Toxic effects of linear alkylbenzene sulfonate on metabolic activity, growth rate, and microcolony formation of Nitrosomonas and Nitrosospira strains

Strong inhibitory effects of the anionic surfactant linear alkylbenzene sul fonate (LAS) on four strains of autotrophic ammonia-oxidizing bacteria (AOB ) are reported. Two Nitrosospira strains were considerably more sensitive t o LAS than two Nitrosomonas strains were, Interestingly, the two Nitrosospi ra strains showed a weak capacity to remove LAS from the medium. This could not be attributed to adsorption or any other known physical or chemical pr ocess, suggesting that biodegradation of LAS took place, In each strain, th e metabolic activity (50% effective concentration [EC50], 6 to 38 mg liter( -1)) was affected much less by LAS than the growth rate and viability (EC50 , 3 to 14 mg liter(-1)) were. However, at LAS levels that inhibited growth, metabolic activity took place only for 1 to 5 days, after which metabolic activity also ceased. The potential for adaptation to LAS exposure was inve stigated with Nitrosomonas europaea grown at a sublethal LAS level (10 mg l iter(-1)); compared to control cells, preexposed cells showed severely affe cted cell functions (cessation of growth, loss of viability, and reduced NH 4+ oxidation activity), demonstrating that long-term incubation at subletha l LAS levels was also detrimental. Our data strongly suggest that AOB are m ore sensitive to LAS than most heterotrophic bacteria are, and we hypothesi ze that thermodynamic constraints make AOB more susceptible to surfactant-i nduced stress than heterotrophic bacteria are. We further suggest that AOB may comprise a sensitive indicator group which can be used to determine the impact of WS on microbial communities.