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 an d anoxic decay of three marine phytoplankton (diatom Thalassiosira wei ssflogii, cyanobacterium Synechococcus sp., and dinoflagellate Proroce ntrum minimum) during simulated sedimentation. In the stationary phase of phytoplankton growth, 36-100% of total hydrolyzable amino acids we re comprised of amino acids associated with greater than or equal to 2 000 Da molecular weight fraction. After cell death, proteins degraded at similar rates, independent of which phytoplankton species they orig inated from, but dependent on oxygen. Protein degradation was rapid un der oxic conditions, with 2-8% of the initial greater than or equal to 2000 Da protein amino acid fraction remaining at the end of the incub ations. Under anoxic conditions, 17-46% of the greater than or equal t o 2000 Da protein amino acid fraction remained. Throughout oxic and an oxic incubations, amino acids normalized to particulate nitrogen paral leled carbon normalized values. 51-82% of the particulate nitrogen cou ld be attributed to proteins, polypeptides and/or bound amino acid mon omers, suggesting that these nitrogen-rich compounds can be refractory . A comparison of the changes in concentrations of constituent amino a cids of the total amino acid and greater than or equal to 2000 Da prot ein amino acid fractions suggests that all amino acids are lost at com parable rates during the degradation sequence. For the dinoflagellate decay, two non-protein amino acids, beta-alanine and gamma-aminobutyri c acid, exhibited rate constants similar to those observed for total h ydrolyzable amino acids; relative abundances of beta-alanine and gamma -aminobutyric acid in degraded material did not significantly differ f rom that in fresh material. At the conclusion of oxic and anoxic diato m decay and oxic cyanobacterial decay, the slight increase in these tw o non-protein amino acids suggests sorption to the remaining organic m atrix. (C) 1997 Published by Elsevier Science Ltd.