Magadi Tilapia Research Articles

Genetic structure of Lake Magadi tilapia populations

The control region of the mitochondrial DNA haplotype frequencies were significantly different among the two separate lagoon populations of Oreochromis alcalicus grahami in Lake Magadi and of O. a. alcalicus from lake Natron, and DNA fingerprint similarity indices were significantly higher for intra-population comparisons of the two Magadi lagoon populations and the Lake Natron population than the inter-population similarity indices among these populations. A modified Fst measure indicated population sub-division and the phylogeographic partitioning of the VNTR fragments observed were unique to specific populations further indicating substantial genetic differentiation. The lagoon populations within Lake Magadi demonstrated the same degree of genetic differentiation as either of these populations did to the outgroup (the Lake Natron population). There appears to be limited gene flow between Lake Magadi tilapia populations and this population structure has important implications for protecting locally adapted populations within this unique ecosystem.

Genetic structure of Lake Magadi tilapia populations

The control region of the mitochondrial DNA haplotype frequencies were significantly different among the two separate lagoon populations of Oreochromis alcalicus grahami in Lake Magadi and of O. a. alcalicus from lake Natron, and DNA fingerprint similarity indices were significantly higher for intra‐population comparisons of the two Magadi lagoon populations and the Lake Natron population than the inter‐population similarity indices among these populations. A modified Fst measure indicated population sub‐division and the phylogeographic partitioning of the VNTR fragments observed were unique to specific populations further indicating substantial genetic differentiation. The lagoon populations within Lake Magadi demonstrated the same degree of genetic differentiation as either of these populations did to the outgroup (the Lake Natron population). There appears to be limited gene flow between Lake Magadi tilapia populations and this population structure has important implications for protecting locally adapted populations within this unique ecosystem.

Urea production and transport in teleost fishes.

Teleosts appear to have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureotelic (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It was surprising, therefore, to find the expression of four urea cycle enzymes during early life stages of rainbow trout. The urea cycle may play a role in ammonia detoxification during a critical time of development. Exposure to alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) increased urea excretion by several-fold in trout embryos, free embryos and alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney.

The adaptations of fish to extremely alkaline environments

The Lake Magadi Tilapia (MT; Oreochromis alcalicus grahami, the Lahontan cutthroat trout (LCT; Oncorhynchus clarki henshawi) and the tarek (Ct; Chalcalburnus tarichi) have evolved unique strategies that allow them to overcome problems associated with ammonia excretion (JAmm) and acid-base regulation in their alkaline environments. In Lake Magadi, Kenya (pH 10), the MT circumvents problems associated with JAmm by excreting virtually all (>90%) of its waste-nitrogen as urea. Base excretion appears to be facilitated by modified seawater-type gill chloride cells, through apical Cl−/HCO3− exchangers and an outwardly directed OH−/HCO3−/CO3= excretion system. The LCT avoids potentially toxic increases in internal ammonia by permanently lowering ammonia production rates following transfer into alkaline (pH 9.4) Pyramid Lake, Nevada, from its juvenile freshwater (pH 8.4) environment. Greater apical exposure of LCT gill chloride cells, presumably the freshwater variety, probably facilitates base excretion by elevating Cl−/HCO3− exchange capacity. In Lake Van, Turkey (pH 9.8) high ammonia tolerance enables C. tarichi to withstand the high internal ammonia concentrations that it apparently requires for the facilitation of JAmm. It also excretes unusually high amounts of urea. We conclude that adjustments to nitrogenous waste metabolism and excretion patterns, as well as modifications to gill functional morphology, are necessary adaptations that permit these animals to thrive in environments considered unsuitable for most fishes.

