Isotopic evidence for chemosynthesis-based nutrition of macrobenthos: The lightness of being at Pacific methane seeps

The importance of chemosynthetic nutritional pathways was examined for macrofaunal invertebrates (>300 mum) from methane seeps in the Gulf of Alaska (4,413-4,443 m), on the Oregon margin (590 m), and on the northern California slope [Eel River margin] (520 m) by use of natural abundance stable isotopic data. Seep macrofauna exhibited lighter delta(13)C and delta(15)N values than those in nonseep sediments, but isotopic signatures varied among seep sites. Macrofaunal isotopic signatures indicated chemosynthetically fixed carbon sources with a significant contribution from methane-derived carbon (MDC) in macrofauna from sediments of pogonophoran fields (average delta(13)C, -46.44parts per thousand, 32%-51% MDC) and Calyptogena phaseoliformis beds (average delta(13)C, -40.89parts per thousand, 12%-40% MDC) in the Gulf of Alaska and in microbial mat sediments on the Oregon margin (average delta(13)C, -43.80parts per thousand, 20%-44% MDC). Lesser influence of MDC was noted in macrofauna from sediments of Calyptogena pacifica beds on the Oregon (average delta(13)C, -33.38parts per thousand, 0%-27% MDC) and California (delta(13)C, -25.10parts per thousand, 0%-22% MDC) margins and from California microbial mat sediments (delta(13)C, -22.23%o, 0%-5% MDC). Although most macrofauna appeared to be heterotrophic, light delta(15)N and delta(13)C values together provided evidence for chemoautotrophic symbioses in selected taxa. Carbon isotopic signatures were consistent with consumption of methane-oxidizing archaea by some dorvilleid polychaetes (delta(13)C, -90.62parts per thousand and -73.80parts per thousand) and with grazing on filamentous sulfur bacteria by gastropods and polychaetes from the Oregon and California seeps. The importance of chemosynthetic trophic pathways varies regionally and among microhabitats, taxonomic groups, and feeding guilds.