Changes in compound specific δ15N amino acid signatures and d/l ratios in marine dissolved organic matter induced by heterotrophic bacterial reworking

Compound-specific δ15N analysis of individual amino acids (δ15N-AA) represents a potentially important new tool which may reveal the molecular-level basis for δ15N signature of dissolved organic nitrogen (DON) in the ocean, as well indicate DON sources and specific mechanisms of alteration. Past work has indicated that δ15N-AA may be effective at indicating the effects of microbial heterotrophy, however the influence of bacterial degradation on δ15N-AA patterns has never been directly investigated. Here we measured molecular-level changes in δ15N-AA patterns in freshly produced algal high molecular weight (HMW) DON due to heterotrophic bacterial reworking, together with linked changes in enantiomeric (D vs. L) AA ratios and also the AA molar percentage-based degradation index (DI). Our results show a strong increase in degradation with microbial consumption of dissolved organic carbon (DOC), consistent with previous studies. The δ15N-AA data show systematically higher δ15N values for most individual AA after DOC bacterial reworking, resulting in average increases of 3–6‰ in δ15N of total proteinaceous material. The average deviation in the δ15N values of all AA (ΣV parameter) also increased with degradation, indicating an increase in δ15N-AA pattern complexity, most likely due to selected microbial resynthesis of specific AA. These results show that δ15N-AA patterns have the ability to directly track the effects of microbial resynthesis in DON. They indicate that δ15N-AA represents a highly specific tracer that provides independent, and yet strongly complimentary, information vs. existing AA-based degradation indicators. Together, our data suggests that heterotrophic microbial degradation in the ocean would be expected to increase δ15N values of the oceanic DON pool vs. autotrophic sources. This conclusion is consistent with recent results on δ15N signatures of total and HMWDON pool in the open sea, however it also strongly implicates bacterial sources as the likely mechanism for δ15N-DON changes. Reevaluating existing DON isotopic data in light of these results may improve our understanding of the influence and mechanism of bacterial reworking on DON long-term preservation in the marine water column.

► Heterotrophic bacterial δ15N fractionation patterns of individual amino acids
► New Tracer for bacterial degradation/proteinaceous material sources
► Bacterial influence on bulk δ15N value of dissolved organic nitrogen
► Mechanism for refractory DON formation