Links between chemosynthetic production and mobile predators on the Louisiana continental slope: stable carbon isotopes of specific fatty acids

Abstract

The Gulf of Mexico supports chemoautotrophic communities associated with hydrocarbon seeps. The chemoautotrophic symbiont-containing metazoans are dominated by mussels and tube worms that harbor bacteria which utilize reduced carbon (CH 4) and sulfur (H 2S) compounds as an energy source. In the deep sea, nutritional input from photosynthetic production is scarce, and chemoautotrophic production may be a significant source of nutrients to mobile, benthic predators. Large and significant differences exist between the stable carbon isotope signatures of chemosynthetic and photosynthetic primary production. This isotopic difference makes it possible to determine the importance of each type of primary production to heterotrophs because heterotrophs incorporate the isotope signature of their food. Here, we examine the carbon isotope signatures of specific fatty acids in heterotrophic predators caught both within and approximately 2 km from known chemosynthetic communities. Heterotroph fatty acid signatures were compared to those observed in chemoautotrophic symbiont-containing fauna in order to evaluate degree of usage. Most organisms had at least a 6‰ range in their fatty acids. This reflected patterns of de novo synthesis, with essential and precursor fatty acids being 13C enriched (in heterotrophs) and highly unsaturated fatty acids being 13C depleted. The fatty acid δ 13C values show that heterotrophs had a wide range in their utilization of chemosynthetic production. Bathynomus giganteus (giant isopod), captured off-site, did not utilize chemosynthetic production. Its nonessential fatty acids ranged from −26.3‰ to −18.8‰ (δ 13C), reflecting kinetic isotope effects during de novo synthesis. Essential omega-3 fatty acids, which are directly incorporated from diet, originated from photosynthetic production (δ 13C=−16.2‰). Sclerasterias cf. tanneri (starfish), captured on-site, relied predominantly on chemosynthetic production. Its fatty acid δ 13C values ranged from −30.5‰ to −42.2‰, reflecting chemosynthetic production. Other predators such as Eptatretus sp. (hagfish) and Rochina crassa (spider crab) derived variable percentages of their fatty acid pool from chemosynthetic production, estimated to be 38% and 5%, respectively. Bulk analysis of δ 15N and δ 34S show chemosynthetic production usage patterns generally consistent with the specific fatty acid δ 13C data.