Oreochromis Alcalicus Grahami Research Articles

SPECIAL COLLECTION: DOGMAS AND CONTROVERSIES IN THE HANDLING OF NITROGENOUS WASTES

Science is full of dogmas. But during the process of discovery, some dogmas face challenges, and the dogmas in nitrogen waste management are no different. As all biology students know, the dogma holds that waste proteins are broken down and excreted from the body in a variety of forms, such as

The highly specialized secretory epithelium in the buccal cavity of the alkalinity adapted Lake Magadi cichlid, Oreochromis alcalicus grahami (Teleostei: Cichlidae): a scanning and transmission electron microscope study

Oreochromis alcalicus grahami is a small cichlid fish that lives in the hot, highly alkaline, highly saline peripheral lagoons of Lake Magadi, Kenya. The fish faces profound diurnal oscillations of oxygen concentration. During the day, from photosynthetic activity of cyanobacteria (blue-green algae), the water is supersaturated with oxygen but after sunset when photosynthetic activity stops the water is virtually anoxic as a result of bacterial respiration. During the night and after explosive exercise, O. a. grahami characteristically skims the surface of the water with the mouth agape, suggesting that the buccal cavity is used as a gas-exchange organ. Contrary to expectation, however, the buccal cavity was found to be conspicuously non-vascularized: the surface epithelial lining was fundamentally of a mucus secretory type. In addition, certain cells in the deeper layers showed extensive lateral labyrinths similar to the epithelium of the renal tubules. These morphological features respectively indicated roles of secreting a protective film and regulation of ions taken across the epithelial lining of the buccal cavity. The allocation of gas-exchange to the gills and the air-bladder, osmoregulation essentially to the gills, and mucus secretion/protection to the buccal cavity displays an adaptive trade-off process in an elite animal. Effective use of the buccal cavity as a gas-exchanger would entail air-gulping followed by brief retention of it in the cavity to allow oxygen uptake. During such interval, both the gills and the air-bladder would of necessity be rendered temporarily non-functional. Skimming the top layer of water with the mouth open ensures that the gills are passively ventilated with well aerated water and the air-bladder is simultaneously used for gas-exchange, a strategy that should enhance oxygen acquisition, especially at higher ambient temperatures.

Functional morphology of the gas-gland cells of the air-bladder of Oreochromis alcalicus grahami (Teleostei: Cichlidae): an ultrastructural study on a fish adapted to a severe, highly alkaline environment

Oreochromis alcalicus grahami is a small cichlid fish that lives in shallow peripheral lagoons of Lake Magadi, Kenya. The internal surface of the air-bladder is highly vascularized, illustrating possible utilization as an accessory respiratory organ. The wall of the bladder consists of five distinct tissue layers. From the outer to the inner surfaces are: a squamous, undifferentiated epithelial cell; a collagen-elastic tissue space; a smooth muscle tissue band; an inner connective tissue space; and columnar gas-gland cells projecting into the lumen. The cell membrane over the perikarya of the gas-gland cells was sporadically broken. The disruptions were ascribed to possible physical damage by discharging gas-bubbles. Juxtaluminally, the gas-gland cells attached across tight junctions. The cells have highly euchromatic nuclei and conspicuously large intracytoplasmic secretory bodies. On the blood capillary facing (basal) aspect, the cell membrane of the gas-gland cells is highly amplified. The cells insert onto a smooth muscle tissue band. The morphological features and the topographical arrangement of the gas-gland cells in O. a. grahami are indicative of an operative exchange of materials between them and the underlying tissue components especially the blood capillaries. For a fish that subsists in hot, highly saline and alkaline water heavily invested by avian predators and where the partial pressure of oxygen diurnally shifts from virtual anoxia to hyperoxia, development of a versatile air-bladder for efficient buoyancy control conforms to the functional demands placed on it by a unique environment. Illustratively, instead of the gas-gland morphology in O. a. grahami resembling that in the freshwater fishes, the group from which the fish has evolved, it compares more closely to that of the marine fish. This similarity suggests amazing parallel evolutionary adaptation to biophysically corresponding aquatic milieus.

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.

