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 individ ual compounds (fatty acids, sterols and hydrocarbons) were quantified over time. Oxic decay constants of total lipid and POC showed good agr eement between the two phytoplanktors, suggesting that changes in comp osition at the molecular level during degradation would be similar. De tailed analysis of individual lipids, however, revealed significant di fferences, 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 abs ence of oxygen decreasing the decay rate for all lipids. Diatom sterol s showed the largest reduction in degradative rate when oxygen was abs ent (almost 13-fold). Contrary to expectations, individual lipids comm on to both phytoplankton did not always show similar patterns of decay , suggesting that factors other than chemical structure may control de gradative rate. Carbon isotopic analysis of the total lipid pool over the time course of all incubations revealed that the residual lipid po ol shifted to lighter values, opposite of that typically observed for POC. The observed isotopic and molecular changes during degradation su ggest that the residual lipid pool, which is preserved yet difficult t o characterize, is Similar to the fraction which is preserved in sedim ents. (C) 1997 Published by Elsevier Science Ltd.