Amino Acid Enantiomeric Ratios in Semi-Labile vs. Refractory Dissolved Organic Matter: Implications for a Microbial N Pump

Microbes are key moderators in the cycling of marine dissolved organic matter (DOM), most of which remains unidentifiable at the molecular level. A major current question, however, is to what degree heterotrophic bacteria directly mediate C and N sequestration in the deep sea. Amino acids (AA) represent almost all organic N that can be identified at the molecular level, while D-AA enantiomers represent unique source-specific biomarkers for prokaryotes, known to be highly enriched in ocean DOM. If increasing bacterial-sourced material ultimately leads to millennial scale sequestration of refractory DOM (RDOM), then one would expect a clear correlation between bacterial biomarkers and DOM radiocarbon (Δ¹⁴C) ages, in particular in the deep sea. Here we directly test this idea for the first time, by measuring D-AA abundance and distribution in isolated Δ¹⁴C young vs. old DOM from the central north pacific gyre. We used a coupled ultrafiltration/solid phase extraction approach to isolate semi-labile vs. RDOM, based on known molecular weight and Δ¹⁴C age correlations. The Δ¹⁴C ages of isolated material ranged from 205-275 ybp for surface semi-labile DOM, to 6680-6740 ybp for deep ocean RDOM. We measured enantiomeric (D/L) AA ratios, as well as AA molar percentages to estimate traditional degradation parameters (DI and RI), in semi-labile vs. RDOM fractions from the surface to deep ocean at HOT, Station ALOHA. We interpret our results in terms of the hypothesis of a microbial N pump, examining the similarity of microbial source signatures (D/L distributions) and calculated organic N contributions from heterotrophic bacteria, in the context of the Δ¹⁴C age of each fraction. Finally, we also evaluate these data vs. common AA molar percentage based indices of degradation.