The implementation of a stereoselective Michael addition with water as substrate is still a major challenge by classical, chemical means. Inspired by nature's ability to carry out this attractive reaction with both high selectivity and efficiency, the interest in hydratases (EC 4.2.1.x) to accomplish a selective water addition is steadily rising. The gram-positive bacterial genus Rhodococcus is known as biocatalytic powerhouse and has been reported to hydrate various Michael acceptors leading to chiral alcohols. This study aimed at the in-depth re-investigation of the hydration potential of Rhodococcus whole-cells towards Michael acceptors. Here, two concurrent effects responsible for the hydration reaction were found: while the majority of substrates was hydrated in an oxygen-independent manner by amino-acid catalysis, an enzyme-catalysed water addition to (E)-4-hydroxy-3-methylbut-2-enoic acid was proven to be oxygen-dependent. ¹⁸O₂-labelling studies showed that no ¹⁸O₂ was incorporated in the product. Therefore, a novel O₂-dependent hydratase distinct from all characterised hydratases so far was found.