SHORT COMMUNICATION C.R. Fisher 7 I.R. MacDonald 7 R. Sassen C.M. Young 7 S.A. Macko 7 S. Hourdez R.S. Carney 7 S. Joye 7 E. McMullin Methane Ice Worms: Hesiocaeca methanicola Colonizing Fossil Fuel Reserves Received: 16 August 1999 / Accepted in revised form: 19 January 2000 C.R. Fisher 7 S. Hourdez 7 E. McMullin Department of Biology, Pennsylvania State University, University Park, PA 16802, USA I.R. MacDonald 7 R. Sassen Geochemical and Environmental Research Group, Texas A&M University, College Station, TX 77845, USA C.M. Young Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Fort Pierce, FL 35946, USA S.A. Macko Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA R.S. Carney Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803, USA S. Joye Department of Marine Sciences, University of Georgia, Athens, GA 30605, USA Abstract During a research cruise in July 1997 in the Gulf of Mexico we discovered a gas hydrate approxi- mately 1 m thick and over 2 m in diameter which had recently breached the sea floor at a depth of 540 m. The hydrate surface visible from the submarine was consid- erably greater than that of any other reported hydrate. Two distinct color bands of hydrate were present in the same mound, and the entire exposed surface of the hy- drate was infested (2500 individuals/m 2 ) with 2 to 4 cm- long worms, since described as a new species, Hesiocae- ca methanicola, in the polychaete family Hesionidae (Desbruyères and Toulmond 1998). H. methanicola tis- sue stable isotope values are consistent with a che- moautotrophic food source. No evidence of chemoau- totrophic symbionts was detected, but geochemical data support the presence of abundant free living bacteria on the hydrate. The activities of the polychaetes, graz- ing on the hydrate bacteria and supplying oxygen to their habitats, appears to contribute to the dissolution of hydrates in surface sediments. Gas hydrates are icelike crystalline solids composed of gasses, predominantly methane, but also hydrogen sul- fide (Kastner et al. 1998), encased within rigid cages of water molecules. Formed under pressure, hydrate sta- bility depends on temperature, pressure, and chemical composition (MacDonald et al. 1994). Globally, me- thane associated with hydrates comprises on the order of 10 4 gigatons of carbon and may represent the most abundant store of fossil fuel on the earth (Kvenvolden 1993, 1995). No nonmicrobial life has previously been reported living in or on hydrate structures. Most hy- drate deposits are deeply buried by marine sediments and have been stable during recent geological time. Hy- drates also occur in surface sediments in areas of active hydrocarbon seepage (Brooks et al. 1991, 1984), where formation and dissociation of hydrates is dynamic ow- ing to interactions of the hydrate with bottom water (MacDonald et al. 1994). Analysis of small-volume water samples taken from the surface of the discovered hydrate and molecular analysis of pore water samples from the overlying sedi- ments confirm that the hydrate is decomposing. Sam- ples taken adjacent to polychaetes contained up to 900 mM methane and 100 mM hydrogen sulfide, which are not present in micromolar quantities in the ambient bottom water. Furthermore, the elevated concentra- tions and low d 13 C values of the carbon dioxide in the sediments reflect bacterial oxidation of hydrocarbon gasses over the hydrate (Table 1). The yellow color in the upper band of the gas hydrate is correlated with the presence of high molecular weight hydrocarbons (oil) in this band, whereas the lower white band lacked oil. The molecular characteristics of the hydrate gases from both bands are quite similar and consistent with struc- ture II gas hydrates formed from an unaltered single source of thermogenic vent gas (Fig. 1, Table 1) (Sassen et al. 1998). The faunal aggregation on the entire exposed sur- face of the hydrate, and surfaces revealed by removal of sediment by the submersible, was monospecific H. methanicola (Fig. 2). Most individuals were between 2 Naturwissenschaften (2000) 87 : 184–187 Q Springer-Verlag 2000