<jats:p>Sponges produce distinct fatty acids (FAs) that (potentially) can be used as chemotaxonomic and ecological biomarkers to study endosymbiont-host interactions and the functional ecology of sponges. Here, we present FA profiles of five common habitat-building deep-sea sponges (class Demospongiae, order Tetractinellida), which are classified as high microbial abundance (HMA) species. <jats:italic>Geodia hentscheli</jats:italic>, <jats:italic>G</jats:italic>. <jats:italic>parva</jats:italic>, <jats:italic>G</jats:italic>. <jats:italic>atlantica</jats:italic>, <jats:italic>G</jats:italic>. <jats:italic>barretti</jats:italic>, and <jats:italic>Stelletta rhaphidiophora</jats:italic> were collected from boreal and Arctic sponge grounds in the North-Atlantic Ocean. Bacterial FAs dominated in all five species and particularly isomeric mixtures of mid-chain branched FAs (MBFAs, 8- and 9-Me-C<jats:sub>16:0</jats:sub> and 10- and 11-Me-C<jats:sub>18:0</jats:sub>) were found in high abundance (together ≥ 20% of total FAs) aside more common bacterial markers. In addition, the sponges produced long-chain linear, mid- and <jats:italic>a(i)-</jats:italic>branched unsaturated FAs (LCFAs) with a chain length of 24‒28 C atoms and had predominantly the typical Δ<jats:sup>5,9</jats:sup> unsaturation, although the Δ<jats:sup>9,19</jats:sup> and (yet undescribed) Δ<jats:sup>11,21</jats:sup> unsaturations were also identified. <jats:italic>G</jats:italic>. <jats:italic>parva</jats:italic> and <jats:italic>S</jats:italic>. <jats:italic>rhaphidiophora</jats:italic> each produced distinct LCFAs, while <jats:italic>G</jats:italic>. <jats:italic>atlantica</jats:italic>, <jats:italic>G</jats:italic>. <jats:italic>barretti</jats:italic>, and <jats:italic>G</jats:italic>. <jats:italic>hentscheli</jats:italic> produced similar LCFAs, but in different ratios. The different bacterial precursors varied in carbon isotopic composition (δ<jats:sup>13</jats:sup>C), with MBFAs being more enriched compared to other bacterial (linear and <jats:italic>a</jats:italic>(<jats:italic>i</jats:italic>)-branched) FAs. We propose biosynthetic pathways for different LCFAs from their bacterial precursors, that are consistent with small isotopic differences found in LCFAs. Indeed, FA profiles of deep-sea sponges can serve as chemotaxonomic markers and support the concept that sponges acquire building blocks from their endosymbiotic bacteria.</jats:p>