Mikrobiologie, Fakultät für Biologie, Schänzlestraße 1, D-79104 Freiburg, Germany.
Appl Environ Microbiol. 2011 Sep;77(17):6181-8. doi: 10.1128/AEM.00705-11. Epub 2011 Jul 15.
Chloroflexus aurantiacus is a facultative autotrophic green nonsulfur bacterium that grows phototrophically in thermal springs and forms microbial mats with cyanobacteria. Cyanobacteria produce glycolate during the day (photorespiration) and excrete fermentation products at night. C. aurantiacus uses the 3-hydroxypropionate bi-cycle for autotrophic carbon fixation. This pathway was thought to be also suited for the coassimilation of various organic substrates such as glycolate, acetate, propionate, 3-hydroxypropionate, lactate, butyrate, or succinate. To test this possibility, we added these compounds at a 5 mM concentration to autotrophically pregrown cells. Although the provided amounts of H(2) and CO(2) allowed continuing photoautotrophic growth, cells immediately consumed most substrates at rates equaling the rate of autotrophic carbon fixation. Using [(14)C]acetate, half of the labeled organic carbon was incorporated into cell mass. Our data suggest that C. aurantiacus uses the 3-hydroxypropionate bi-cycle, together with the glyoxylate cycle, to channel organic substrates into the central carbon metabolism. Enzyme activities of the 3-hydroxypropionate bi-cycle were marginally affected when cells were grown heterotrophically with such organic substrates. The 3-hydroxypropionate bi-cycle in Chloroflexi is unique and was likely fostered in an environment in which traces of organic compounds can be coassimilated. Other bacteria living under oligotrophic conditions acquired genes of a rudimentary 3-hydroxypropionate bi-cycle, possibly for the same purpose. Examples are Chloroherpeton thalassium, Erythrobacter sp. strain NAP-1, Nitrococcus mobilis, and marine gammaproteobacteria of the OM60/NOR5 clade such as Congregibacter litoralis.
橙光合细菌是一种兼性自养的绿色非硫细菌，它在热泉中通过光合作用生长，并与蓝细菌形成微生物垫。蓝细菌在白天（光呼吸）产生乙醇酸，并在夜间排出发酵产物。橙光合细菌利用 3-羟基丙酸生物循环进行自养碳固定。人们认为这条途径也适合于各种有机基质的共同化，如乙醇酸、乙酸盐、丙酸盐、3-羟基丙酸、乳酸盐、丁酸盐或琥珀酸盐。为了验证这一可能性，我们将这些化合物以 5mM 的浓度添加到自养预培养的细胞中。尽管提供的 H(2)和 CO(2)量允许继续进行光自养生长，但细胞立即以与自养碳固定相同的速率消耗大部分底物。使用 [(14)C]乙酸盐，一半的标记有机碳被掺入细胞物质中。我们的数据表明，橙光合细菌利用 3-羟基丙酸生物循环，与乙醛酸循环一起，将有机基质导入中心碳代谢。当细胞以这些有机底物异养生长时，3-羟基丙酸生物循环的酶活性受到轻微影响。Chloroflexi 中的 3-羟基丙酸生物循环是独特的，可能是在可以共同化痕量有机化合物的环境中形成的。其他在寡营养条件下生活的细菌获得了原始 3-羟基丙酸生物循环的基因，可能出于同样的目的。例如 Chloroherpeton thalassium、Erythrobacter sp. strain NAP-1、Nitrococcus mobilis 和海洋 γ-变形菌 OM60/NOR5 分支的 Congregibacter litoralis 等。
