Gagen Emma J, Wang Jiakun, Padmanabha Jagadish, Liu Jing, de Carvalho Isabela Pena Carvalho, Liu Jianxin, Webb Richard I, Al Jassim Rafat, Morrison Mark, Denman Stuart E, McSweeney Christopher S
CSIRO Agriculture, St Lucia, Australia.
School of Agriculture and Food Sciences, The University of Queensland, Gatton, Australia.
BMC Microbiol. 2014 Dec 11;14:314. doi: 10.1186/s12866-014-0314-3.
Forestomach fermentation in Australian marsupials such as wallabies and kangaroos, though analogous to rumen fermentation, results in lower methane emissions. Insights into hydrogenotrophy in these systems could help in devising strategies to reduce ruminal methanogenesis. Reductive acetogenesis may be a significant hydrogen sink in these systems and previous molecular analyses have revealed a novel diversity of putative acetogens in the tammar wallaby forestomach.
Methanogen-inhibited enrichment cultures prepared from tammar wallaby forestomach contents consumed hydrogen and produced primarily acetate. Functional gene (formyltetrahydrofolate synthetase and acetyl-CoA synthase) analyses revealed a restricted diversity of Clostridiales species as the putative acetogens in the cultures. A new acetogen (growth on H2/CO2 with acetate as primary end product) designated isolate TWA4, was obtained from the cultures. Isolate TWA4 classified within the Lachnospiraceae and demonstrated >97% rrs identity to previously isolated kangaroo acetogens. Isolate TWA4 was a potent hydrogenotroph and demonstrated excellent mixotrophic growth (concomitant consumption of hydrogen during heterotrophic growth) with glycerol. Mixotrophic growth of isolate TWA4 on glycerol resulted in increased cell densities and acetate production compared to autotrophic growth. Co-cultures with an autotrophic methanogen Methanobrevibacter smithii revealed that isolate TWA4 performed reductive acetogenesis under high hydrogen concentration (>5 mM), but not at low concentrations. Under heterotrophic growth conditions, isolate TWA4 did not significantly stimulate methanogenesis in a co-culture with M. smithii contrary to the expectation for organisms growing fermentatively.
The unique properties of tammar wallaby acetogens might be contributing factors to reduced methanogen numbers and methane emissions from tammar wallaby forestomach fermentation, compared to ruminal fermentation. The macropod forestomach may be a useful source of acetogens for future strategies to reduce methane emissions from ruminants, particularly if these strategies also include some level of methane suppression and/or acetogen stimulation, for example by harnessing mixotrophic growth capabilities.
在诸如小袋鼠和袋鼠等澳大利亚有袋动物中，前胃发酵虽然类似于瘤胃发酵，但产生的甲烷排放量较低。深入了解这些系统中的氢营养作用有助于制定减少瘤胃甲烷生成的策略。还原型产乙酸作用可能是这些系统中一个重要的氢汇，先前的分子分析已经揭示了帚尾袋鼩前胃中假定产乙酸菌的新多样性。
从帚尾袋鼩前胃内容物制备的产甲烷菌抑制富集培养物消耗氢气并主要产生乙酸盐。功能基因（甲酰四氢叶酸合成酶和乙酰辅酶A合成酶）分析表明，梭菌目物种作为培养物中的假定产乙酸菌具有有限的多样性。从培养物中获得了一种新的产乙酸菌（以氢气/二氧化碳为生长底物，乙酸盐为主要终产物），命名为菌株TWA4。菌株TWA4属于毛螺菌科，与先前分离的袋鼠产乙酸菌的16S rRNA基因序列一致性>97%。菌株TWA4是一种高效的氢营养菌，在以甘油为底物时表现出优异的兼养生长能力（在异养生长过程中同时消耗氢气）。与自养生长相比，菌株TWA4在甘油上的兼养生长导致细胞密度增加和乙酸盐产量提高。与自养产甲烷菌史氏甲烷短杆菌的共培养表明，菌株TWA4在高氢气浓度（>5 mM）下进行还原型产乙酸作用，但在低浓度下不进行。在异养生长条件下，与预期的发酵生长生物相反，菌株TWA4在与史氏甲烷短杆菌的共培养中并未显著刺激甲烷生成。
与瘤胃发酵相比，帚尾袋鼩产乙酸菌的独特特性可能是帚尾袋鼩前胃发酵中产甲烷菌数量减少和甲烷排放降低的促成因素。有袋动物前胃可能是未来减少反刍动物甲烷排放策略中有用的产乙酸菌来源，特别是如果这些策略还包括一定程度的甲烷抑制和/或产乙酸菌刺激，例如利用兼养生长能力。
