inorganic substrates present in the hydrothermal fluids might 

 represent a localized source of food for the vent-associated 

 animal community. In this ecosystem model, the particulate 

 materials consumed by the benthic invertebrates would be derived 

 from bacterial production in the hydrothermal waters rather than 

 from plankton production in the surface waters of the ocean. The 

 deep-sea vent community would, therefore, be supported ultimately 

 by energy supplied to the ocean in the form of geothermal energy. 

 This "heat-based" system, maintained by the radioactive decay of 

 long-lived isotopes of uranium, thorium and potassium and coupled 

 to bacterial chemolithoautotrophic growth at the expense of 

 geothermally reduced chemical compounds, would then represent 

 an alternative to conventional sunlight-photosynthesis based 

 communities. At the time of the discovery of deep-sea vents, all 

 life processes on earth were thought to be sustained ultimately 

 by photosynthetic processes. 



These two fundamentally independent hypotheses regarding the 

 distribution and abundance of animal life at deep-sea vents have 

 provided the incentive and initiative for a decade of extensive 

 hydrothermal vent investigation. While the buoyant plume 

 ( "thermal advection" ) hypothesis has received some support 

 (Enright, Newman, Hessler, and McGowan 1981) it is unlikely that 

 this process, by itself, could supply the amount of utilizable 

 organic matter that would be required by the dense, 

 metabolically-active vent animal communities. Furthermore, 

 direct measurements of the <5 ^C and natural ^C content of 

 selected vent animal tissues indicated that the utilization of 

 photosynthetically derived organic matter is quantitatively 

 insignificant (Williams, Smith, Druffel, and Linick 1981; Rau 

 1981, 1985). These results have focused attention on the second 

 hypothesis, and the potential for bacterial chemolithoautotrophy 

 (i.e., the reduction of inorganic carbon at the expense of 

 reduced inorganic compounds) at deep-sea hydrothermal vents. 



The initial microbiological investigations conducted at the 

 Galapagos Rift provided evidence for the presence of high 

 concentrations of bacterial cells in the discharged hydrothermal 

 vent fluids (Corliss et al . 1979 [samples analyzed by J. Baross] ; 

 Jannasch and Wirsen 1979; Karl, Wirsen, and Jannasch 1980; 

 Jannasch and Wirsen 1981). These data supported, but by no means 

 confirmed, the hypothesis of localized chemolithoautotrophic 

 bacterial production. However, since 1980, most scientists have 

 tacitly accepted as fact the previously mentioned hypothesis. If 

 true, it could have important implications for oceanic carbon 

 cycles and would establish a global precedent for a non-solar 

 based food web. It is, therefore, a hypothesis of fundamental 

 importance and clearly deserving of a rigorous experimental 

 evaluation. One of the authors, David M. Karl, has recently 

 summarized the extant data base on bacterial production at 

 deep-sea hydrothermal vents with particular emphasis on the 

 bacterial chemolithoautotrophy production hypothesis (Karl 1987). 



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