A major success of Leg 193 was confirmation that microbes including actinomycetes (filamentous or dendritic bacteria), cocci, and bacilli flourish in the shallow subsurface portion of the PACMANUS hydrothermal system. Bacteria were found by direct counting of total cells in core samples from depths to ~70 mbsf in Hole 1188A and ~80 mbsf in Holes 1189A and 1189B (Kimura et al., 2003). Adenosine triphosphate (ATP) analysis detected live cells in cores recovered to ~50 mbsf under Site 1188 and ~40 mbsf under the high-temperature chimney field at Site 1189. Cell counts fell progressively with depth at both sites.

Growth occurred of colonial and dispersed individual bacteria in anaerobic cultivation experiments at 60C in samples from 60 to 88 mbsf at Site 1188 and from 50 to 99 mbsl at Site 1189 (Kimura et al., 2003). In experiments at 90C, growth occurred in samples from 69 to 107 mbsf at Site 1188 and from 68 to 129 mbsf at Site 1189. This extends the range of microbial habitats relative to those detected by the ATP analysis. In neither experiment set was growth observed in shallower or deeper samples than the ranges cited. Higher cultivation temperatures favor microbes from deeper core samples. Using 16S ribosomal deoxyribonucleic acid (rDNA) analysis, Kimura et al. (2003) identified bacteria from the 60 and 90C cultures as new subspecies of Geobacillus sp. and Deinococcus sp., respectively, the latter being previously regarded as a more typically aerobic strain.

Within the PACMANUS hydrothermal alteration system Asada et al. (2003) found clay minerals to be the preferred microbial habitat, suggesting these may act as a buffer against hydrothermal fluids and as a source of K+ and H2O nutrients via bacterial dissolution. In the unaltered volcanic caprock, glass is the preferred microbe habitat. Chemical analyses of microbial samples revealed relatively low carbon contents (150–1270 ppm; mean = 320 ppm), whereas nitrogen was not detected. Paucity in C and N, or limited flux of nutrient-bearing hydrothermal fluids, may restrict microbe abundances.

Transmission electron microscope evidence for biomineralization found within cores from 87.9 and 106.8 mbsf in Hole 1188A includes clay flakes covering the outer cell walls and mineralized cell interiors of two bacteria and a thick gridlike cell wall covered by excretal exopolymers in a third (Asada et al., 2003).

Below 129 mbsf the PACMANUS hydrothermal system is evidently sterile. Unfortunately, temperature logging was inadequate for translating this into a maximum temperature limit for microbial life.