Erythromycin and GC7 fail as domain-specific inhibitors for bacterial and archaeal activity in the open ocean

Domain-specific metabolic inhibitors are currently used to differentiate archaeal from bacterial activity. However, studies testing the specificity of these inhibitors are sparse or are based on cultured strains. We determined the inhibition specificity of erythromycin (EMY) and N1-guanyl-1,7-diaminoheptane (GC7) on bacterial and archaeal communities in the North Atlantic. EMY and GC7 are assumed to inhibit bacterial and archaeal activity, respectively. Heterotrophic prokaryotic activity was estimated via 3H-leucine incorporation on the cell-specific level using catalyzed reporter deposition fluorescence in situ hybridization combined with microautoradiography (MICRO-CARD-FISH). In the water masses studied, the contribution of Thaumarchaeota to total picoplankton abundance ranged from 5 to 24% while Euryarchaeota contributed 2 to 6%; the relative abundance of Bacteria ranged from 29 to 48%. The addition of EMY and GC7 reduced the bulk leucine incorporation by ∼77% and ∼41%, respectively. Evaluation of the inhibition efficiency of EMY on a cell-specific level showed no difference between Archaea (76.0 ± 14.2% [SD]) and Bacteria (78.2 ± 9.5%). Similarly, the reduction of substrate uptake in GC7-treated samples was similar in Archaea (59.9 ± 24%) and Bacteria (47.2 ± 9.6%). Taken together, our results suggest that in complex open-ocean prokaryotic communities neither EMY nor GC7 is efficient as a domain-specific inhibitor.