Accumulation of deaminated peptides in anoxic sediments of Santa Barbara Basin

Proteins represent the most abundant class of biomolecules in marine sinking particles and microbial biomass, yet their cycling in marine sediments is not fully understood. To investigate whether some portion of hydrolyzed proteins escapes complete remineralization and accumulate in the pore waters, we analyzed dissolved organic matter from the anoxic sediments of Santa Barbara Basin, California, by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS). The results showed an increase in the molecular diversity and abundance of dissolved organic nitrogen (DON) formulas with depth. A comparison of the detected DON formulas to a database of small peptides (2-4 amino acid sequences) returned 119 matches, and these formulas were most abundant near the sediment surface. When we compared our detected formulas to all possible structures that would result from deamination of peptides in the database, we found 680 formula matches. However, these molecular formulas can represent hundreds of different structural isomers (in the present case as many as 3257 different deaminated peptide structures), which cannot be distinguished by the FTICR-MS settings that were used. Analysis of amino acid sequences suggests that these deaminated peptides may be the products of selective degradation of source proteins in marine sediments. We hypothesize that these deaminated peptides accumulate in the pore waters due to extracellular proteinases being inhibited from completely hydrolyzing specific peptides to free amino acids. We suggest that anaerobic microbes deaminate peptides largely to produce H₂, which is ultimately used as a reducing agent by other sediment microbes (e.g. CO₂ reduction by methanogens). Simple calculations suggest that deaminated peptides may represent ∼25-45% of DOC accumulating in these sediment pore waters. Unlike rapid remineralization of free amino acids, peptide deamination leaves behind the peptide carbon skeleton. Molecular structures of these remnant carbon skeletons may hold important clues about specific microbial processes influencing organic matter remineralization and accumulation.