Nitrogen stable isotope patterns in free and protein-bound amino acids of the anoxygenic phototroph, Rhodopseudomonas palustris

Abstract

Compound-specific isotope analysis of amino acids (AAs) can reveal the sources and transformations of nitrogen in ecological and environmental studies. However, there is limited information on microbial patterns of ¹⁵N distributions in AAs and the intracellular biochemical processes that determine these patterns. This leads to gaps in understanding the underlying mechanisms that determine the ¹⁵N content of key N-containing biomarkers. Here we measured δ¹⁵N values of both free and protein-bound amino acids in metabolically flexible Rhodopseudomonas palustris grown photomixotrophically under anoxic conditions. Patterns of ¹⁵N distribution in protein-bound amino acids of R. palustris resemble aerobic phototrophs, implying these microbial groups may be indistinguishable in environmental samples. Intriguingly, free amino acids consistently are ca. 5 ‰ enriched in ¹⁵N relative to protein-bound amino acids; this pattern may provide a new window to understanding intracellular fractionating processes during metabolite recycling and protein synthesis.