By mapping translated metagenomic reads to a microbial metabolic network, we show that ruminal ecosystems that are rather dissimilar in their taxonomy can be considerably more similar at the metabolic network level. Using a new network bi-partition approach for linking the microbial network to a bovine metabolic network, we observe that these ruminal metabolic networks exhibit properties consistent with distinct metabolic communities producing similar outputs from common inputs. For instance, the closer in network space that a microbial reaction is to a reaction found in the host, the lower will be the variability of its enzyme copy number across hosts. Similarly, these microbial enzymes that are nearby to host nodes are also higher in copy number than are more distant enzymes. Collectively, these results demonstrate a widely expected pattern that, to our knowledge, has not been explicitly demonstrated in microbial communities: namely that there can exist different community metabolic networks that have the same metabolic inputs and outputs but differ in their internal structure.

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Among them, great efforts have been made to elucidate the correlations between the reaction performance and the specific physical properties of the catalyst. During this period, some secondary concepts were introduced and widely discussed such as strong metal-support interaction (SMSI), [8][9][10][11][12] hydrogen spillover, [13,14] and the confinement effect, [15][16][17][18] etc.The "confinement effect" is a vital factor affecting the reaction, focusing prominently on porous materials. It mainly includes the physical constraint effect, [19,20] the electronic effect, [21] and the molecular enrichment effect. [22][23][24] The confinement effect was introduced for the first time on zeolite catalysts by constructing a spherical confined model to estimate van der Waals adsorption energies. [25] Since then, some other zeolites, [18,26] carbon nanotubes (CNTs), [27][28][29][30] and metal-organic frameworks [31,32] (MOFs) have been stepwisely involved in this specific topic to explain the correlation between the properties of a specific local environment and the reaction activity. The most prominent feature in confinement effect is the separation of catalyst on substrate molecules based on the size of the molecules, which is also called the physical constraint effect. The catalytic performance depends on a great extent on the precise match between the size/shape of confined space and substrate, while the other effects exist in confined space due to the nature of catalysts. The structural, physical, and chemical properties of zeolites, CNTs, and MOFs are summarized in Table 1. Albeit they all have some characteristics such as well crystallinity, high surface area, and uniform pores/channels, they differ in many significant aspects as discussed below.The confinement effect in the solid acid-base catalytic system has been substantially described and summarized by Iglesia [33][34][35][36][37][38] and Lercher et al. [39,40] Zeolite, as the most representative crystalline solid acid catalyst with periodically designed pore structure, has been widely applied in both fundamental and applied catalytic research. It exhibits diverse frameworks and tunable pore structures, [41,42] and features exceptional chemical and thermal stability, as well as unique Lewis and Brønsted acid sites. [43] Iglesia et al. suggested that the confined location of Brønsted acid sites in zeolites played an essential role in the catalytic consequences. [23] Furthermore, through extensive works, using zeolite as a host, they concluded

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