The results of AODC and CARD-FISH analysis of black shale and sediment reference samples are shown in Figure F1 and Table T1. Prokaryotes could be detected with AODC as well as with CARD-FISH in 2 of 13 black shale samples but not in the 5 reference sediment samples. The number of total prokaryotes and living bacteria in the two black shale samples are comparable to those found for Leg 201 sediment samples from the equatorial Pacific and the Peru margin (D´Hondt, Jørgensen, Miller, et al., 2003; D´Hondt et al., 2004; Schippers et al., 2005). The CARD-FISH numbers represent a minimum of the total living bacteria because living cells with very low activity and ribosome contents were probably not detected. A few living cells of archaea could be detected, but their number was too low to be quantified using CARD-FISH.
The absence of prokaryotes in most of the black shale and all reference sediment samples indicates that the highly compacted Leg 207 sediment layers studied here do not provide suitable conditions for a thriving microbial community (in contrast to Leg 201 sediments, for example). Bulk densities ranged from ~1.5 to ~2.1 g/cm3 for Leg 207 samples and from ~1.2 to ~1.8 g/cm3 for Leg 201 sediments. Porosities were always <70% for Leg 207 samples studied here but showed values of >75% for all Leg 201 sediments. However, the occurrence of prokaryotes in at least two Leg 207 black shale samples suggests a patchy distribution of the cells probably living in larger pores or cracks. Because contamination tests were not carried out during Leg 207 as described for Leg 201 (D´Hondt, Jørgensen, Miller, et al., 2003), contamination of the two black shale samples can not be excluded, but it is unlikely because of the high numbers of prokaryotes detected.
The interstitial water geochemistry of Leg 207 sediments indicates microbial activity at the top of the black shale sequence. At all sites (except Site 1261), sulfate and ammonium gradients are essentially linear from the sediment/seawater interface to the top of the black shale sequence (Erbacher, Mosher, Malone, et al., 2004). Unfortunately, we have not received samples from these depth intervals to verify the presumably high abundance of prokaryotes there.