Inhibition of the archaeal ammonia oxidation by 1,5HD
AOA are strongly inhibited by low concentrations (10 µM) of short-chain alkynes (≤ C5), but resistant to inhibition by longer-chain-length alkynes (≥ C6), even at concentrations that can completely inhibit ammonia oxidation activity in AOB12,22. Due to these compounds’ contrasting inhibition profiles of AOA and AOB, long-chain alkynes have been used in many ecological studies as differential inhibitors to distinguish bacterial and archaeal contributions to ammonia oxidation in soil21,35,36.
In this study, we investigated whether the diyne counterparts to these alkynes displayed the same differential inhibition pattern in AOA and AOB. An earlier study18 found that 1,7OD effectively inhibited ammonia and alkane oxidation in all tested bacterial strains, but was inefficient in binding to the archaeal AMO, which is consistent with previous reports that AOA were resistant to 1-octyne21. As short-chain alkynes are effective inhibitors of ammonia oxidation in AOA, we hypothesized that short-chain diynes may be better suited for activity-based probing in AOA. 1,5HD is the shortest chain-length diyne commercially available and was therefore chosen for characterization on a pure culture of N. franklandus. Previous studies found 1-hexyne to be less inhibitory to AOA than < C5 alkynes, but surprisingly, ammonia consumption by the AOA N. franklandus was completely inhibited by the addition of 100 µΜ 1,5HD, while 1 mM ammonium was stoichiometrically oxidized to nitrite in the absence of the inhibitor (Fig. 1A, B). No effect of 1,5HD on hydroxylamine oxidation was observed (Fig. 1C, D), confirming the specific interaction of 1,5HD with the AMO enzyme. These results indicated that 1,5HD, at least at 100 µΜ concentration, is able to efficiently inhibit ammonia oxidation in archaea, and thus could potentially be used for activity-based labelling of AOA.
Inhibition of bacterial ammonia and alkane oxidation by 1,5HD
To investigate the effect of 1,5HD on bacterial ammonia oxidation, we performed inhibition assays with a pure culture of N. europaea. Similar to N. franklandus, ammonia oxidation in N. europaea was completely suppressed by 100 µΜ 1,5HD (Fig. 2A, B). Again, hydroxylamine oxidation was not affected (Fig. 2C), demonstrating that 1,5HD specifically inhibits the AMO also in canonical AOB.
Comammox Nitrospira possess a phylogenetically distinct AMO9,10,29 and thus might react differently to ammonia oxidation inhibitors. To test the effect of 1,5HD on comammox bacteria, a highly enriched culture of Ca. N. kreftii was used. In the absence of the inhibitor, the culture stoichiometrically oxidized 120 µM ammonium to nitrate, whereas addition of 100 µΜ 1,5HD resulted in complete inhibition of ammonia oxidation (Fig. 3A, B). While the influence on hydroxylamine oxidation was not tested in this enrichment culture, no influence of 1,5HD on nitrite oxidation was observed (Fig. 3C, D), again showcasing the specific reaction of 1,5HD with the AMO.
Besides the ammonia monooxygenases, the CuMMO family includes enzymes that catalyze diverse reactions like the oxidation of alkanes such as methane and other short-chain (C2–C4) hydrocarbons, but still exhibit a high degree of structural similarity9–11. Thus, the ability of 1,5HD to inhibit methane-oxidizing bacteria was tested. Indeed, methane oxidation by the type Ib methanotroph M. oryzae was fully inhibited in the presence of 100 µΜ 1,5HD, while no effect on subsequent methanol oxidation was observed (Fig. 4). Thus, together with the successful labelling of additional ammonia- and alkane-oxidizing microorganisms (see below), our results indicate that 1,5HD can be employed as a universal inhibitor of CuMMO-containing microorganisms.
In situ activity-based fluorescent labelling of ammonia- and alkane-oxidizers
The diyne 1,7OD irreversibly inactivates bacterial CuMMOs via a suicide inactivation mechanism23 and has successfully been employed as a bifunctional enzyme probe that, in combination with a subsequent CuAAC reaction, allows the activity-based fluorescent staining of ammonia- and alkane-oxidizing bacteria18. However, AOA are only partly inhibited by octyne (≤ 40 µM) and this inhibition is fully reversible, indicating a different, non-covalent interaction with the archaeal AMO12. Consequently, activity-based staining of AOA cells was not possible using 1,7OD18.
