Cell‐surface oxidation of amino acids in seawater

Recently, it was shown that certain phytoplankton species in laboratory culture can use cell‐surface enzymes to oxidize amino acids and amines and take up ammonium produced during the oxidation. Previously, we developed a fluorescent analog of the amino acid lysine (LYA‐lysine) to act as a tracer of cell‐surface oxidation processes. In the present study, a kinetic approach demonstrated that these fluorescent probes effectively compete with natural amino acids and are thus similar to the natural compounds. Field studies showed that naturally occurring phytoplankton and bacteria‐size organisms oxidized the fluorescent analog, extending the discovery of this oxidation process to natural waters. Parallel experiments measuring uptake of 14C‐labeled amino acids and oxidation of LYA‐lysine showed that oxidation rates could be as much as 40% of the total removal rate. This finding suggests that removal of amino acids from seawater might be significantly underestimated if uptake across the cell membrane were the only process considered. However, cell‐surface oxidative deamination was detected only in summer, when temperatures exceeded 22°C.