Inhibition of Nitrous Oxide Reduction in Forest Soil Microcosms by Different Forms of Methanobactin

Copper plays a critical role in controlling greenhouse gas emissions as it is a key component of both the particulate methane monooxygenase and nitrous oxide reductase. To collect copper, several alphaproteobacterial methanotrophs produce and excrete a small modified peptide (< 1,300 Da) - methanobactin (MB) – that has an extremely high affinity for copper. As MB is secreted by methanotrophs for copper collection, it is possible that such activity may limit the ability of microbes (e.g., denitrifiers) to take up copper, thereby decreasing their activity, as well as also affecting overall microbial community composition. Here we show that different forms of MB - one from Methylosinus trichosporium OB3b (MB-OB3b) and another from Methylocystis sp. strain SB2 (MB-SB2) affected nitrous oxide (N2O) accumulation as well caused significant shifts in microbial community composition in three different sets of forest soil microcosms. Such effects, however, were mediated by the amount of copper in the soils, with microcosms constructed from soils with the lowest amount of copper showing the strongest response to the presence of either form of MB. In addition, the presence of MB-SB2 had a stronger effect, likely due to its higher affinity for copper. Finally, it was also found that the presence of either form of MB affected the accumulation of nitrite in all three forest soils, and that the addition of MB affected the presence of genes encoding for either a copper-dependent nitrite reductase (nirK) vs. an iron-containing form (nirS). These data indicate the methanotrophic-mediated production of strong copper chelating agents can significantly impact not only methane consumption, but also multiple steps of the denitrification arm of the nitrogen cycle as well as have broad effects on microbial community composition of forest soils.

铜在温室气体排放管控中发挥关键作用，因其是颗粒性甲烷单加氧酶（particulate methane monooxygenase）与一氧化二氮还原酶的核心组成成分。为摄取铜元素，部分α-变形菌门的甲烷氧化菌会合成并分泌一种分子量小于1300道尔顿的修饰肽——甲烷菌素（methanobactin，MB），该物质对铜具有极高的结合亲和力。由于甲烷氧化菌通过分泌甲烷菌素获取铜，这类活动可能会限制其他微生物（如反硝化菌）对铜的摄取能力，进而削弱其活性，同时也会改变整体微生物群落结构。

本研究表明，两种不同来源的甲烷菌素——分别来自孢囊甲基杆菌（Methylosinus trichosporium）OB3b（MB-OB3b）与囊甲基菌属（Methylocystis）菌株SB2（MB-SB2）——会在三组不同的森林土壤微宇宙体系中，影响一氧化二氮（nitrous oxide，N₂O）的积累，并显著改变微生物群落组成。不过这类效应受土壤铜含量调控：以铜含量最低的土壤构建的微宇宙体系，对两种甲烷菌素的响应均最为强烈。此外，MB-SB2展现出更强的调控效应，这可能与其对铜的结合亲和力更高有关。

本研究还发现，两种甲烷菌素均会改变三类森林土壤中亚硝酸盐的积累量，且甲烷菌素的添加会影响编码铜依赖性亚硝酸盐还原酶（nirK）与含铁型亚硝酸盐还原酶（nirS）的基因的相对丰度。上述研究结果表明，甲烷氧化菌介导产生的强效铜螯合剂，不仅会显著影响甲烷消耗过程，还会对氮循环中反硝化通路的多个环节造成影响，并对森林土壤的微生物群落结构产生广泛调控作用。