Mid-chain branched alkanoic acids from “living fossil” demosponges: a link to ancient sedimentary lipids?

The lipid assemblages of the “living fossil” stromatoporoid Astrosclera willeyana (Great Barrier Reef) and the demosponge Agelas oroides (Mediterranean Sea) were investigated. Large amounts of branched carboxylic acids are present in the sponges studied. These compounds include terminally branched carboxylic acids (iso-/anteiso-) and abundant mid-chain branched carboxylic acids (MBCA) which are characterized by an intriguing variety of structural isomers present in the C15–C25 range. The most prominent MBCA are comprised of isomeric methylhexadecanoic acids and methyloctadecanoic acids. A second cluster of MBCA includes methyldocosanoic acids and methyltetracosanoic acids, but other homologues are also present. Methyl branching points were generally observed between the ω5- and ω9-positions. These complex isomeric mixtures apparently derive from symbiotic bacteria living exclusively in demosponges. Comparison with hydrocarbon compositions of ancient carbonates reveals evidence that the MBCA found are potential lipid precursors of mid-chain branched monomethylalkanes often observed in fossil sediments and oils. As a working hypothesis, we suggest that their bacterial source organisms have been widespread in the geological past, and are found “inherited” in the protective environment of distinctive sponge hosts in recent marine ecosystems. Furthermore, both sponges contain abundant linear, long-chain C24–C26 dienoic “demospongic” acids. The demospongic acid distribution and the presence of phytanic acid in A. willeyana match the patterns found in A. oroides and other members of the Agelasida. These findings confirm the systematic position of A. willeyana within this demosponge taxon.