Figure 3.

Carbon and nitrogen isotopic compositions of targeted aggregates. Aggregates classification scheme based on description in text as well as indicated in Figure 2 and Figure 3. Dashed line shows the approximate maximum ¹⁵N values observed in the non-active control incubation (∼150‰). The top panel shows aggregates from the sulfate control. The bottom panel shows aggregates from the manganese incubation. Shell aggregates are active in the sulfate incubation, but not active in the manganese as indicated by incorporation of ¹⁵N. Mixed/mixed cluster and archaea sarcina are active in the manganese incubation (as shown by incorporation of ¹⁵N). Aggregates in the manganese (birnessite) incubation (bottom panel) are less active than those in the sulfate incubation (top panel) as expected due to the slower rate of manganese dependent AOM as compared to sulfate dependent AOM. Typical carbon isotopic composition of shell and mixed and mixed-cluster aggregates in the ERB range from −100 to −60‰ (although some have been found as enriched as −20‰). Archaea sarcina have typical ¹³C values ranging from −80 to −20‰. Bacteria are typically heavier, with ¹³C values typically between −45‰ and −20‰ [1,2,9]. Because we do not know the ¹³C value of the target aggregates before incubation with ¹³CH₄, we cannot conclusively state which aggregates show minor incorporation of ¹³C. However, our data indicate that the more active aggregates (shown in an oval) are also slightly enriched in ¹³C as compared to other active aggregates in our incubation, suggesting that they have incorporated minor amounts of ¹³C from methane.