Beyond Parent-Compound Removal: Microbial Transformation Pathways, Physiological Constraints, and Evidence...

Steroid hormones occur widely in wastewater, sludge, sediments, soils, and receiving waters, where exposure at low concentrations can elicit pronounced endocrine effects. Microbial transformation is an important removal process, yet loss of a parent compound may reflect phase partitioning, structural modification, steroid-ring cleavage, or mineralization. For synthetic steroids such as 17α-ethinylestradiol (EE2) and synthetic progestins, structural compatibility with entry enzymes and downstream metabolic modules further constrains transformation depth. This review organizes current knowledge around transformation depth, linking substrate recognition to aerobic and anaerobic ring-cleavage pathways, HIP-related downstream metabolism, physiological function, and community carbon flow. Aerobic 9,10-seco and estrogen 4,5-seco pathways and the denitrifying 2,3-seco pathway are supported by metabolite, enzymatic, and genetic evidence, whereas Fe(III)-reducing, sulfate-reducing, and methanogenic systems mainly document reversible redox and stereochemical transformations of natural estrogens. We further distinguish growth-linked utilization, cometabolic modification, and potential detoxification-related homeostatic responses, and examine how community functional organization may extend pathway continuity. Finally, evidence is integrated across metabolic potential, active transformation, and steroid-derived carbon flow, while endocrine-risk outcomes are treated as a parallel environmental endpoint. This framework emphasizes that parent-compound removal alone does not establish degradation depth or risk reduction.