Inhibition of ammonia oxidation in the thermophilic ammonia‐oxidizing archaeon Nitrosocaldus yellowstonii (617.6)

Abstract

Molecular data suggests ammonia‐oxidizing archaea (AOA) play an important role in nitrogen cycling. In particular, organisms related to the thermophilic archaeon Nitrosocaldus yellowstonii appear to be the only nitrifiers present in terrestrial hydrothermal systems at temperatures above 65°C. To better understand the contribution these thermophilic archaeal nitrifiers have in geochemical processes such as nitrogen cycling, we need tools to manipulate their metabolic activity in laboratory cultures and in situ. Nitrosocaldus yellowstonii HL72 is a thermophilic AOA cultivated from a hot spring in Yellowstone National Park (Wyoming, USA). To understand how N. yellowstonii contributes to nitrogen cycling in hot springs, we must first characterize the effect of nitrification inhibitors on its metabolic function. In this study, we have focused on allylthiourea (ATU), a commonly‐used inhibitor of nitrification. We have shown that ATU is stable at temperatures up to 72°C, the growth temperature of N. yellowstonii. Ammonia oxidation in N. yellowstonii is completely inhibited by ATU concentrations of 1 mM, concentrations that are approximately 100 times higher than those required to inhibit ammonia oxidation in bacteria. Addition of 1 mM ATU decreased the rate of nitrite production of N. yellowstonii from 0.039 ± 0.001 mM/h‐1 prior to addition, to 0.001 ± 0.001 mM/h‐1. Because nitrite is used for a proxy of growth, quantitative PCR was used to measure cell numbers for the single copy gene amoA per cell. There was no change in amoA copy number after the addition of 1 mM ATU while the unamended control continued to exponentially increase, further confirming that the addition of 1 mM ATU halted cell growth completely. RT‐qPCR showed amoA expression decreased 23.5% and 47.9% compared to the unamended control, two and four hours after ATU addition. We conclude that ATU is an effective inhibitor of AOA cell growth and is will be a useful tool to manipulate metabolic activity of AOA in hot spring systems.Grant Funding Source: Supported by NIH MARC T34‐GM00857