We have yet to sample the living micro-organisms at vents 

 where these iron/silica deposits are forming, and no traces of 

 bacterial cellular constituents have been observed in existing 

 samples. Nonetheless, the precipitation of iron and silica on an 

 existing filamentous structure offers an explanation of the 

 nature of these deposits. Further study of this problem will 

 require in situ sampling to verify the presence or non-presence 

 of living microbial filaments. 



OVERVIEW 



Vent organisms are able to both actively and passively 

 influence mineral deposition. Active effects, that is direct 

 chemical transformation of mineral forming elements, result from 

 organism metabolism and detoxication. Passive effects are 

 produced by the physical presence of biological structures and 

 debris which trap mineral particles, control geochemical 

 gradients and provide surfaces for mineral precipitation. In the 

 few examples we have studied to date, passive effects appear to 

 have a greater quantitative influence on mineral deposition than 

 active effects. Even where possible active effects were 

 identifiable, such as in iron deposition by bacteria on worm 

 tubes or within filamentous mats, they occurred in conduction 

 with surface or structural effects that likely greatly enhanced 

 their quantitative significance. 



Biological effects are most evident at sites where mineral 

 deposition is the greatest, rather than where biomass is 

 highest. In fact, biomass is usually greater where little 

 deposition is occurring and physico-chemical conditions are less 

 severe. Thus, the importance of biomineralization appears to 

 depend on the potential of a vent to deposit minerals, which is 

 likely related to the properties of vent fluids (Tunnicliffe and 

 Fontaine 1987). 



The intimate involvement of vent organisms with mineral 

 deposition has both positive and negative effects on their 

 ability to survive and reproduce. Negative effects are the most 

 apparent. Mineral particle fouling of branchial surfaces has been 

 observed in Vestimentif era (unpublished observation) and probably 

 affects the respiratory surfaces of most vent organisms. The 

 abundance of mineral particles likely poses detoxication problems 

 for grazing and deposit feeding animals that ingest a variety of 

 metals along with particulate organic material (Tunnicliffe et 

 al. 1985). Although entombment of live vent organisms by mineral 

 deposition has never been observed, this process may be an 

 important cause of mortality for sessile organisms in areas of 

 heavy mineral deposition. A certain degree of mineralization can 

 have fortuitous positive effects. There are noticeably fewer 

 signs of predation in northeast Pacific vestimentiferan 

 populations that have visibly mineralized tubes (V. Tunnicliffe, 

 unpublished observation). The major predator of these tube-worms 



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