Compound-specific nitrogen isotope analysis of amino acids (¹⁵N-AA) is a potentially powerful technique for investigating mechanistic pathways of microbial degradation of organic material (OM) in aquatic environments. The technique leverages differences in isotopic fractionation among individual amino acids during metabolic processing. Four general microbial metabolic patterns of ¹⁵N-AA fractionation have been proposed: 1) algal-like de novo synthesis; 2) animal-like heterotrophy, 3) selective microbial resynthesis, and 4) extracellular hydrolysis. However, only a few of these patterns have been clearly demonstrated outside of the laboratory. We investigated ¹⁵N-AA fractionation in biogeochemically contrasting wastewater treatment plants (WWTPs) employing enhanced primary (physical-chemical flocculation) and secondary (sedimentation plus conversion of ammonia to nitrate) treatment. Patterns of ¹⁵N-AA were similar in the influent particulate OM at both WWTPs, but diverged with successive treatment, becoming lighter by an average -3‰ across all AAs in the primary WWTP (pattern 1) and heavier by +6‰ in the secondary WWTP (pattern 4). The concentrations of total hydrolysable AAs, D/L ratios, and the “sum-V” parameter for pattern (3) remained relatively unchanged among treatment stages, suggesting that metabolisms involving de novo synthesis or extracellular hydrolysis may be invisible to these degradation proxies in nutrient-rich environments. These results add new empirical evidence for metabolically distinct ¹⁵N-AA fractionation patterns in detrital organic material.

Primary Presenter: Owen Sherwood, Dalhousie University (owen.sherwood@dal.ca)

Authors:

Rachel Noddle, Dalhousie University  (rachel.noddle@dal.ca)

Amina Stoddart, Dalhousie University  (Amina.Stoddart@Dal.Ca)

Carolyn Buchwald, Dalhousie University  (cbuchwald@dal.ca)

Matthew McCarthy, University of California, Santa Cruz  (mdmccar@ucsc.edu)

Owen Sherwood, Dalhousie University  (owen.sherwood@dal.ca)