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 bacterial biomass and diagnostic fatty acids during degradation of two marine phytoplankton, the diatom Thalassiosira weissflogii and the coccoid cyanobacterium Synechococcus sp. Flow-through systems were used to simulate sedimentation, while macrozooplankton grazers were excluded. Bacterial 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 biomass. Bacterial carbon increased over 3.7-fold during the period when maximal loss of diatom derived POC was observed. In the oxic cyanobacterial decay experiment bacterial biomass showed a different pattern, with a rapid increase prior to the time period of particulate carbon loss and decrease thereafter. The total concentration of “bacterial” fatty acids for all incubations increased from time zero, with highest concentrations (164 μg l−1) in oxic incubations and coinciding with periods of highest bacterial carbon. Bacterial carbon accounted for up to 32.6% of total POC during the early stages of anoxic diatom decay, with lower maximum amounts (21.4%) in parallel incubations under oxic conditions. Carbon isotopic signatures of individual fatty acids representing algal, bacterial and mixed sources were determined during oxic diatom decay, and showed a small enrichment (average of +1.6‰) in 13C after the period of major POC loss. A comparison of total fatty acids attributed to bacteria and bacterial carbon calculated from measured bacterial cells showed no strong relationship, suggesting that fatty acids considered diagnostic of bacteria remain a qualitative tool for the estimation of bacterial contributions to sediments.