Stable carbon isotope composition (δ13C values) of the halogenated monoterpene MHC-1 as found in fish and seaweed from different marine regions

Abstract

Six samples of the red seaweed Plocamium cartilagineum (two both from Helgoland/Germany, Ireland, and the Antarctic) were analyzed by stable isotope analysis. The bulk δ¹³C values (−31 to −38‰) confirmed that this seaweed was exceptionally highly depleted in ¹³C. The δ¹⁵N values were in the reported range for algae, and a moderate correlation between ¹³C and ¹⁵N content was observed. These measurements indicated that the season had a higher impact on the δ¹³C values than the location. Compound specific carbon isotope analysis by gas chromatography coupled with isotope ratio mass spectrometry (GC-IRMS) was used to verify an even stronger depletion in ¹³C in the halogenated natural product (HNP) MHC-1 isolated from the seaweed samples. The δ¹³C values of MHC-1 in the range −42 to −45‰ are the lightest determined to date for polyhalogenated compounds in the molecular mass range between 250 and 800 Da. MHC-1 was also isolated from two fish samples. The much higher ¹³C content in MHC-1 from the fish samples (−35.6 to −39.2‰) indicated carbon isotope enrichment due to partial transformation. In processes such as the reductive debromination, the partial transformation is associated with an increase in the ¹³C content in the remaining non-metabolized share of a compound. Stable isotope analysis may thus provide insights into the degree of transformation of HNPs in environmental samples.