matrix of their sheaths (Fig. 4b and c). This observation later 

 proved important in interpretation of iron-rich encrustations on 

 tubes from sites where mineral deposition was much heavier. 

 Tubes from these areas were encrusted with a red-orange material 

 which qualitative EDS analyses revealed to consist mainly of Fe 

 and P (Fig. 5a). Sheathed bacteria accumulating Fe were abundant 

 on these tubes (Fig. 5b), as were aggregations of sheathed 

 bacteria that formed rounded Fe-rich particles of 6-8 m 

 diameter. Tunnicliffe and Fontaine (1987) have proposed that 

 formation of iron encrustations is initiated by the process 

 observed on the "clear" tubes, that is, accumulation of fine iron 

 particles by the sheathed bacteria. Where sheathed bacteria are 

 more abundant and more closely spaced, as they are on the 

 encrusted tubes, fine Fe particles will begin to accumulate 

 between sheaths and then accumulate in a non-specific way on the 

 growing Fe particle. This type of autocatalytic accumulation of 

 metals following biocatalysis is known from a variety of aquatic 

 habitats. Ghiorse (1984) in a review of Fe and Mn accumulating 

 bacteria describes how iron can associate non-specif ically with 

 acidic extracellular polymers (bacterial slime), such that once 

 iron oxides have been formed by microbial biocatalysis further 

 binding and oxidation of iron can occur autocatalytically . The 

 tube-surface microflora and iron encrustations are most developed 

 at sites where iron deposition is heaviest and presumably venting 

 fluids are transporting the most reduced iron. The relationship 

 between microbial growth and ambient iron concentration remains 

 to be investigated. 



Structural Effects 



Growth of tube worm assemblages over or adjacent to hot 

 water openings clearly affects the mixing of vent fluid and 

 bottom seawater. Tunnicliffe et al. (1985) used a simple series 

 of point temperature measurements to illustrate the retention of 

 vent fluid within a tube worm thicket. At vents where mineral 

 deposition is significant there are a number of mechanisms by 

 which it can be enhanced within the microenvironment formed by 

 tube worm aggregations. Surface effects such as those described 

 in the preceding section would be magnified since tube structures 

 both provide extensive surface area and retard dilution of metals 

 being transported in vent fluids. Partial retention of venting 

 waters within this microenvironment may also increase cooling of 

 the fluids, further facilitating chemical saturation and 

 precipitation of metals. Unoccupied tubes appear to persist for 

 a long time. Groves of empty tubes seen in a 1984 cruise were 

 still present at the same vent in 1987 (personal communication, 

 Wm. Normark, U.S. Geological Survey, 1987); micro-spires of 

 sulfides were forming around these tubes. Presently this effect 

 is being studied using a time-lapse camera to record the 

 relationship between the location of worm tubes on a growing 

 sulfide mound and the addition of new sulfides to this deposit. 



105 



