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 a n upwelling site on the Peru Margin. Samples were obtained from the se diment surface to 8020 cm below sea floor (cbsf). Bacterial concentrat ions increased from near surface to a maximum at 12.5 cbsf, then rapid ly decreased to 20 cbsf and a slower decrease to 8020 cbsf. The bacter ial population at 8020 cbsf was 9.5% of the near surface value, but at 3.3 x 10(8) cells/cm3 it was still very significant. The presence of dividing bacterial cells indicated that a portion of the population wa s active and this was confirmed by radiotracer measurements. Biopolyme rs were overestimated in the surface 4 cm (138% of the TOC). By 5.5 cb sf, however, organic matter uncharacterized as biopolymers was present . This increased rapidly to 17% of the TOC at 10 cbsf and then more sl owly to 79% at 2229 cbsf. Bacterial populations and activity were sign ificantly correlated with depth changes in both TOC and biopolymers (p < 0.002). Calculation of rates of anaerobic carbon metabolism from th e 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, whe re bacterial activity was intense. These data confirm the importance o f anaerobic bacterial processes in these high organic matter sediments . At deeper intervals, 22.5-480 and 480-8020 cbsf, the decrease in ''c arbohydrate'' and ''protein'' only accounted for 40% and 15% respectiv ely of the anaerobic bacterial metabolism and hence the significant ba cterial population present must have been utilizing a portion of the i ncreasing uncharacterized organic matter. There was a negative relatio nship between the total bacterial population and the percentage of unc haracterized organic matter, suggesting that a portion of the uncharac terized organic matter may have come from dead bacterial cells. Estima tes of bacterial necromass production from rates of thymidine incorpor ation 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 incorpor ation technique underestimates bacterial productivity in anoxic sedime nts it is likely that a much greater proportion of the increase in unc haracterized organic matter should be attributed to ''bacterial necrom ass'' 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 regi ons.