Carere Carlo R, McDonald Ben, Peach Hanna A, Greening Chris, Gapes Daniel J, Collet Christophe, Stott Matthew B
Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand.
Scion, Te Papa Tipu Innovation Park, Rotorua, New Zealand.
Front Microbiol. 2019 Aug 16;10:1873. doi: 10.3389/fmicb.2019.01873. eCollection 2019.
Metabolic flexibility in aerobic methane oxidizing bacteria (methanotrophs) enhances cell growth and survival in instances where resources are variable or limiting. Examples include the production of intracellular compounds (such as glycogen or polyhydroxyalkanoates) in response to unbalanced growth conditions and the use of some energy substrates, besides methane, when available. Indeed, recent studies show that verrucomicrobial methanotrophs can grow mixotrophically through oxidation of hydrogen and methane gases respiratory membrane-bound group 1d [NiFe] hydrogenases and methane monooxygenases, respectively. Hydrogen metabolism is particularly important for adaptation to methane and oxygen limitation, suggesting this metabolic flexibility may confer growth and survival advantages. In this work, we provide evidence that, in adopting a mixotrophic growth strategy, the thermoacidophilic methanotroph, sp. RTK17.1 changes its growth rate, biomass yields and the production of intracellular glycogen reservoirs. Under nitrogen-fixing conditions, removal of hydrogen from the feed-gas resulted in a 14% reduction in observed growth rates and a 144% increase in cellular glycogen content. Concomitant with increases in glycogen content, the total protein content of biomass decreased following the removal of hydrogen. Transcriptome analysis of sp. RTK17.1 revealed a 3.5-fold upregulation of the Group 1d [NiFe] hydrogenase in response to oxygen limitation and a 4-fold upregulation of nitrogenase encoding genes () in response to nitrogen limitation. Genes associated with glycogen synthesis and degradation were expressed constitutively and did not display evidence of transcriptional regulation. Collectively these data further challenge the belief that hydrogen metabolism in methanotrophic bacteria is primarily associated with energy conservation during nitrogen fixation and suggests its utilization provides a competitive growth advantage within hypoxic habitats.
好氧甲烷氧化细菌（甲烷营养菌）中的代谢灵活性可增强细胞在资源可变或有限情况下的生长和存活能力。例如，在生长条件不平衡时会产生细胞内化合物（如糖原或聚羟基脂肪酸酯），并且在有可用资源时，除了甲烷外还会利用一些能量底物。事实上，最近的研究表明，疣微菌门甲烷营养菌可以通过分别氧化氢气和甲烷气体进行混合营养生长，呼吸膜结合的1d型[NiFe]氢化酶和甲烷单加氧酶参与其中。氢代谢对于适应甲烷和氧气限制尤为重要，这表明这种代谢灵活性可能赋予生长和存活优势。在这项工作中，我们提供了证据，即嗜热嗜酸甲烷营养菌 在采用混合营养生长策略时，会改变其生长速率、生物量产量以及细胞内糖原储备的产生。在固氮条件下，从进料气中去除氢气导致观察到的生长速率降低14%，细胞糖原含量增加144%。随着糖原含量的增加，去除氢气后生物量的总蛋白质含量下降。对 的转录组分析显示，响应氧气限制，1d型[NiFe]氢化酶上调3.5倍，响应氮限制，固氮酶编码基因（ ）上调4倍。与糖原合成和降解相关的基因组成性表达，未显示转录调控的证据。这些数据共同进一步挑战了甲烷营养细菌中的氢代谢主要与固氮过程中的能量守恒相关的观点，并表明其利用在缺氧生境中提供了竞争生长优势。
