The coupling of surface seawater organic nitrogen and the marine aerosol as inferred from enantiomer-specific amino acid analysis

Atmospheric deposition is now well recognised as a major source of both inorganic and organic nitrogen for many aquatic ecosystems. In the present study HPLC was used to determine D and L amino acids in remote Atlantic aerosol samples and surface seawater particulate material. This work provides molecular evidence for both microbial and phytoplankton contributions to the organic nitrogen content of the atmospheric aerosol. Samples of aerosol and surface water particulate material were collected from the Atlantic Ocean during the Atlantic Meridional Transect AMT 12 cruise. Samples were leached with water and the extracts analysed for free amino acids (DFAA) and hydrolysable amino acids (DHAA). The residual (non-leachable) particulate amino acids (PAA) were also determined in each sample. The peptidoglycan derived D-enantiomers of aspartic acid (D-Asp), glutamic acid (D-Glu), serine (D-Ser), and alanine (D-Ala) were found in significant amounts in all dissolved and suspended surface water and aerosol samples. Plant derived D-enantiomers of leucine (D-Leu) and isoleucine (D-Ile) were found mainly in the surface water samples. It is estimated that in the aerosol samples 60% of particulate amino acids, 55% of leachable combined amino acids and 8% of leachable free amino acids derive from bacterial sources. The bacterial contribution to the amino acid inventory of surface water particulate material ranged between 15 and 30% of the different fractions studied. Similarities between the composition of surface particulate material and marine aerosol provide evidence that these systems are coupled and that consequently the concepts of ‘new’ and ‘recycled’ nitrogen may apply across the air–sea interface as well as within ocean waters.