CATALYSTS OR CONTRIBUTORS - TRACKING BACTERIAL MEDIATION OF EARLY DIAGENESIS IN THE MARINE WATER COLUMN

A series of oxic and anoxic incubations examined the changes in bacter ial biomass and diagnostic fatty acids during degradation of two marin e phytoplankton, the diatom Thalassiosira weissflogii and the coccoid cyanobacterium Synechococcus sp. Flow-through systems were used to sim ulate sedimentation, while macrozooplankton grazers were excluded. Bac terial abundance was strongly coupled to the decay sequence for algal carbon under both oxic and anoxic conditions, with loss of particular organic carbon (POC) accompanied by a rapid increase in bacterial biom ass. Bacterial carbon increased over 3.7-fold during the period when m aximal loss of diatom derived POC was observed. In the oxic cyanobacte rial decay experiment bacterial biomass showed a different pattern, wi th a rapid increase prior to the time period of particulate carbon los s and decrease thereafter. The total concentration of ''bacterial'' fa tty acids for all incubations increased from time zero, with highest c oncentrations (164 mu g l(-1)) in oxic incubations and coinciding with periods of highest bacterial carbon. Bacterial carbon accounted for u p to 32.6% of total POC during the early stages of anoxic diatom decay , with lower maximum amounts (21.4%) in parallel incubations under oxi c conditions. Carbon isotopic signatures of individual fatty acids rep resenting algal, bacterial and mixed sources were determined during ox ic diatom decay, and showed a small enrichment (average of +1.6 parts per thousand) in C-13 after the period of major POC loss. A comparison of total fatty acids attributed to bacteria and bacterial carbon calc ulated from measured bacterial cells showed no strong relationship, su ggesting that fatty acids considered diagnostic of bacteria remain a q ualitative tool for the estimation of bacterial contributions to sedim ents. (C) 1997 Elsevier Science Ltd.