Organic Carbon Production and Remineralization in the Oligotrophic Water Column Assessed Using Compound-specific Stable Isotopic Composition of Particulate Fatty Acids

Lipid molecular and carbon isotopic composition (δ¹³C) can elucidate sources, recycling, and remineralization of organic carbon (OC) in the oceanic water column. We analyzed the δ¹³C of particulate fatty acids at the Bermuda Time Series Site in the oligotrophic Sargasso Sea to evaluate OC cycling processes, from the surface to the seafloor at 4500 m depth. Here we focus on seasonal comparison of δ¹³C depth profiles of even-chained saturated fatty acids (SFA, non-specific sources), polyunsaturated fatty acids (PUFAs, synthesized by phytoplankton and deep ocean piezophilic bacteria) and odd/branched chained fatty acids (ODD/BR, synthesized by bacteria). During April post-bloom conditions, SFA δ¹³C is highly depleted in the euphotic zone (-25 to -29 per mil), rapidly increases by 8-10 per mil in the upper mesopelagic zone of intense carbon remineralization, and then remains relatively constant in deeper waters. During November low productivity conditions, SFA δ¹³C in the euphotic zone is enriched by ~4 per mil relative to April, reflecting the higher relative abundance of recycled OC, and increases to isotopic values similar to April in the deep water column. PUFAs also are most isotopically depleted in the euphotic zone, with a comparable but smaller 5 per mil enrichment in the upper mesopelagic. In the deep water column, PUFA δ¹³C also shows no depth trends but is more variable, with slightly depleted values associated with more labile particulate OC. PUFA δ¹³C profiles provide no clear evidence of significant PUFA biosynthesis by piezophilic bacteria in deep waters. ODD/BR δ¹³C profiles are the most diverse. The δ¹³C of br15:0 FAs is most depleted at the chlorophyll maximum, while n15:0 and br19:0 FAs exhibit no Chl max depletion but are ~2 per mil isotopically enriched relative to br15:0 isomers. Overall, δ¹³C profiles of diagnostic fatty acids evidence the significant isotopic fractionation that is associated with OC remineralization in the mesopelagic zone.