The cells of blue-green algae are lysed by high concentrations of acid (pH 1.9–1.4) in the stomach of Tilapia nilotica. After lysis, cell contents are digested in the intestine. Acid secretion follows a diurnal cycle related to feeding, and thus there is a cycle from zero to maximum digestion each day. Some of the digestive enzymes have been studied.
Blue-green algae are lysed by a high concentration of acid in the stomach. pH 1.4 is the lowest value recorded in the stomach of T. nilotica; lysis is most effective at this pH.
Enzymatic digestion occurs in the intestine, after the algae are lysed.
Acid secretion in the stomachs of T. nilotica and H. nigripinnis in Lake George, Uganda, follows a diurnal cycle associated with feeding. Secretion starts when feeding starts in the morning. In T. nilotica, low pH values are not obtained throughout the stomach until feeding ceases in the evening. Thus there is a gradation through the day, from zero to maximum in number of blue-green algal cells lysed, and therefore digested and assimilated.
Colour changes in the ingested phytoplankton can be used as an indicator of gastric acid secretion by fish in the laboratory. Acid is not secreted by stressed fish.
Pepsinogen was detected in the stomach wall of T. nilotica. Evidence for the absence of peptic digestion in the stomach of T. nilotica is presented.
A pancreatic α-amylase, pH optimum 7.0 to 8.0, and requiring chloride, is present in the intestinal juice of T. nilotica.
Trypsin and chymotrypsin are also present, but carboxypeptidases A and B could not be detected.
Esterase activity, but no lipase, was detected in the intestine juice.