The degradation of the xenobiotic phthalic acid esters by microorganisms is initiated by the hydrolysis to the respective alcohols and ortho-phthalate (phthalate). In aerobic bacteria and fungi oxygenases are involved in the conversion of phthalate to protocatechuate, the substrate for ring cleaving dioxygenases. In contrast, anaerobic bacteria activate phthalate to the extremely unstable phthaloyl-CoA that is decarboxylated by oxygen-sensitive UbiD-like phthaloyl-CoA decarboxylase (PCD) to the central benzoyl-CoA intermediate. Here we demonstrate that the facultatively anaerobic, denitrifying Thauera chlorobenzoica 3CB-1 and Aromatoleum evansii KB740 strains use phthalate as growth substrate under aerobic and denitrifying conditions. In vitro assays with extracts from cells grown aerobically with phthalate demonstrated the succinyl-CoA dependent activation of phthalate followed by decarboxylation to benzoyl-CoA. In T. chlorobenzoica 3CB-1, we identified PCD as highly abundant enzyme in both aerobically and anaerobically grown cells, whereas genes for phthalate dioxygenases are missing in the genome. PCD was highly enriched from aerobically grown T. chlorobenzoica cells, and identified as identical enzyme produced under denitrifying conditions. The results obtained indicate that the initial steps of aerobic phthalate degradation in denitrifying bacteria are accomplished by the anaerobic enzyme inventory, whereas the benzoyl-CoA oxygenase dependent pathway is used for further conversion to central intermediates. Such a hybrid pathway requires intracellular oxygen homeostasis at concentrations low enough to prevent PCD inactivation but sufficiently high to supply benzoyl-CoA oxygenase with its co-substrate.

IMPORTANCE Phthalic acid esters (PAEs) are industrially produced on a million-ton scale per year, and are predominantly used as plasticizers. They are classified as environmentally relevant xenobiotics with a number of adverse health effects including an endocrine disrupting activity. Biodegradation by microorganisms is considered as the most effective process to eliminate PAEs from the environment. It is usually initiated by the hydrolysis of PAEs to alcohols and o-phthalic acid. Degradation of the latter fundamentally differs in aerobic and anaerobic microorganisms: aerobic phthalate degradation heavily depends on dioxygenase-dependent reactions, whereas anaerobic degradation employs the oxygen-sensitive key enzyme phthaloyl-CoA decarboxylase. We demonstrate that aerobic phthalate degradation in facultatively anaerobic bacteria proceeds via a previously unknown hybrid degradation pathway involving oxygen-sensitive and oxygen-dependent key enzymes. Such a strategy is essential for facultatively anaerobic bacteria that frequently switch between oxic and anoxic environments.

微生物对异种生物邻苯二甲酸酯的降解是通过水解为相应的醇和邻苯二甲酸酯（邻苯二甲酸酯）而引发的。在好氧细菌和真菌中，加氧酶参与了邻苯二甲酸酯向原儿茶酸酯的转化，原儿茶酸酯是环裂解双加氧酶的底物。相反，厌氧细菌将邻苯二甲酸酯活化为极其不稳定的邻苯二甲酰辅酶A，邻苯二甲酰辅酶A被氧敏感的UbiD样邻苯二甲酰辅酶A脱羧酶（PCD）脱羧至中心苯甲酰辅酶A中间体。在这里，我们证明了兼性厌氧，反硝化的Thauera chlorobenzoica 3CB-1和埃文氏香气菌KB740菌株在好氧和反硝化条件下使用邻苯二甲酸酯作为生长底物。体外邻苯二甲酸盐需氧生长的细胞提取物的含量测定表明邻苯二甲酸盐的琥珀酰辅酶A依赖性活化，然后脱羧成苯甲酰辅酶A。在T. chlorobenzoica 3CB-1中，我们在需氧和需氧的细胞中都鉴定出PCD是高度丰富的酶，而基因组中缺少邻苯二甲酸酯双加氧酶的基因。PCD是高度从有氧生长富集T. chlorobenzoica细胞，并鉴定为在反硝化条件下产生的相同酶。获得的结果表明，反硝化细菌中好氧邻苯二甲酸酯降解的初始步骤是通过厌氧酶库存完成的，而苯甲酰辅酶A加氧酶依赖性途径可用于进一步转化为中心中间体。这种杂化途径需要细胞内氧稳态，其浓度应足够低以防止PCD失活，但浓度要足够高以为其共底物提供苯甲酰辅酶A加氧酶。

重要邻苯二甲酸酯（PAE）的工业生产规模为每年一百万吨，主要用作增塑剂。它们被归类为与环境有关的异生物素，具有许多不利健康影响，包括破坏内分泌的活性。微生物的生物降解被认为是从环境中消除PAE的最有效方法。它通常是由PAE水解为醇和邻苯二甲酸酯引发的-邻苯二甲酸。后者的降解在需氧和厌氧微生物中根本不同：需氧邻苯二甲酸酯的降解很大程度上取决于双加氧酶依赖性反应，而厌氧降解则采用对氧敏感的关键酶邻苯二甲酰辅酶A脱羧酶。我们证明，兼性厌氧细菌中的好氧邻苯二甲酸酯降解通过涉及氧敏感和氧依赖性关键酶的先前未知的杂化降解途径进行。对于经常在有氧和无氧环境之间切换的兼性厌氧细菌而言，这种策略至关重要。