Physiological insights into largemouth bass (Micropterus salmoides) survival during long-term exposure to high environmental ammonia.

Waterborne ammonia is an environmental pollutant that is toxic to all aquatic animals. However, ammonia induced toxicity as well as compensatory mechanisms to defend against high environmental ammonia (HEA) are not well documented at present for largemouth bass (Micropterus salmoides), a high value fish for culture and sport fisheries in the United States. To provide primary information on the sensitivity of this species to ammonia toxicity, a 96 h-LC50 test was conducted. Thereafter, responses at physiological, ion-regulatory and transcript levels were determined to get insights into the underlying adaptive strategies to ammonia toxicity. For this purpose, fish were progressively exposed to HEA (8.31 mg/L representing 25% of 96 h-LC50 ) for 3, 7, 14, 21 and 28 days. Temporal effects of HEA on oxygen consumption rate (MO2 ), ammonia and urea dynamics, plasma ions (Na+ , Cl- and K+ ), branchial Na+ /K+ -ATPase (NKA) and H+ -ATPase activity, muscle water content (MWC), energy store (glycogen, lipid and protein) as well as branchial mRNA expression of Rhesus (Rh) glycoproteins were assessed. Probit analysis showed that 96 h-LC50 of (total) ammonia (as NH4 HCO3 ) at 25 °C and pH 7.8 was 33.24 mg/L. Results from sub-lethal end-points shows that ammonia excretion rate (Jamm ) was strongly inhibited after 7 days of HEA, but was unaffected at 3, 14 and 21 days. At 28 days fish were able to increase Jamm efficiently and concurrently, plasma ammonia re-established to the basal level. Urea production was increased as evidenced by a considerable elevation of plasma urea, but urea excretion rate remained unaltered. Expression of Rhcg isoform (Rhcg2) mRNA was up-regulated in parallel with restored or increased Jamm , suggesting its ammonia excreting role in largemouth bass. Exposure to HEA also displayed pronounced augmentations in NKA activity, exemplified by a rise in plasma [Na+ ]. Furthermore, [K+ ], [Cl- ] and MWC homeostasis were disrupted followed by recovery to the control levels. H+ -ATPase activity was elevated but NKA did not appear to function preferentially as a Na+ /NH4 + -ATPase. From 14 days onwards MO2 was depressed, potentially an attempt towards minimizing catabolism. Glycogen content in liver and muscle were temporarily depleted, whereas a remarkable increment in protein was evident at the last exposure period. Overall, these data suggest that ammonia induced toxicity can disturb several biological processes in largemouth bass, however, it can adapt to the long-term sub-lethal ammonia concentrations by activating various components of ammonia excretory, ion-regulatory and metabolic pathways.