Scaling of oxygen consumption of Lake Magadi tilapia, a fish living at 37°C

Scaling of oxygen consumption of Lake Magadi tilapia, a fish living at 37°C

Scaling of oxygen consumption of Lake Magadi tilapia, a fish living at 37°C

Rates of oxygen consumption were measured in the geothermal, hot spring fish, Oreochromis alcalicus grahami by stopped flow respirometry. At 37° C, routine oxygen consumption followed the allometric relationship: V o2=0.738 M0.75, where V o2 is ml O2 h −1 and M is body mass (g). This represents a routine metabolic rate for a 10 g fish at 37° C of 0.415 ml O2 g−1 h −1 (16.4 μmol O2 g −1 h −1). Acutely increasing the temperature from 37 to 42° C significantly elevated the rate of O2 consumption from 0.739 to 0.970 ml O2 g −1 h −1 (Q10=l.72). In the field, O. a. grahami was observed to be ‘gulping’ air from the surface of the water especially in hot springs that exceeded 40° C. O. a. grahami may utilize aerial respiration when O2 requirements are high.

UREA PRODUCTION, ACID-BASE REGULATION AND THEIR INTERACTIONS IN THE LAKE MAGADI TILAPIA, A UNIQUE TELEOST ADAPTED TO A HIGHLY ALKALINE ENVIRONMENT

The Lake Magadi tilapia, Oreochromis alcalicus grahami, thrives in highly alkaline geothermal springs and pools surrounding Lake Magadi, Kenya (control pH=9.9, CCO2=173 mmol l-1), has a functional hepatic ornithine­urea cycle (OUC) and excretes all nitrogenous waste as urea-N at variable rates (JUrea) related to O2 consumption (M·O2). The mean value of JUrea/M·O2 (N/O2=0.183) was high for fish but below the theoretical maximum (approximately 0.27) for 100 % aerobic respiration of protein, so an exogenous source of substrates is not required to explain the observed JUrea. JUrea was insensitive to thiourea. Urea excretion occurred largely (80 %) through the gills, but urea-N was also present in bile and urine. Control blood pHe, pHi and [HCO3-] (approximately 8.1, 7.6 and 15 mmol l-1, respectively, at approximately 32°C) were extremely high. When fish were exposed to lake water titrated with HCl and aerated to remove CO2, N/O2 progressively declined. At a lake water pH of 7.05 and CCO2 of 0 mmol l-1, N/O2 was reduced by 80 % and an intense metabolic acidosis occurred (pHe=7.04, [HCO3-]=1.5 mmol l-1). Restoration of control water pH 9.9 at a CCO2 of 0 mmol l-1 resulted in intermediate levels of N/O2 and internal acid­base status. Additional experiments confirmed that urea production was inhibited by low pHe, was dependent on blood [HCO3-] with a Km of 3.06 mmol l-1 and was insensitive to acetazolamide. While metabolic acidosis clearly inhibited OUC ureagenesis, the system appeared to be saturated with HCO3- under control conditions so that additional basic equivalent loading would not stimulate ureagenesis. Urea production in the Lake Magadi tilapia does not appear to remove exogenous HCO3- or to play a role in normal acid­base regulation.

Nitrogen Excretion and Enzyme Pathways for Ureagenesis in Freshwater Tilapia (Oreochromis niloticus)

The tilapia of Lake Magadi (pH 10), Oreochromis alcalicus grahami, excrete all nitrogen wastes as urea and possess the ornithine urea cycle (OUC), unlike the majority of teleost fish. It was proposed that liver OUC enzymes could be induced in the freshwater ammoniotelic tilapia Oreochromis niloticus in response to environmental or dietary manipulations that stimulate urea excretion in other animals. Elevated water ammonia (1 mM NH₄Cl) inhibited ammonia excretion over the first 12 h and increased urea excretion rates three- to fivefold over 7 d. The OUC was not functional in control fish, as carbamoyl phosphate synthetase III activity was not detectable and ornithine carbamoyltransferase activity was relatively low. No changes in OUC enzyme activities were detected in NH₄Cl-exposed fish, but the activity of the uricolytic enzyme allantoicase was twofold higher. Exposure to pH 10 water had only a transient effect on nitrogen excretion, and OUC enzymes and allantoicase activities were unchanged. The rate of urea and ammonia excretion increased with an increase in the dietary protein content, but OUC or uricolytic enzymes were not induced. These results demonstrate that O. niloticus, unlike Lake Magadi tilapia, do not recruit the hepatic OUC, even under environmental or dietary conditions that enhance urea excretion. Rather, uricolysis and argininolysis appear to be the major routes for ureagenesis.

