alone, the ocean would be as green as a rainforest —or whatever colour the photosynthetic pigment of this hypothetical life were to be. Light gives power, carbon can be fixed from carbon dioxide and amino groups can be obtained from nitrogen gas —with some trouble—, while phosphorus tends to be insoluble so comes from rocks: there are no rocks on the open ocean surface.

There is a counter-intuitive phenomenon called eutrophication where, when nitrogen and/or phosphorus get dumped into a river or lake, an algal bloom occurs, which blocks the light to the bottom of the watermass and the decomposing dead alga result in oxygen deprivation. This just show how some elements are the rate limiting step for a green watermass. Adding nutrients before the tipping point, results in a more lush ecosystem. There are a lot of pipe-dreamy talks of phosphorous and iron fertilisation of the oceans to combat CO₂ levels: forest occupy only 7% of the planets surface, whereas oceans occupy 71%, so there is a lot of carbon fixing that could be going on that isn't. What interests me, however, is how could Nature overcome the issue.

Ammonia causes eutrophication because it is limiting as nitrogen fixation is neither simple or cheap, but it can and is made, albeit slowly. The major issue is the fact that splitting N₂ costs 4 pairs of high energy electrons and O₂ quenches the reaction in nearly all nitrogenases.

Whereas strategies to avoid oxygen sensitivity in nitrogenases can (and maybe has) evolve, phosphate and iron limitation is an insurmountable issue.

Iron is a big problem, for starters no iron means no heam and no iron-sulfur clusters, which catalyse several complex reactions, including that of nitrogenase. The problem is actually greater as except for alkali metals, there are a few parts per billion or less of most metals. The reactions vary, but not many of them could be substituted by hypothetical delocalised pi orbitals, other larger elements and similar. So the metal dependence is unavoidable.
Also, it is a fairly safe guess that life will not evolve to be phosphorous-less any time soon: the genome (DNA) needs it, the transcriptome (RNA) needs it and the cellular energetic currency (ATP) needes it. The best evolution can do is to streamline the genome to the bare minimum. There is a lot of talk about nucleic acid analogous with carbon-based backbones, most which do not form helices but tangled messes —hey, they are still amazing. So engineering such a phosphorous-free organism is out of the realm of what is current feasible in synthetic biology and more in the realm of science fiction. Plus, if the oceans were dominated by a (planet-saving) spooky GM organism, prince Charles would never shut up.
I am not aware of any other oxygenated multivalent ions that would fit the bill or that have even been tested, for example nitrate-backboned DNA would be very explosive, whereas sulfate-backboned DNA would be neutrally charged, but the question if an element other than phosphorous would work is pretty cool and is comparable to the debate of the fictional arsenic bacterium.
If there were some non-phosphorous system that did form helices… the genome would still be different, requiring new polymerases and XNA-binding proteins. The transcriptome would be different as RNA would not fold making riboswitches, tRNA and other quirky RNA structures unusable. The transcription machinery would have to be fully proteinaceous. The energy storing bond would have to be something different (acetyl-CoA or something radically new)... but rather cool.