Phosphate is an element. And outside of nuclear fission or fusion you can not create more phosphate. You can only extract phosphate which is already there. Nitrate on the other hand is molecules which are created from oxygen and nitrogen. The primary natural sources of nitrates are bacteria in the soil which extract oxygen and nitrogen from the air and use a lot of energy to create nitrate. However there are nitrogen in the ocean as well as oxygen. It is also possible to get oxygen atoms from water. So while it is more efficient to create nitrates in soil the microalgae will still make nitrate and not just extract it.
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