correct me if im wrong and this can get complicated but the chemical makeup of decomposing organics and waste from other organisms in our saltwater aquariums produce ammonium (NH4) which is broken down by ammonia oxidizing bacteria and nitrifying organisms which converts ammonia to nitrites (NO2) Then other bacterial species are responsible for the oxidation of the nitrites into nitrates (NO3) then with the introduction of a macroalga or some macroalgae into the cycle the nitrates are assimilated (absorbed) into the algae to the form of glutamate (C5 H9 NO4) and is converted into complex amino acids, nucleic acids, chlorophyll and proteins. i have never studied this in a lab nor will i but from the biochemistry books and online resources I have studied the ammonia (NH3) ammonium (NH4) nitrites (NO4) and nitrates (NO3) do not stay in there chemical compound once assimilated into the algae, they are converted into other complex chemical compounds due to reactions with enzymes and protiens that provide a biocatalysis where the nitrogen source (ie.ammonia, ammonium, nitrites, nitrates) that has been assimilated into the algae is converted into usefull organic compounds like chlorophyll (C55 H70 O6 N4 Mg) and Amines (H2NCH (x) COOH).
the cycle will restart and you will get a spike of all the toxic nitrogeneous compounds like nitrates if the algae is allowed to die and decompose.
when the organism breaks down the the nitrogenious compounds can and will easily be converted back into the toxic compounds that we are constantly trying to remove. so only by harvesting the macroalgae would you be removing the nitrogenious compounds