Respiratory Physiology of the Lake Magadi Tilapia (Oreochromis alcalicus grahami), a Fish Adapted to a Hot, Alkaline, and Frequently Hypoxic Environment

The tilapia Oreochromis alcalicus grahami is a unique ureotelic teleost, the only fish that lives in the alkaline hotsprings of Lake Magadi, Kenya. Physical conditions and fish behavior were monitored in the Fish Springs Lagoon area, a site where the tilapia were particularly abundant. Water Po₂ and temperature fluctuated more or less in parallel in a diurnal cycle from less than 20 Torr and less than 25° C at night to greater than 40° Torr and 38° C during the day, whereas pH remained constant at approximately 9.8. Field laboratory tests demonstrated that routine $\dot{M}o_{}$ (under normoxia) increased greatly from 27° C to 36° C ($Q_{10}$ = 62) but then stabilized at a very high level (~34.5 μmol $g^{-1}$ $h^{-1}$) up to the lethal temperature (~ 42.5° C), a pattern that was adaptive to the natural diurnal regime. The Po₂ threshold for survival during acute exposure (≤ 1 h) was approximately 16 Torr. $\dot{M}o_{2}$ from water was well maintained down to a Po₂ of 60 Torr, below which it declined. Under ...

A comparative allometric study of the morphometry of the gills of an alkalinity adapted cichlid fish,Oreochromis alcalicus grahami, of Lake Magadi, Kenya

A morphometric analysis of the gills ofOreochromis alcalicus grahami has been carried out on specimens from ecologically distinct lagoons and a water-holding tank of Lake Magadi, a highly alkaline salt lake situated in a volcanically active region of the southern part of the Great Rift Valley in Kenya. The data were compared with those fromOreochromis niloticus, a close relative that lives in fresh water and with data from other fresh water and marine fish. Our primary goal was to identify the possible adaptive features which enable the fish to survive in an environment characterized by severely fluctuating levels of oxygen, a condition exacerbated by factors such as high temperature, alkalinity and osmolarity. The specimens ofO. a. grahami from the south-western lagoons of the lake had gills better adapted for gas exchange with a body mass specific diffusing capacity for oxygen which was about 2 times greater than that of the gills of the specimens from the fish spring lagoons and 2.5 times that of those from the water-holding tanks. Some parameters of the gills ofO. a. grahami, e.g. the gill filament length and number of gill filaments are significantly greater than those ofO. niloticus but the number of secondary lamellae, area of secondary lamellae and the diffusing capacity of the gills are similar in the two species. Compared with most other fish, the gills ofO. a. grahami appear to be particularly well adapted for gas exchange especially by having a thin water-blood barrier. Perhaps in no other extant fish have the gills had to be so exquisitely designed to meet environmental extremes and regulate complex and at times conflicting functions such as gas exchange, iono-regulation, acid-base balance and nitrogenous waste excretion as inO. a. grahami

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.

Preliminary morphometric study of the gills of Oreochromis alcalicus grahami from Lake Magadi and a comparison with O. niloticus

Morphometric features, including shrinkage, were determined on a small sample of Oreochromis alcalicus grahami. Comparison with more extensive data for O. niloticus, using identical methodology, indicates that O. alcalicus grahami has a greater lamellar surface area and is even more adapted for the transfer of oxygen from the water to the blood.

Gill structure of a fish from an alkaline lake: effect of short-term exposure to neutral conditions

The morphology and morphometry of the gills of Oreochromis alcalicus grahami, a unique ureogenic teleost that lives in the alkaline environment of Lake Magadi, Kenya (pH 10, [Formula: see text], temperature 30 – 40 °C) were examined by transmission electron, scanning electron and light microscopy. Fish were examined in normal Lake Magadi water and 2 – 3 or 24 h after transfer to Lake Magadi water neutralized to pH 7 with HCl (i.e., [Formula: see text] replaced with Cl−), a treatment that caused severe reductions in urea excretion and O2uptake, internal acidosis, and ionoregulatory disturbance. In Lake Magadi water, the organization of the filament epithelium of the gill was similar to that of sea water teleosts. Indeed, chloride cells were located at the bottom of pits bordered by overlying pavement cells and flanked by typical accessory cells. Total numbers of chloride cells remained unchanged after transfer to pH 7, but after 2 – 3 h, many were covered by pavement cells, restricting their communication with the external milieu. At 24 h, this trend was reversed, an observation indicative of a reactivation of chloride cells. Mucous cells were located at maximum density on the trailing edge of the filament; most of them were empty after 24 h at pH 7. The harmonic mean thickness of the lamellar epithelium (blood-to-water diffusion pathway) was very small and not altered by acute or longer term exposure to pH 7. A model of alterations in ion and acid – base transport accompanying the morphological changes is presented.

