Chronic Exposure to Sublethal Concentrations of Aquatic Nitrite Change Nose and Gut Morphology and Associated Microbiomes in Channel Catfish.

Nitrite is a common pollutant that can enter waterways via agricultural runoff or build up in aquaculture ponds when nitrogenous waste is converted to nitrite. Preliminary studies by the Huertas lab found that exposure to sublethal concentrations of aquatic nitrite alters the microbiome of goldfish and bioaccumulates in the olfactory epithelia, gills and brain. These studies also showed a negative impact on the olfactory tissue of exposed fish, which could lead to higher susceptibility to nasal pathogens. Our objective is to assess nitrite bioaccumulation in tissues and the alteration of the microbiome in the key aquaculture species channel catfish (Ictalurus punctatus). We hypothesize that the relative abundance and diversity of bacteria in the nose and gut microbiomes of channel catfish will be altered due to elevated nitrite exposure and that nitrite will bioaccumulate in tissues. Channel catfish were exposed to 0 mM (control), 0.2 mM, and 2 mM concentrations of nitrite. A total of six 500 liter tanks were used in a two-phase experiment. For phase one, 16 fish per tank were placed into three 0 mM nitrite tanks and three 2 mM nitrite tanks. The 2 mM fish were kept for 12 hours before sampling. For phase two, 24 of the phase one control fish were placed into three 0.2 mM tanks and the remaining 24 control fish were left in the three control tanks for a total of 8 fish per tank. Phase two lasted 30 days with sub-samplings at day five and 30. All fish were sampled for nose, gut, muscle, gill, brain, blood, liver, and kidney tissues for biochemical, microbial, or histological analysis. Upon dissection, fish from the 2 mM tanks exhibited marked methemoglobinemia due to nitrite-induced oxidation of hemoglobin. The mortality rates were 0.04%, 0.08%, and 92% for the control, 0.2 mM, and 2 mM treatments respectively. The nose and gut microbiomes will be sequenced using whole genome shotgun sequencing to determine the microbial composition and measure changes in relative abundance. Histological analysis showed oxidative damage in all tissues from individuals exposed to 2 mM and partial damage in tissues of the 0.2 mM treatment. Because the microbiome is a powerful barrier against pathogens, disruption of the microbiome could lead to higher rates of disease infection and transmission in fish. Alteration of the nasal microbiome may increase mortality from the bacteria that cause septicemia in catfish, Edwardsiella ictaluri, whose common routes of infection are the nose or gut. Changes to the microbiome could be the missing link between nitrite and increased susceptibility to E. ictaluri infections in intensive farming. In the long term, this research aims to provide guidelines for water quality management in order to decrease the rate of bacterial infection and transmission in aquaculture systems.