δ13C values of lipids from phototrophic zone microplankton and bathypelagic shrimps at the Azores sector of the Mid-Atlantic Ridge

The lipid composition and δ13C values of phototrophic zone microplankton, and species of bathypelagic shrimp that are not associated with hydrothermal vents, were determined for samples collected from the water column above the Mid-Atlantic Ridge. These analyses were compared with similar previously published data for vent bresiliid shrimp to address the hypothesis that deep-sea hydrothermal vent ecosystems are reliant on specific dietary nutrients produced by photosynthetic organisms. Microplankton (<200 μm) sampled from the surface layer (∼4 m deep) and from the region of maximum light scattering (LSM, 48–75 m deep) were analysed to determine δ13C values of individual fatty acids in particulate matter. The distributions of fatty acids in total lipid from the surface layer and from the LSM were very similar, with high levels (∼45% of the total) of saturated fatty acids, particularly 14 : 0, 16 : 0 and 18 : 0, and moderate amounts (∼31% of the total) of polyunsaturated fatty acids (PUFA), dominated by 22 : 6(n-3). δ13C values of fatty acids from the surface layer and LSM were also very similar (mean values of −27.6 and −28.8‰, respectively), with a range of values from −25.0 to −32.2‰ and PUFA being somewhat depleted in 13C relative to saturated and monounsaturated fatty acids. Total lipid of abdominal muscle from three species of bathypelagic decapod shrimp, Ephyrina bidentata, Parapasiphaea sulcatifrons and Sergia japonicus collected from 2000 m contained 18 : 1(n-9), 16 : 0, 22 : 6(n-3) and 20 : 5(n-3) as major fatty acids in all cases. The fatty acids in total lipid from the wax ester-rich hepatopancreas of all three shrimps were dominated (∼50% of the total) by 18 : 1(n-9) and contained substantially lower levels of PUFA than muscle lipid. Total lipids from the hepatopancreas of E. bidentata and S. japonicus contained high levels of 22 : 1 alcohols and 16 : 0 alcohol, respectively, whereas total hepatopancreatic lipid from P. sulcatifrons contained mainly 18 : 1 alcohols and 16 : 0 alcohol. δ13C values for the fatty acids in the three species ranged from −22.4 to −30.7‰ for muscle (mean value −25.7‰) and from −23.9 to −31.9‰ for hepatopancreas (mean value −26.1‰), with PUFA again being depleted in 13C relative to saturated and monounsaturated fatty acids. δ13C values for the fatty alcohols in the three species ranged from −25.7 to −29.5‰ (mean value −27.1‰). These lipid analytical and δ13C data are consistent with PUFAs in bathypelagic shrimps originating unmodified from PUFAs in the photic zone and wax esters in bathypelagic shrimps originating substantially from animal biosynthetic activity.