deposits is from the hydrothermal solutions themselves (see Fig. 

 3) rather than being derived from seawater. 



The SEM-EDS analysis (Fig. 11) indicated that the 

 bacterial mat was comprised predominantly of Fe with significant 

 contributions from P and Ca. Si also appeared as a minor 

 constituent of bulk sample excitation (i.e., sample magnification 

 of <100 x) but was not present as a compositional constituent of 

 a higher magnification (and excitation) of individual filaments. 



These results were confirmed by quantitative microprobe 

 analyses which revealed that the mat had an Fe:P ratio (by 

 weight) of 6-7 to 1 (M. Garcia, pers. coram.). The percent P 

 content, as determined by microprobe, was generally higher than 

 that estimated by chemical methods. X-ray diffraction ( XRD ) 

 analysis of the bacterial mat material indicated the general 

 absence of crystalline material in the dried preparations. The 

 Ca peak (Fig. 11) is most likely due to the presence of 

 contaminating seawater as trace amounts of halite (NaCl) and 

 bassanite (CaSC>4 • 0.5 H2O) were detected in the XRD analysis 

 (J. Schoonmaker, pers. coram. ). The presence of amorphous Fe 

 (oxides/hydroxides) is consistent with the suggestion of a 

 microbiological origin of the filamentous structures. Metal 

 associations, especially amorphous Fe, are also known to occur in 

 other biological/ microbiological hydrothermal vent deposits (Alt 

 1986; Juniper, Thompson, and Calvert 1986). 



Ferrous iron oxidation is well known in the microbial world 

 (e.g., Thiobacillus f errooxidans , Sulfolobus acidocaldarius , 

 Leptospir ilium f errooxidans , Gallionella f erruginea , Leptothrix 

 ochracea , Siderocapsa geminata , Sphaerotilus natans , 

 Metal logenium ; Cullimore and McCann 1977, Ehrlich 1981, Ghiorse 

 1984). Not all iron-oxidizing bacteria couple the oxidation to 

 C02 reduction and in some cases the oxidation of iron is only an 

 indirect metabolic process (i.e., non-enzymatic). However, 

 obligate chemolithoautotrophic growth utilizing Fe+2 has been 

 demonstrated for selected species. Waters collected from Pele's 

 Vent at Loihi Seamount contain >700 ymoles dissolved Fe l -1 (G. 

 McMurtry, pers. coram. ). The molar Fe/Mn ratio decreases in the 

 Loihi hydrothermal vent system from approximately 50 in the 

 warmest anoxic waters (30°C) to a value of 20 at lower water 

 temperatures (2-5°C). These data suggest iron deposition is 

 perhaps mediated by micro-organisms. 



Absence of Benthic Macrofauna at Pele's Vent 



A hallmark of nearly all hydrothermal vent systems studied 

 to date is the presence of dense populations of benthic 

 invertebrates which thrive on the localized source of bacterial 

 production. These deep-sea oases contrast sharply with the 

 depauperate nature of non-vent deep-sea benthic populations, a 



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