In this study, we were able to demonstrate that the short-chain diyne 1,5HD efficiently inhibited ammonia oxidation in the AOA N. franklandus. Thus, its ability to be used for in situ activity-based labelling of AOA was further investigated. For this, active cultures of N. koreense, N. franklandus and Ca. N. chungbukensis were used. Following incubation with 1,5HD, ethanol fixation, and CuAAC reactions, all cultures were efficiently fluorescently stained (Fig. 5), indicating that 1,5HD can function as a bifunctional enzyme probe for the fluorescent labelling of phylogenetically diverse AOA. As the inhibition by 1,5HD was AMO-specific and did not interfere with hydroxylamine oxidation (Fig. 1), the observed fluorescence can be attributed to specific binding of 1,5HD to the AMO enzyme, rather than alternative targets in the cell. Additionally, the fact that its addition is required for fluorescence suggests the binding of 1,5HD to be covalent and irreversible. This makes 1,5HD only the third known irreversible inhibitor of the archaeal AMO besides acetylene and phenylacetylene22. Being able to fluorescently label active AOA will be a useful tool in environmental studies, as it will enable the identification of active members of mixed microbial communities based on monooxygenase enzyme activity.
The successful inhibition of ammonia- and alkane-oxidizing bacteria by 1,5HD (Figs. 2–4) also confirmed interaction of 1,5HD with the bacterial CuMMO enzymes. Consequently, when 1,5HD-treated biomass was subjected to the CuAAC protocol, strong fluorescent labelling was achieved for ammonia-oxidizing N. europaea, methane-oxidizing M. oryzae, propane-oxidizing Rhodococcus sp. ZPP and butane-oxidizing Thauera butanivorans cells (Fig. 6). In contrast, no background or unspecific labelling was observed when E. coli cells were subjected to the same protocol (Figure S2). This efficient and specific labelling indicated that 1,5HD can be used as a universal probe for the function-based detection of CuMMO-containing microorganisms. For instance, coupling activity-based probing with 1,5HD to fluorescence-activated cell sorting and sequencing-based approaches could yield novel insights into the ecology and metabolism of ammonia- and alkane-oxidizing microorganisms.
Localization of the AMO enzyme in AOA
Like its bacterial homolog, the archaeal AMO enzyme is believed to reside on and be strongly associated with the cytoplasmic membrane5 and fluorescent signals derived from AMO labelling are thus expected to colocalize with the cytoplasmic membrane. This assumption is supported by all characterized members of the CuMMO superfamily being membrane-bound10, by the archaeal AMOs containing alpha helices predicted to span the membrane16, by the native archaeal AMO complex being recovered from the membrane fraction of N. viennensis16, and by the fact that AOA lack the membrane stacks found in AOB. However, until now the archaeal AMO had not been visualized in its native environment. To visualize the subcellular AMO localization, three AOA cultures were subjected to the activity-based labelling protocol in combination with DAPI counter-staining. As expected, the localization of the AMO-derived signal was observed to coincide with the cytoplasmic membrane in high-resolution deconvolution micrographs in all cells (Fig. 7). Surprisingly, in addition to the plasma membrane in the periphery of the cells, some fluorescent labelling was evident inside the cells of N. franklandus (Fig. 7A). This distribution of fluorescent signal in N. franklandus was more akin with bacterial ammonia oxidizers (Fig. 8) than with the other AOA tested in this study. While most AOA do not contain intracellular compartments or structures, N. franklandus has been described to have some intracellular compartmentalization, including vesicle-like structures of unknown function37. While fluorescence microscopy cannot conclusively proof that these vesicle-like structures harbor AMO, it is tempting to speculate based on the signal distribution we observed that they serve a similar function in increasing available membrane space available for ammonia oxidation as has been proposed for AOB. This should be further explored in future studies using, e.g., electron microscopy approaches.
Furthermore, high-resolution deconvolution microscopy of the AOB N. europaea and the type Ib methanotroph M. oryzae revealed that the respective CuMMO-derived signals were localized along the intracytoplasmic membrane stacks and cytoplasmic membranes present in these organisms (Fig. 8), in agreement with previous studies showing the cytoplasmic membrane-associated localization of the AMO and pMMO enzymes38,39. Thus, in addition to its usability for the targeted detection of ammonia- and alkane-oxidizing microorganisms in complex environmental samples, the activity-based CuMMO labelling method presented here also has great potential to study their cellular organization using high-resolution microscopy.