Kinetics of phytoplankton decay during simulated sedimentation: changes in lipids under oxic and anoxic conditions

A series of oxic and anoxic incubations examined lipid degradation in two marine phytoplankton, the diatom Thalassiosira weissflogii and the coccoid cyanobacterium Synechococcus sp., using flow-through systems without macrozooplankton grazers. Total extractable lipids and individual compounds (fatty acids, sterols and hydrocarbons) were quantified over time. Oxic decay constants of total lipid and POC showed good agreement between the two phytoplanktors, suggesting that changes in composition at the molecular level during degradation would be similar. Detailed analysis of individual lipids, however, revealed significant differences, with unsaturated moieties being degraded more rapidly than their saturated counterparts. Calculated turnover times for individual lipids ranged from 8.8 days for unsaturated alkenes of diatoms under oxic conditions to over 142 days for phytol under anoxia, with the absence of oxygen decreasing the decay rate for all lipids. Diatom sterols showed the largest reduction in degradative rate when oxygen was absent (almost 13-fold). Contrary to expectations, individual lipids common to both phytoplankton did not always show similar patterns of decay, suggesting that factors other than chemical structure may control degradative rate. Carbon isotopic analysis of the total lipid pool over the time course of all incubations revealed that the residual lipid pool shifted to lighter values, opposite of that typically observed for POC. The observed isotopic and molecular changes during degradation suggest that the residual lipid pool, which is preserved yet difficult to characterize, is similar to the fraction which is preserved in sediments.