Influence of nitrogen substrates and substrate C:N ratios on the nitrogen isotopic composition of amino acids from the marine bacterium Vibrio harveyi

Nitrogen (N) isotopic compositions of individual hydrolysable amino acids (δ 15 N AAs ) in N pools have been increasingly used for trophic position assessment and evaluation of sources and transformation processes of organic matter in marine environments. However, there are limited data about variability in δ 15 N AAs patterns and how this variability influences marine bacteria, an important mediator of trophic transfer and organic matter transformation. We explored whether marine bacterial δ 15 N AAs profiles change depending on the type and C:N ratio of the substrate. The δ 15 N AAs profile of a marine bacterium, Vibrio harveyi , was examined using medium containing either glutamate, alanine or ammonium as the N source [substrate C:N ratios (range, 3 to 20) were adjusted with glucose]. The data were interpreted as a reflection of isotope fractionations associated with de novo synthesis of amino acids by bacteria. Principal component analysis (PCA) using the δ 15 N offset values normalized to glutamate + glutamine δ 15 N revealed that δ 15 N AAs profiles differed depending on the N source and C:N ratio of the substrate. High variability in the δ 15 N offset of alanine and valine largely explained this bacterial δ 15 N AAs profile variability. PCA was also conducted using bacterial and phytoplankton (cyanobacteria and eukaryotic algae) δ 15 N AAs profile data reported previously. The results revealed that bacterial δ 15 N AAs patterns were distinct from those of phytoplankton. Therefore, the δ 15 N AAs profile is a useful indicator of biochemical responses of bacteria to changes in substrate conditions, serving as a potentially useful method for identifying organic matter sources in marine environments.