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.

ARE MITOCHONDRIA SUBJECT TO EVOLUTIONARY TEMPERATURE ADAPTATION?

Thermal tolerance and the respiratory properties of isolated red muscle mitochondria were investigated in Oreochromis alcalicus grahami from the alkaline hot-springs, Lake Magadi, Kenya. Populations of O. a. grahami were resident in pools at 42.8 °C and migrated into water reaching temperatures of 44.8 °C for short periods. The maximum respiration rates of mitochondria with pyruvate as substrate were 217 and 284 natom O mg-1 mitochondrial protein min-1 at 37 °C and 42 °C, respectively (Q10=1.71). Fatty acyl carnitines (chain lengths C8, C12 and C16), malate and glutamate were oxidised at 70­80 % of the rate for pyruvate. In order to assess evolutionary temperature adaptation of maximum mitochondrial oxidative capacities, the rates of pyruvate and palmitoyl carnitine utilisation in red muscle mitochondria were measured from species living at other temperatures: Notothenia coriiceps from Antarctica (-1.5 to +1 °C); summer-caught Myoxocephalus scorpius from the North Sea (10­15 °C); and Oreochromis andersoni from African lakes and rivers (22­30 °C). State 3 respiration rates had Q10 values in the range 1.8­2.7. At the lower lethal temperature of O. andersoni (12.5 °C), isolated mitochondria utilised pyruvate at a similar rate to mitochondria from N. coriiceps at 2.5 °C (30 natom O mg-1 mitochondrial protein min-1). Rates of pyruvate oxidation by mitochondria from M. scorpius and N. coriiceps were similar and were higher at a given temperature than for O. andersoni. At their normal body temperature (-1.2 °C), mitochondria from the Antarctic fish oxidised pyruvate at 5.5 % and palmitoyl-dl-carnitine at 8.8 % of the rates of mitochondria from the hot-spring species at 42 °C. The results indicate only modest evolutionary adjustments in the maximal rates of mitochondrial respiration in fish living at different temperatures.

The thermostability of haemoglobins from the hot-spring fish, Oreochromis alcalicus grahami: Comparisons with antarctic and temperate species

1. 1. The thermostability of haemoglobin was measure in three species of fish living at different environmental temperatures ( ET). 2. 2. The time (min) for 50% denaturation ( T 1/2 d) of the haemoglobin at 2 mg ml -1 in 50 mM phosphate buffer, pH 7.3 was 46.4±1.7 for Oreochromis alcalicus grahami ( ET, 35 to 42°C), 43.1±1.9 for Oreochromis niloticus ( ET, 18 to 26°C) and 19.2±0.3 for the Antarctic teleost, Notothenia coriiceps ( ET, -1.5 to 1°C) (Mean ±SEM, N=5–7 preparation). In contrast, T 1/2 d for haemoglobin from birds and mammals is usually in excess of 500 min. 3. 3. These results suggest that the lower thermostability of haemoglobins in fish relative to birds and mammals is not solely a function of differences in body temperature.

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.

The Effects of Reducing Water pH and Total CO2 on a Teleost Fish Adapted to an Extremely Alkaline Environment

ABSTRACT The teleost fish Oreochromis alcalicus grahami is well adapted to an extremely alkaline environment (pH 10, total CO2 184 mmol I−1) in Lake Magadi, Kenya. O. a. grahami excretes all nitrogenous wastes as urea, and exposure to neutral water (pH7, total CO2<1mmol l−1) results in a complete inhibition of urea excretion. In the present study, we further characterized the physiological effects of transferring O. a. grahami from alkaline to neutral water. Exposure to neutral water resulted in a significant decrease in blood pH, a large reduction in plasma HCO3− levels, a severe impairment of swimming ability, an increase in Na+ influx, but no change in O2 consumption. A closely related species of tilapia, O. nilotica, living in neutral waters of the Sagana River, was relatively unaffected by acute exposure to tapwater at pH 10, but showed a marked alkalosis in Lake Magadi water prior to death. These results demonstrate that O. a. grahami thrive in an environment of high pH and bicarbonate/carbonate concentration, conditions that are fatal to other species adapted to neutrality. O. a. grahami, however, is extremely unusual amongst aquatic animals in its adverse response to neutral water.

