KINETICS OF PHYTOPLANKTON DECAY DURING SIMULATED SEDIMENTATION: CHANGES IN BIOCHEMICAL COMPOSITION AND MICROBIAL ACTIVITY UNDER OXIC AND ANOXIC CONDITIONS

Abstract

A series of oxic and anoxic incubations examined the decay of two marine phytoplankton, the diatom Thalassiosira weissflogii and the coccoid cyanobacterium Synechococcus sp, in flow-through systems without macrozooplankton grazers. The major biochemical fractions of algal carbon (protein, carbohydrates, and lipid) were quantified over time together with bacterial abundance and activity. Oxic decay constants of bulk and individual biochemical fractions showed good agreement between both phytoplankters, suggesting that composition at the molecular level within a particular biochemical class does not influence decay rate as much as differences among the major biochemical fractions. Large differences in decay rates did exist among biochemical classes, with carbohydrates utilized most rapidly under oxic conditions, followed by protein and then lipid. Turnover times among the particulate pools ranged from 10.7 days for diatom and cyanobacterial carbohydrates under oxic conditions to over 160 days for cyanobacterial lipids under anoxia, with oxygen having a substantial effect on overall rates of algal carbon decomposition. PON values tracked POC with an average POC:PON ratio of 4.99 +/- 0.52 for diatoms and 4.48 +/- 0.66 for cyanobacteria throughout the experiments. Bacterial abundances and activity varied substantially over the course of the incubations with greatest activity during periods of greatest particulate loss. Bacterial abundances and metabolism were comparable under oxic and anoxic conditions even though the amount of material degraded under anoxic conditions was significantly less than when oxygen was present, suggesting that oxygen increased rates of particulate material degradation.