Mid-chain methoxylated fatty acids within the chemocline of the Cariaco Basin: A chemoautotrophic source?

A series of C₁₅-C₂₀ mid-chain methoxylated fatty acids (FAs) has been tentatively identified in suspended particulate matter samples from within the chemocline of the Cariaco Basin. The concentrations appeared to be highest (50 ng/l) within the core of the chemocline (245-256 m), where they constituted 25% of the total FAs, 9-methoxy-C_16:0 and 10-methoxy-C_16:0 being 73% of the total methoxy acids. Mid-chain methoxy acids in the chemocline had δ¹³C values of ca. -34‰. In stable isotope probing experiments using ¹³C labeled bicarbonate and various sulfur species (S₂ O₃^{2 -}, SO₃^{2 -} and particulate S⁰), up to 80% of 9-/10-methoxy-C_16:0 were ¹³C labeled when thiosulfate was provided. Therefore, we propose that mid-chain methoxy FAs are key membrane lipids in sulfur-metabolizing chemoautotrophs within the core of the chemocline. Sulfur-oxidizing bacteria belong to both the ε- and β-proteobacteria; half of the bacteria within the chemocline are either ε-proteobacteria (∼40%) or β-proteobacteria (∼11%), providing a possible clue to the phylogeny of chemoautotrophs and a link to the production of chemoautotrophic biomarkers. We thus demonstrate an association between the presence and activity of sulfur-utilizing chemoautotrophs and occurrence of mid-chain methoxylated FAs within the chemocline of the Cariaco Basin.