Fixed nitrogen is shorthand for the sum of ammonium and nitrate, the two most important forms of nitrogen which all phytoplankton can absorb. The sketch above shows how the use of dissolved inert N2 by N2-fixers ultimately leads to an increase in the amount of fixed nitrogen. N2-fixers such as Trichodesmium take up N2 and break apart the two N atoms in the molecule before using those N atoms to make various essential proteins.
But that intra-cellular N is eventually lost back to the external environment. This can occur either through leakage from the phytoplankton cell of dissolved organic N (DON) compounds, or when cells are eaten by zooplankton (small animals which eat phytoplankton). The faecal pellets of zooplankton are decomposed by bacteria (as is DON), leading to release of ammonium (NH4). Although not shown in the diagram, specialised guilds of bacteria can also break the NH4 down further to nitrate (NO3).
