A quantitative study of microbial decomposition of biopolymers in Recent sediments from the Peru Margin

The relationship between depth changes in total organic matter (TOC), biopolymers (“carbohydrates”, “proteins” and lipids) and bacterial populations and their activity, were investigated on two cruises to an upwelling site on the Peru Margin. Samples were obtained from the sediment surface to 8020 cm below sea floor (cbsf). Bacterial concentrations increased from near surface to a maximum at 12.5 cbsf, then rapidly decreased to 20 cbsf and a slower decrease to 8020 cbsf. The bacterial population at 8020 cbsf was 9.5% of the near surface value, but at 3.3 × 108 cells/cm3 it was still very significant. The presence of dividing bacterial cells indicated that a portion of the population was active and this was confirmed by radiotracer measurements. Biopolymers were overestimated in the surface 4 cm (138% of the TOC). By 5.5 cbsf, however, organic matter uncharacterized as biopolymers was present. This increased rapidly to 17% of the TOC at 10 cbsf and then more slowly to 79% at 2229 cbsf. Bacterial populations and activity were significantly correlated with depth changes in both TOC and biopolymers (p < 0.002). Calculation of rates of anaerobic carbon metabolism from the measured rates of sulphate reduction and methanogenesis demonstrated that these processes could account for all (101%) of the decrease in “carbohydrate” and “protein” in the 2.5 to 22.5 cbsf interval, where bacterial activity was intense. These data confirm the importance of anaerobic bacterial processes in these high organic matter sediments. At deeper intervals, 22.5–480 and 480–8020 cbsf, the decrease in “carbohydrate” and “protein” only accounted for 40% and 15% respectively of the anaerobic bacterial metabolism and hence the significant bacterial population present must have been utilizing a portion of the increasing uncharacterized organic matter. There was a negative relationship between the total bacterial population and the percentage of uncharacterized organic matter, suggesting that a portion of the uncharacterized organic matter may have come from dead bacterial cells. Estimates of bacterial necromass production from rates of thymidine incorporation were significantly correlated with increases in uncharacterized organic matter (p < 0.05, 2.5–197.5 cbsf) and could account for 16% of the increase within the top 25 cm. However, as the thymidine incorporation technique underestimates bacterial productivity in anoxic sediments it is likely that a much greater proportion of the increase in uncharacterized organic matter should be attributed to “bacterial necromass” production. It is suggested that bacterial necromass production may be an important mechanism for the production of recalcitrant, and hence preserved, organic matter in sediments in high productivity regions