Free amino acids in marine rains: evidence for oxidation and potential role in nitrogen cycling

Abstract

Previous studies of dissolved organic nitrogen (DON) in precipitation have addressed various aspects of nutrient transport and global nitrogen cycling¹. In most of these studies however, the detailed chemical composition of DON was not determined. Analyses of specific organic nitrogen compounds within precipitation can yield new information about sources and transformations of DON as well as about heterogeneous oxidative processes in the atmosphere. Dissolved free amino acids (DFAA) are a class of compounds for which surprisingly few analyses in precipitation have been published1. We report here the first detailed analyses of DFAA and primary amines in marine rains. Unexpectedly high concentrations of total DFAA were measured, averaging about 6.5 μM (16 marine rain samples) and ranging from 1.1 to 15.2 μM (0.015 to 0.21 p.p.m. N); these values are similar to those obtained for inorganic nitrogen species². Amino acids are predominantly found in rain as their L optical isomers and therefore are most likely biological in origin; however, the exact sources and modes of enrichment in rain over the open ocean are not known. There is evidence that some of the amino acids, in particular methionine, are oxidized in the atmosphere possibly by a heterogeneous photochemical pathway. The finding of high DFAA and primary amine concentrations in marine rain may have important implications in global nitrogen cycling and also may contribute locally to available nitrogen at the sea surface.