measured total Si by atomic absorption spectrometry (McMurtry, 

 Karl, Tilbrook, and Sedwick 1987). Our analyses revealed that 

 there was an excellent correspondence between reactive Si (by 

 autoanalyzer ) and total Si (by atomic absorption) for samples 

 with total Si < 600 yM. At higher concentrations, the 

 autoanalyzer methods underestimated the total dissolved Si 

 contained in the vent water samples. All nutrient determinations 

 were made within 1 week of sample collection. 



We also attempted to measure dissolved organic nitrogen 

 (DON) and dissolved organic phosphorus ( DOP ) in Pele's Vent water 

 samples but these analyses were unsuccessful because of the 

 precipitation of dissolved Fe during the 60°C UV oxidation 

 procedure . 



Particulate ATP 



The particulate materials contained in Pele's Vent water 

 samples (100-250 ml) were concentrated onto Whatman glass fiber 

 filters (GF/F type) and the filters were immediately placed into 

 a test tube containing 5 ml of boiling phosphate buffer (60 mM, 

 pH 7.4; Karl and Craven 1980). The samples were extracted for 5 

 minutes, and stored frozen for subsequent laboratory analysis of 

 adenosine-5 ' -triphosphate (ATP) by the firefly bioluminescence 

 assay procedure (Karl and Holm-Hansen 1978). 



Chemical composition of bacterial mat 



A frozen subsample of the fresh bacterial mat was also sent 

 to Dr. Greg Rau (National Aeronautics and Space Administration, 

 Moffett Field, CA ) for stable carbon isotope analysis. In his 

 laboratory, a portion of the mat sample was washed with 0.25 M 

 HC1 (to remove carbonates) and dried for several days at 50°C. 

 Subsamples were placed into precombusted quartz tubes, mixed with 

 organic-free CuO and Cu particles (30 mesh), sealed and combusted 

 at 800°C for 6 hr. The resulting CO2 was purified cryogenically 

 and the stable C isotope ratios were measured using a Nuclide 6- 

 60 ratio mass spectrometer. 



For phosphorus determinations, 10 mg subsamples of the dried 

 mat material were placed into three clean glass test tubes and 



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