Heterotrophic growth on formate is dependent on the maintenance of intracellular pH for the thermoacidophilic methanotroph Methylacidiphilum sp RTK17.1.

In this work, we investigated intracellular pH homeostasis within the thermoacidophilc methanotroph Methylacidiphilum sp. RTK17.1. Our findings show the proton motive force for this species is primarily generated by a pH gradient across the cellular membrane. In batch experiments, the addition of formate resulted in no observable cell growth and, correspondingly, acidification of the cytosol, decreased formate dehydrogenase activity and (presumably) cell-death. Nevertheless, we were able to demonstrable growth on formate as the sole source of metabolizable energy was possible in steady-state (continuous) cultures following the transition from methanol to formate. Under these conditions, biomass productivity yields on formate were 63% less than for growth on methanol. Transcriptome analysis revealed key genes associated with pH homeostasis, methane, methanol and formate metabolism were significantly regulated in response to growth on formate. Collectively, these results suggest environmental formate represents a utilisable source of energy/carbon to the acidophilic methanotrophs during periods of methane starvations and highlights potential short-comings of traditional batch-culture physiological characterisation studies in acidophilic species. Overall design: Chemostat cell samples (3 total) of Methylacidiphilum sp. RTK17,1 were grown methylotrophically (pH 2.5, 45C) under oxygen limiting conditions in the presence of NH4+. Following the establishment of a steady state condition, methanol was replaced with an equimolar concentration of formate

本研究针对嗜热嗜酸甲烷氧化菌（thermoacidophilic methanotroph）Methylacidiphilum sp. RTK17.1的细胞内pH稳态（intracellular pH homeostasis）展开探究。研究结果表明，该菌株的质子驱动力（proton motive force）主要由细胞膜两侧的pH梯度产生。在分批培养实验（batch experiments）中，添加甲酸盐（formate）后未观测到细胞生长，同时伴随细胞质基质（cytosol）酸化、甲酸脱氢酶（formate dehydrogenase）活性下降，且（推测）出现细胞死亡。尽管如此，在从甲醇切换至甲酸盐的稳态（连续）培养（steady-state (continuous) cultures）体系中，我们证实可实现以甲酸盐作为唯一可代谢能源的细胞生长。在此条件下，以甲酸盐为碳源的生物质产率较甲醇生长条件低63%。转录组分析（transcriptome analysis）结果显示，与pH稳态、甲烷、甲醇及甲酸盐代谢相关的关键基因，在以甲酸盐为碳源生长时发生了显著调控。综合来看，上述结果表明，在甲烷匮乏时期，环境中的甲酸盐可作为嗜酸甲烷氧化菌的可利用能源/碳源；同时本研究也揭示了传统分批培养生理特性表征研究在嗜酸菌株中存在的潜在局限性。实验整体设计：将Methylacidiphilum sp. RTK17.1的恒化器（chemostat）培养细胞样本（共3份）以甲基营养型方式培养，培养条件为pH 2.5、温度45℃，处于限氧环境且存在铵离子（NH4+）。待培养体系达到稳态后，用等摩尔浓度的甲酸盐替换甲醇。