Schmitt-Wagner D, Brune A
Fakultät für Biologie, Mikrobielle Okologie, Universität Konstanz, 78457 Konstanz, Germany.
Appl Environ Microbiol. 1999 Oct;65(10):4490-6. doi: 10.1128/AEM.65.10.4490-4496.1999.
It has been shown that the coexistence of methanogenesis and reductive acetogenesis in the hindgut of the wood-feeding termite Reticulitermes flavipes is based largely on the radial distribution of the respective microbial populations and relatively high hydrogen partial pressures in the gut lumen. Using Clark-type microelectrodes, we showed that the situation in Cubitermes orthognathus and other soil-feeding members of the subfamily Termitinae is different and much more complex. All major compartments of agarose-embedded hindguts were anoxic at the gut center, and high H(2) partial pressures (1 to 10 kPa) in the alkaline anterior region rendered the mixed segment and the third proctodeal segment (P3) significant sources of H(2). Posterior to the P3 segment, however, H(2) concentrations were generally below the detection limit (<100 Pa). All hindgut compartments turned into efficient hydrogen sinks when external H(2) was supplied, but methane was formed mainly in the P3/4a and P4b compartments, and in the latter only when H(2) or formate was added. Addition of H(2) to the gas headspace stimulated CH(4) emission of living termites, indicating that endogenous H(2) production limits methanogenesis also in vivo. At the low H(2) partial pressures in the posterior hindgut, methanogens would most likely outcompete homoacetogens for this electron donor. This might explain the apparent predominance of methanogenesis over reductive acetogenesis in the hindgut of soil-feeding termites, although the presence of homoacetogens in the anterior, highly alkaline region cannot yet be excluded. In addition, the direct contact of anterior and posterior hindgut compartments in situ permits a cross-epithelial transfer of H(2) or formate, which would not only fuel methanogenesis in these compartments, but would also create favorable microniches for reductive acetogenesis. In situ rates and spatial distribution of H(2)-dependent acetogenic activities are addressed in a companion paper (A. Tholen and A. Brune, Appl. Environ. Microbiol. 65:4497-4505, 1999).
研究表明，以木材为食的黄胸散白蚁后肠中甲烷生成与还原型乙酸生成的共存很大程度上基于各自微生物种群的径向分布以及肠腔内相对较高的氢气分压。使用克拉克型微电极，我们发现直颚古白蚁及白蚁亚科其他以土壤为食的成员的情况有所不同且更为复杂。琼脂包埋后肠的所有主要区域在肠中心均为缺氧状态，碱性前部区域较高的氢气分压（1至10千帕）使混合段和第三直肠段（P3）成为氢气的重要来源。然而，在P3段之后，氢气浓度通常低于检测限（<100帕）。当供应外部氢气时，所有后肠区域都变成了高效的氢气汇，但甲烷主要在P3/4a和P4b区域生成，且仅在添加氢气或甲酸盐时才在后者区域生成。向气体顶空添加氢气会刺激活体白蚁的甲烷排放，这表明内源性氢气产生在体内也限制了甲烷生成。在后肠后部较低的氢气分压下，产甲烷菌很可能会在这种电子供体的竞争中胜过同型产乙酸菌。这或许可以解释在以土壤为食的白蚁后肠中甲烷生成明显优于还原型乙酸生成的现象，尽管尚不能排除在前部高碱性区域存在同型产乙酸菌的可能性。此外，前后肠区域在原位的直接接触允许氢气或甲酸盐进行跨上皮转移，这不仅会为这些区域的甲烷生成提供燃料，还会为还原型乙酸生成创造有利的微环境。关于依赖氢气的产乙酸活性的原位速率和空间分布在一篇配套论文中进行了探讨（A. 托伦和A. 布鲁内，《应用与环境微生物学》65:4497 - 4505，1999年）。
