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.