The effect of microorganisms and seasonal factors on the isotopic composition of particulate organic carbon from the Black Sea

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1 998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the C-12 isotope, as compared to th e POC of the oxycline and anaerobic zone. In the C-layer waters, the concur rent presence of dissolved oxygen and hydrogen sulfide and an increased rat e of dark CO2 fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically li ght organic matter of the biomass of autotrophic bacteria involved in the s ulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows a n inference to be made about the considerable seasonable variability of the POC delta(13)C value, which depends on the ratio of terrigenic and plankto nogenic components in the particulate organic matter.