Effects of Ammonia on Survival, Swimming and Activities of Enzymes of Nitrogen Metabolism in the Lake Magadi Tilapia Oreochromis Alcalicus Grahami

ABSTRACT The Lake Magadi tilapia, Oreochromis alcalicus grahami, is remarkable among teleosts in that it flourishes under extremely well-buffered alkaline water conditions (pH10, 180mmol l−1) at temperatures of 30–40°C (Wood et al. 1989). As expected from current models in teleosts, ammonia excretion into such water would be difficult at best (Wood, 1993). Part of the survival strategy of the Lake Magadi tilapia is that it has a complete ornithine–urea cycle (O–UC) in the liver and excretes virtually all of its waste nitrogen as urea (Randall et al. 1989). Ammonia toxicity in ammoniotelic teleosts has been studied extensively, and typical values for unionized ammonia (NH3) 96h LC 50 (the concentration at which half of test subjects die after 96h) are well below 100 μmol l−1 (Haywood, 1983; Thurston et al. 1983a,b; Campbell, 1991). Surprisingly, no ammonia LC50 values are available for ureogenic teleost fish, and one would predict that fish synthesizing and excreting urea for whatever purpose would have higher LC50 values than their ammoniotelic counterparts. Additionally, since ammonia exposure has been implicated in the functional response of urea excretion in the Lake Magadi tilapia (Wood et al. 1989) and another ureogenic teleost (the gulf toadfish Opsanus beta) (Walsh et al. 1990), we reasoned that ammonia exposure in the Lake Magadi tilapia might reveal insights into the biochemical regulation of the O–UC in this species; in particular that it might induce enzyme activity. We report here that the Lake Magadi tilapia has a rather high ammonia LC50 compared to values for other teleosts, but that short-term ammonia exposure has very limited effects on the activities of the enzymes of nitrogen metabolism and on swimming performance.

Ammonia and urea dynamics in the Lake Magadi tilapia, a ureotelic teleost fish adapted to an extremely alkaline environment

The tilapia Oreochromis alcalicus grahami, which thrives under harshly alkaline conditions in Lake Magadi, Kenya, was studied in its natural environment (pH = 10, total CO 2 = 180 mmol/L, osmolality = 525 mOsm/kg, 30–36.5 ° C). At rest, this species excretes all nitrogenous waste as urea. This is the first known instance of complete ureotelism in an entirely aquatic teleost fish. Very small ‘apparent’ ammonia excretion (<5% of overall N excretion) was attributable to faecal/bacterial production. Ammonia excretion could not be induced by feeding, reduced temperature, or exposure to pH 7. Exhaustive exercise induced only a small efflux of ammonia. Urea output was inhibited completely by pH 7 water and partly by exhaustive exercise, and greatly stimulated by exposure to 500 μmol/L NH 3 (at pH 10). A related species, nominally Oreochromis nilotica, which lives in freshwater at circumneutral pH in the same geographic region, excretes 85% ammonia-N and 15% urea-N at pH 7 in the standard teleost fashion. Urea-N efflux increased to 33% upon transfer of O. nilotica to pH 10 in freshwater. Urea output in this species was only marginally stimulated by exposure to 500 μmol/L NH 3 (at pH 7). Plasma and white muscle urea levels were 4- to 5-fold higher in O. a. grahami than in O. nilotica, and plasma levels increased between caudal and cardiac sampling sites, indicating hepatic ureagenesis. Blood pH and P NH3 levels, when corrected for sampling artifact, were unusually high in O. a. grahami. We hypothesize that complete ureotelism in O. a. grahami in evolutionary response to the problems of excreting ammonia into highly buffered water at pH 10 and/or acid-base balance in this extreme environment.

PH, Salinity and Temperature Tolerance of Lake Magadi Tilapia

THERE are several species of Tilapia isolated in the alkaline lakes of the Great Rift Valley of Africa, living in extreme conditions of temperature, salinity and pH. One of these fish, the Lake Magadi Tilapia (Tilapia grahami), lives in the lagoons and alkaline volcanic springs around the margin of Lake Magadi in Kenya1. The main salts of the springs, and the lake, are carbonate and bicarbonate of sodium.