Characterization of macromolecular organic matter in sediment traps from the northwestern Mediterranean Sea

Abstract

Curie point-pyrolysis-gas chromatography (CuPy-GC) and Curie point-pyrolysis-gas chromatography-mass spectrometry (CuPy-GC/MS) were applied to chemically characterize the macromolecular content of large particles collected from sediment traps in the northwestern Mediterranean Sea at 100, 200, 1000, and 2000 m depth. The samples were also examined by scanning electronic microscopy (SEM). A field sample rich in the diatom Biddulphia sinensis was used as a reference for contribution of photosynthetic organisms and compared to the four sediment trap samples. Several n-alkanes and n-alk-1-enes as well as aliphatic alkyl nitriles could be assigned to aliphatic resistant biopolymers. Bound fatty acids were present in the upper traps ( 100 and 200 m) and absent in the lower ones (1000 and 2000 m), indicating the absence of esterified acid moieties in the latter. These observations indicated that in the sediment trap material, ester hydrolysis has been completed probably via hydrolytic enzymes, and that in the lower trap samples, the alk-1-enes and alkanes may be derived exclusively from aliphatic resistant macromolecules, whereas in the upper samples, they represent aliphatic resistant macromolecules and esterified moieties in macromolecules. Fragments such as phytadienes and pristenes likely derive from photosynthetic organisms, whereas their natural product precursors are different. In the upper samples, a suite of dipeptide-type pyrolysis products, assumed to be “true protein indicators,” were detected. The same suite of compounds was present in the pyrolysates of the field B. sinensis-rich sample. In all pyrolysates, pyrroles, indoles and aromatic nitriles were also found. The important contributions of these N-containing compounds at greater depth, i.e., 1000 and 2000 m, is noteworthy. Except for the pyrroles, their contribution to the pyrolysate of the B. sinensis-rich sample was also significant. Their abundances did not match those of the “true protein indicators.” This led to the conclusion that they were produced upon pyrolysis of a highly resistant precursor, probably from algae, which is not proteinaceous in nature. In all samples, some relatively stable polysaccharide-containing constituents could be observed. Aromatic hydrocarbons such as alkyl benzenes, naphthalenes, indanes, and indenes present in all pyrolysates may partly be derived from proteins, especially in the upper samples, and from unknown resistant precursors. A phenolic series detected encompassed phenol, 1-, 2-,and 3-methylphenols and dimethylphenols. They were important contributors to all the pyrolysates and were also present in the pyrolysate of the field B. sinensis-rich sample. They are possibly produced from autochthonous resistant macromolecules occurring in algae. Overall, we concluded that the preservation of known and some presently unknown biomacromolecules biosynthesized by algae, either by selective preservation or by rapid sinking, could be a major process determining the quality and quantity of sinking and depositing organic matter in the ocean.