Variability of Total Dissolved Amino Acids in the North Atlantic Ocean Reveal Microbial Transformation of Dissolved Organic Matter on Diel to Seasonal Time Scales

Dissolved organic matter (DOM) production via foodweb processes and subsequent microbial oxidation lead to compositional transformation that can be assessed by examining changes in specific organic components such as total dissolved amino acids (TDAA). The contribution of TDAA to bulk DOM and changes in the mole fraction of individual amino acid monomers provide useful indices of diagenetic alteration of DOM. We analyzed data collected from the interdisciplinary BIOS-SCOPE (31°40’ N at Bermuda Atlantic Time-Series (BATS) site) and NAAMES (39 – 57°N) programs to assess the fate and compositional transformation of DOM (0-300m) on diel to seasonal time-scales. At BATS, the TDAA composition showed a diel pattern shifting from fresh DOM during the day to more degraded DOM at night. Over longer seasonal time scales, DOM dynamics followed the annual trend of accumulation in the surface during spring-summer stratified periods followed by downward flux during winter convective mixing at BATS. Clear transformation of DOM in epipelagic and mesopelagic was observed on a seasonal time-scale at BATS. In addition, short 3-4 day process studies were conducted during the time of deep convective mixing at both BATS (2017) and NAAMES (2016) stations where DOC profiles indicated a local export event. TDAA concentrations and degradation index increased in the mesopelagic water during mixing indicating export of fresh DOM. During and shortly following deep mixing, mesopelagic bacterioplankton growth increased at both the BATS and NAAMES stations. Complementary TDAA and microbial measurements reveal how the diagenetic status of DOM changed before, during and after mixing. Studying temporal and spatial variability of DOC and TDAA helps to understand the carbon flux and microbial alteration of the quantity and quality of DOM in the ocean.