Protein and amino acid cycling during phytoplankton decomposition in oxic and anoxic waters

The fates of proteins and amino acids were followed during the oxic and anoxic decay of three marine phytoplankton (diatom Thalassiosira weissflogii, cyanobacterium Synechococcus sp., and dinoflagellate Prorocentrum minimum) during simulated sedimentation. In the stationary phase of phytoplankton growth, 36–100% of total hydrolyzable amino acids were comprised of amino acids associated with a ≥2000 Da molecular weight fraction. After cell death, proteins degraded at similar rates, independent of which phytoplankton species they originated from, but dependent on oxygen. Protein degradation was rapid under oxic conditions, with 2–8% of the initial ≥2000 Da protein amino acid fraction remaining at the end of the incubations. Under anoxic conditions, 17–46% of the ≥2000 Da protein amino acid fraction remained. Throughout oxic and anoxic incubations, amino acids normalized to particulate nitrogen paralleled carbon normalized values. 51–82% of the particulate nitrogen could be attributed to proteins, polypeptides and/or bound amino acid monomers, suggesting that these nitrogen-rich compounds can be refractory. A comparison of the changes in concentrations of constituent amino acids of the total amino acid and ≥2000 Da protein amino acid fractions suggests that all amino acids are lost at comparable rates during the degradation sequence. For the dinoflagellate decay, two non-protein amino acids, β-alanine and γ-aminobutyric acid, exhibited rate constants similar to those observed for total hydrolyzable amino acids; relative abundances of β-alanine and γ-aminobutyric acid in degraded material did not significantly differ from that in fresh material. At the conclusion of oxic and anoxic diatom decay and oxic cyanobacterial decay, the slight increase in these two non-protein amino acids suggests sorption to the remaining organic matrix.