Stable isotope evidence for chemosynthesis in an abyssal seep community

Abstract

Communities of abundant organisms, similar to those which surround the hydrothermal vents of the East Pacific Rise (EPR), occur in 3,266 m of water in the Gulf of Mexico. They were discovered with the Alvin at 26° 02′ N, 84° 55′ W in an area where pore waters seep from the Cretaceous limestones of the adjacent platform at the contact between the Florida Escarpment and the Holocene hemipelagic sediments of the Abyssal Gulf. Because of the unusual abundance of organisms in this deep-sea environment, and the taxonomic similarities to the chemosynthetically based communities which surround the hydrothermal vents on the EPR, the seep communities are suspected to exist on a local non-photosynthetic food source¹. We have now measured isotope ratios in tissues of organisms which surround these saline seeps to determine the origin of the local food chain. The tissues have extremely negative δ¹³C (−42 to −77‰, PDB) and δ¹⁵N (−2.72 to −9.34‰ air) values. Such highly fractionated carbon and nitrogen isotopes are unknown in food chains based on photosynthesis, suggesting that these communities are chemosynthetic. The cause of this extreme fractionation is attributed either to assimilation from local supplies of isotopically depleted methane and ammonium or to biochemical fractionation. The tissues of these animals contain significant amounts of ¹⁴C (∼60% modern), so the source of this fractionated carbon is not predominantly fossil methane.