Degree Name

Master of Science (MS)

Semester of Degree Completion

1996

Thesis Director

Charles J. Costa

Abstract

The catabolism of amino acids derived from dietary protein results in the production of ammonia and bicarbonate ion, waste products that an animal must eliminate. Often, animals excrete a nitrogenous waste product that is correlated with water availability: Freshwater fish, living where water is abundant, usually excrete ammonia while terrestrial animals excrete urea or uric acid. Recent studies report that fish inhabiting alkaline waters (pH 10) characterized by high bicarbonate concentrations do not excrete ammonia, instead synthesize urea, an energetically expensive process. The conversion to urea excretion by a normally ammonotelic animal is likely to be stimulated by a specific environmental factor.

In order to begin to identify this factor, we separated the contribution of high pH from the elevated HCO3– concentrations. Goldfish and tilapia were acclimated for one week to media buffered 7 mM HCO3– plus a non-HCO3– buffer or to a non- HCO3– buffer alone. Media pH was varied from 7.0 - 9.0 for goldfish and from 8.0 - 9.5 for tilapia. The fish were fed high protein foods daily.

We found that neither pH nor HCO3– caused a dramatic shift in ammonia nitrogen excretion to urea nitrogen excretion in either species of fish. Goldfish increased ammonia nitrogen excretion in bicarbonate containing media, an effect that was strongest at pH 8.0. Increasing bicarbonate concentration from 7 mM to 30 mM did not stimulate a further increase in ammonia excretion.

When tilapia were acclimated to bicarbonate concentrations from 7 mM to 30 mM at pH 9.0, ammonia excretion rates were proportional to bicarbonate concentration. Ammonia nitrogen excretion went from 519 umol/kg-hr for fish acclimated in 7 mM HCO3– to 2746.3 umol/kg-hr for fish acclimated to 30 mM HCO3–. Artificial pond water (APW) formulated with NaCl substituted for NaHCO3, did not stimulate ammonia excretion in a similar manner. An increase in bicarbonate concentrations of the media will inhibit the exchange of Cl–/ HCO3– and cause an increased blood bicarbonate concentration. An increase in blood bicarbonate concentration may stimulate the deamination of amino acids. Urea nitrogen remained a small percentage of nitrogen excretion for all fish regardless of treatments.

Clearly, goldfish and tilapia do not shift their nitrogenous waste product from ammonia to urea in response to alkaline conditions. Neither species appears pre-adapted for life in strongly alkaline environments.

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