Anammox bacteria detected in fish intestinal tract systems. © 2016 Ho Wang Chan, et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by- nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 13 RESEARCH ARTICLE Anammox bacteria detected in fish intestinal tract systems Ho Wang Chan, Han Meng and Ji-Dong Gu * Laboratory of Environmental Toxicology, School of Biological Science, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China Abstract: Anammox bacteria serve an important ecological role in the global nitrogen cycle, enabling the coupling of ammonium and nitrite to yield dinitrogen gas (N 2 ) under anoxic conditions. Based on PCR amplifications of genomic DNA and analysis of the 16S rRNA gene sequence, anammox bacteria was found in intestinal tracts of two types of fish, Siganus fuscescens and Mugil cephalus. Anammox bacteria identified in these fish belonged to the genera Brocadia and Kuenenia, suggesting that the living conditions of S. fuscescens and M. cephalus were under significant influence of wastewater pollution. Our results showed an association between the existence of anammox bacteria in fish intestinal tracts and sediment- or mud-eating habits of the fish involved. The presence of anammox bacteria in an animal system would provide a more comprehensive understanding on ecophysiological characteristics and distribution of anammox bacteria. This discovery might also provide useful information about the living conditions of fish, serving as an envi- ronmental indicator of anthropogenic pollution. Keywords: anammox, fish gut, Siganus fuscescens, Mugil cephalus, black rabbit fish, flathead mullet *Correspondence to: Ji-Dong Gu, Laboratory of Environmental Toxicology, School of Biological Science, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China; Email: jdgu@hku.hk Received: October 21, 2015; Accepted: March 8, 2015; Published Online: April 8, 2016 Citation: Chan H W, Meng H and Gu J-D, 2016, Anammox bacteria detected in fish intestinal tract systems. Applied Environmental and Biotechnology, vol.(1): 13–18. http://dx.doi.org/10.18063/AEB.2016.01.010. 1. Introduction naerobic ammonia oxidation, or anammox, is a novel pathway in the nitrogen cycle which was discovered in 1995 [1] . This pathway is governed by a group of microorganisms known as the anammox bacteria. Nitrite (NO 2 − ) and ammonium (NH 4 + ) would be used as substrates and N 2 gas would be released as the final product. Anammox bacteria serve a significant role in the ecological niche of the global nitrogen cycle. To date, 5 genera of anammox bacteria have been described, and all of them areaffi- liated with a deep branch of the phylum Planctomy- cetes. The 5 genera include Candidatus Scalindua [2] , Ca. Brocadia [3] , Ca. Anamoxoglobus [4] , Ca. Jettenia [5] , and Ca. Kuenenia [6] . Anammox bacteria are still de- scribed as ‘Candidatus’, which means that no pure culture of anammox bacteria could be obtained. Due to the unavailability of pure culture and slow growth rates of anammox bacteria [3] , many studies on anam- mox bacteria are widely based on culture-independent methods. Anammox bacteria have been identified in highly diverse ecosystems including marine sediments [7] , rice paddy fields [8] , coastal wetlands [9] , fresh water terre- strial ecosystems [10–12] , oil fields [13] , and wastewater sludges [5] . Out of the 5 genera of anammox bacteria, Ca. Scalindua is generally related to environments with low nitrogen content such as sediments from un- polluted marine environments. The other 4 genera are A