Gc c 3

Sponge samples were sent to the Isotope Biogeochemistry Laboratory at the University of Hawai'i at Mānoa for δ 15 N and δ 13 C analysis of individual amino acids. Both δ 15 N and δ 13 C, were quantified using gas chromatography/combustion-isotope ratio mass spectrometry (GC/C-IRMS) after amino acid extraction and derivatization. Samples were analyzed using either a Thermo Scientific Delta V Plus or a Thermo Scientific MAT 253 mass spectrometer interfaced with a Thermo Finnigan Trace GC gas chromatograph via a Thermo Finnigan GC-C III. The amino acids measured using this technique were alanine (Ala), glycine (Gly), threonine (Thr), serine (Ser), valine (Val), leucine (Leu), isoleucine (Iso), proline (Pro), aspartic acid (Asp), phenylalanine (Phe) and lysine (Lys). The terminal amide groups in glutamine (Gln) and aspartame (Asn) were cleaved during the chemical isolation of amino acids. This resulted in the conversion of these amino acids to glutamic acid (Glu) and aspartic acid (Asp), respectively. Thus, the isotope value of the combined Glu + Gln was measured (termed Glx), and the isotope value of a combined Asn + Asp was measured (termed Asx). Averaged AA δ 13 C and δ 15 N values from these analyses can be found in Tables S1 andS2, while additional information on CSIA is available in the supplemental document.