Anaerobic Biodegradation of Steroids

Abstract

Steroids, comprising sterols, steroid hormones, and bile acids, are ubiquitous and abundant in nature. Steroids are largely produced by eukaryotes; however, their complete degradation is only accomplished by microorganisms. Steroids degradation is challenging because of their low aqueous solubility and complicated sterane structures. Aerobic steroids catabolism depends on molecular oxygen as a cosubstrate of oxygenases to activate and cleave the recalcitrant sterane. Consequently, steroids degradation in oxygen-limited environments (e.g., freshwater and marine sediments) is generally slow. Obviously, anaerobic biodegradation needs to substitute all oxygenase-mediated reactions by an oxygen-independent strategy to overcome the chemical recalcitrance of steroids. This article presents an overview of current knowledge on anaerobic steroids biodegradation. In the past two decades, several denitrifying proteobacteria capable of utilizing steroids as sole carbon and energy sources were described. The denitrifiers adopt a common metabolic pathway, the steroid 2,3-seco pathway, to degrade cholesterol and testosterone under anoxic conditions. The anaerobic steroid catabolic pathways include various unprecedented catabolic enzymes (e.g., the molybdoenzymes steroid C25 dehydrogenase and 1-testosterone hydratase/dehydrogenase), which introduce the hydroxyl groups in the aliphatic side-chain and sterane of steroid substrates, and the oxygen atoms originate from water. The corresponding genes were found in genomes of steroid-degrading, denitrifying bacteria. Culture-independent investigations indicated that the proposed anaerobic catabolic pathways of steroids are widespread in the environment.