A study of the morphology of the gills of an extreme alkalinity and hyperosmotic adapted teleost Oreochromis alcalicus grahami (boulenger) with particular emphasis on the ultrastructure of the chloride cells and their modifications with water dilution

The general gill morphology of Oreochromis alcalicus grahami, a teleost adapted to high salinity and hyperosmosis, is basically similar to that of other teleostean fish. The species has four pairs of gill arches, all of which have well developed filaments. Each of the arches (holobranchs) has two rows of filaments (hemibranchs). Bilaterally situated secondary lamellae branch from the central axis of the filaments. The lamellae reach their maximum size at the middle of the filament, gradually decrease in size and eventually disappear towards the tip of the filament, which is bare. The leading edge of the gill filament and the immediate interlamellar space is covered by a stratified epithelium consisting of pavement cells, mucous cells, chloride cells and undifferentiated cells. The surface of these cells is made up of concentric microridges. The chloride cells were found only on the primary epithelium (filamental epithelium) and very rarely on the secondary epithelium (lamellar epithelium). Two types of chloride cells were observed in the gills of Oreochromis. The superficial chloride cells have fewer mitochondria concentrated towards the basal aspect of the cell, and a network of tubules towards the apical surface and are less electron dense. These cells intercommunicate with the water through an apical pore. The deep chloride cells have numerous diffuse mitochondria intercalated between a fine profuse tubular network and are more electron dense. These cells are covered by one or more layers of pavement cells and thus do not have access to the external surface. After gradual dilution of the lake water in which the fish were kept, both types of chloride cells remained topographically and ultrastructurally distinct. However, in both kinds of cell the mitochondria decreased in number and size. Initially there was an increase in the diameter and the degree of interdigitation of the tubules followed by a gradual decrease. An increase in the quantity of rough endoplasmic reticulum, particularly at the perinuclear region of the cell, was noted. The morphometric analysis of the branchial system indicated that the gills of Oreochromis are well adapted for gas exchange by having numerous and relatively long gill filaments with a high lamellar density. These features provide a large surface for gas exchange which, when coupled with the notably thin water-blood barrier of an average thickness of only 0.83 micro, would facilitate efficient absorption of oxygen by the gills. Oreochromis alcalicus was observed to be incapable of adapting to freshwater. This may have been due to the progressive degeneration of the chloride cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The interaction between carbon dioxide and ammonia excretion and water pH in fish

The pH of plasma is midway between the pK of the [Formula: see text] and [Formula: see text] reactions. Carbon dioxide excretion across the gill acidifies the water, as CO2 hydrates to form [Formula: see text] and H+ ions. The hydration reaction is catalyzed by carbonic anhydrase in the water boundary layer next to the gills and on the gill epithelium. Acidification of the boundary layer is greater than in the bulk flow, particularly in water of high pH and low buffering capacity. Ammonia excretion will raise water pH but this effect is usually masked by the much larger CO2 excretion. At low water pH, below the pK′ of the CO2 hydration reaction, little [Formula: see text] and H+ will be formed, therefore CO2 will have little effect on downstream water pH. Under these conditions, NH3 excretion will alkalinize water as it passes over the gills. Formation of an acid gill water boundary layer as a result of CO2 excretion will trap NH3 as [Formula: see text] and enhance ammonia transfer. Removal of the acid boundary layer, either by increasing the buffering capacity of the water or by reducing CO2 excretion, decreases ammonia transfer in the isolated head of rainbow trout. Water pH can have a marked effect on CO2 and NH3 gradients across the gills. In alkaline waters, the rise in blood NH3 levels will result in a marked increase in body stores of total ammonia. Tilapia (Oreochromis alcalicus grahami) in Lake Magadi, Kenya, at a water pH of 10, excrete nitrogenous wastes as urea, to avoid the problem of the buildup of toxic levels of ammonia when the fish are exposed to these alkaline conditions.

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.