Bridging a Gap in Marine Sulfur Cycling: Discovery of a d‑Cysteinolic Acid Degradation Pathway

d-Cysteinolic acid is a prominent sulfur-containing
compound
in marine and freshwater ecosystems, yet its metabolic fate has remained
largely uncharacterized. Here, we describe a newly identified d-cysteinolic acid degradation pathway in a marine model bacterium Ruegeria pomeroyi DSS-3. This pathway involves a
PLP-dependent cysteinolic acid racemase (ClaA) that interconverts d- and l-cysteinolic acids, followed by a NAD+-dependent l-cysteinolic acid dehydrogenase (ClaB) that
oxidizes l-cysteinolic acid to l-cysteate. A cysteate
racemase (CuyB) then converts l-cysteate to d-cysteate,
the preferred substrate for the sulfolyase CuyA. Bioinformatic analysis
of the Tara Oceans gene atlas reveals that ClaA and CuyA homologues
are widespread in marine bacterial populations, particularly within
the alphaproteobacterial Roseobacter and SAR116 groups and the gammaproteobacterial
SUP05 clade. Relative abundance of gene homologues correlates with
surface water chlorophyll levels, linking d-cysteinolic acid
degradation to photosynthetic primary production. This discovery advances
our understanding of marine sulfur cycling and highlights d-cysteinolic acid as an important metabolic currency in microbial
networks.