Ammonia tolerance in the slender lungfish (Protopterus dolloi): the importance of environmental acidification

Abstract

Protopterus dolloi Boulenger, 1900 is an obligate air-breather and exhibits ammoniotely (88% ammonia-N excretion, 12% urea-N excretion) under normal aquatic conditions, but tolerates 7 days of exposure to 30 mmol·L–1NH4Cl, a treatment fatal to most other fish. Internal N accumulation is minimal and the subsequent washout of ammonia-N and urea-N after return to control conditions is negligible, indicating that N excretion continues and (or) that N metabolism is markedly depressed. Exposure to 30 mmol·L–1NH4Cl in a closed system without aeration results in depressed urea-N excretion. The lungfish greatly acidifies the external water, a volume 25-fold greater than its own volume. The extent of this acidification increases with time. After several days, the external pH falls from about 7.0 to below 5.0 over a 24-h period, thereby markedly reducing the concentration of NH3(the form that diffuses across biological membranes). CO2excretion is partially responsible for this acidification, because vigorous water aeration reduces but does not eliminate the acidification, and urea-N excretion increases moderately. However, a substantial excretion of titratable acid (non-CO2acidity) also occurs. One exceptional lungfish was able to maintain its aerated environment at a stable pH of 3.7. Environmental acidification may be a less costly strategy for avoiding toxicity than detoxifying ammonia by increasing urea production.