D-Amino Acids in Marine and Terrestrial Environments: Diversity and Uniformity of Their Microbial Catabolism

D-amino acids (DAAs), once considered minor enantiomers, are now recognised as abundant and dynamic components of marine and terrestrial organic matter. While they do not participate in ribosomal protein synthesis, they play crucial roles in microbial physiology, particularly in bacterial cell wall structure. This review systematically synthesises the current understanding of DAA sources, distribution and fate, with a central focus on the microbial catabolic pathways that drive their recycling in marine and terrestrial environments. We show that, despite differences in DAA distribution and bioavailability between marine and terrestrial ecosystems, the core catabolic strategies adopted by bacteria-conversion to α-keto acids, L-amino acids (LAAs) or Gly-are largely conserved. We also evaluate limitations in current studies and major knowledge gaps, including the unclear role of marine fungi in DAA turnover and the relative lack of systematic studies on DAA-catabolising taxa and pathways in terrestrial microbial communities. This review highlights the ecological significance of microbial-mediated DAA recycling in marine and terrestrial environments, offering a better understanding of the global biogeochemical cycling of DAAs.