Nitrogen Excretion and Osmotic Regulation

in Aquatic OrganismsAquatic BiologyAquatic Biology

Biology 450Biology 450Dave McShaffreyDave McShaffrey

Harla Ray Eggleston Department of Biology and Harla Ray Eggleston Department of Biology and Environmental ScienceEnvironmental Science

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Basic Situation:• Many marine organisms can get by with

minimal osmoregulation 1. the oceans are a good environment2. other marine organisms maintain their body fluids

at ionic concentrations different from the surrounding ocean and must actively regulate ions.

• Freshwater also calls for active measures to maintain proper osmotic balance.

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One method to avoid having to deal with osmotic balance is to cover the body with an impermeable membrane.

• Many aquatic organisms do just that!– this protection is necessarily incomplete,

because three other processes involve intimate contact between a water-permeable body membrane and the surrounding fluid.• these three other processes demand large surface

areas in order to occur at sufficient rates to satisfy bodily needs. • These three processes are,

1. respiration, 2. absorption of food, 3. nitrogen excretion.

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• These three processes are, 1. respiration, 2. absorption of food, 3. nitrogen excretion.

• only respiration is required of all aquatic organisms

• plants do not ingest food (although they do need to take up plant nutrients)

• plants do not have to excrete nitrogenous wastes

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Nitrogen Excretion  • Excretion is a necessary consequence of

protein breakdown1. When proteins are converted to carbohydrates

to provide energy, the amino group is removed and must be dealt with.

2. In the body, the amino group is quickly oxidized to form ammonia (or, at high body pH the ammonium ion).

3. Ammonia is highly toxic and highly soluble in water.

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Nitrogen Excretion  • If the organism has a sufficient source of water,

ammonia can simply excreted in the water. 1. This is the course taken by many (if not most)

aquatic organisms, particularly those in freshwater. 2. In any event, ammonia must be dealt with quickly

because of its toxicity. 3. Ammonia will diffuse passively out of respiratory

structures such as gills. 4. It takes a lot of water to dissolve and flush

ammonia, however, and each ammonia molecule carries only one nitrogen.

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Nitrogen Excretion  • Organisms with less fresh water available, such as some marine

organisms and all terrestrial organisms will often invest some energy to convert the ammonia into urea,1. urea is less toxic than ammonia 2. has two nitrogen atoms 3. takes less water to excrete 4. Because it is less toxic, it can be allowed to accumulate in the

blood to some extent• Many organisms have specialized organs to remove urea and

other wastes from the blood and excrete them. • Urea is commonly used as an excretory product in vertebrates,

and is rarely used in invertebrates. • Some organisms, such as sharks and snails, allow urea to

accumulate in their blood to help with overall osmotic balance. – Sharks, for instance, use urea in the blood to make them

hyperosmotic in relation to seawater, • thus they tend to gain water from the ocean and do not have to worry about

dehydration.

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Nitrogen Excretion  • Where water is at a real premium, even the

low toxicity and reduced water loss possible with urea excretion is not enough.1. Uric acid is a purine 2. Uric acid is even less toxic than urea 3. precipitates from solution, allowing the 4

nitrogen atoms per uric acid molecule to be excreted

4. It has evolved in two groups with major water loss problems –

1. terrestrial invertebrates 2. egg-laying vertebrates

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Osmotic RegulationWith the problems of respiration and nitrogen excretion settled (we 

will cover feeding later), we can now deal comprehensively with the issue of osmotic regulation. First, let's review the basic situations that aquatic organisms face.

• Salts:1. The most abundant of the salts found in the oceans

is NaCl, sodium chloride or table salt. 2. We measure salinity in terms of the number of

grams of dissolved salts in 1000 g (one l) of seawater.

3. Seawater ranges in salinity, but a useful approximation is

– 35 g/kg or – 35 parts per thousand (35 ‰) or– 3.5%

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Osmotic Regulation• Organisms in marine environments tend to be isotonic in

relation to the seawater.– In this case, they do not have to regulate ion levels, and are termed

osmoconformers. – They are typically restricted to narrow ranges of salinity (no great

handicap in the ocean, where salinity changes are not common)• stenohaline.

– Many marine organisms, both invertebrate and vertebrate, while they may be close to isotonic, will vary somewhat and need to regulate to a small extent.

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Osmotic Regulation• Major exceptions to the above include sharks and marine

tetrapod vertebrates. – Sharks maintain an internal environment which is hypertonic to

seawater. • They raise their internal osmoticity by retaining urea in their blood. • As a result of being hypertonic, they tend to gain water from the seawater

through their gills and the lining of their guts. • The excess water is excreted as a dilute urine.

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Osmotic Regulation• Marine tetrapod vertebrates, which evolved on land, have blood which is

hypotonic to seawater. – Since they breathe from the atmosphere directly, there are no respiratory surfaces in

contact with the seawater• this reduces the surface area over which water loss can occur.

– Still, these organisms do lose water when excreting urea (mammals) or uric acid (turtles, reptiles, birds)

– Lose water when breathing– they gain salt ions whenever they eat or drink– The only way for these organisms to obtain water is

1. metabolically from the breakdown of carbohydrates2. by drinking seawater

– This still leaves them facing net water loss and ion gain. • There are two basic solutions to this problem.

1. Turtles and birds have special salt glands (concentrations of chloride cells) near their eyes a) actively pump Cl- ions out of the body; Na+ ions follow. b) Thus, birds and turtles can drink seawater and pump the excess ions out of their bodies, 

retaining the water. c) Similar cells are located on the gills of those marine fish (or invertebrates) with hypotonic 

body fluids.2. Marine mammals have some of the most efficient kidneys known. 

a) Their kidneys can resorb most of the water from the urineb) leaving a very concentrated solution of urea and salts to be excretedc) They also are very efficient at removing water from the rectum, so that food wastes pass out 

with a minimum of waterd) By minimizing water loss in this way, marine mammals are able to survive on metabolic water.

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Osmotic Regulation• Freshwater organisms (and many estuarine organisms) are hypertonic in

relation to the water – face a constant influx of water from the surrounding hypotonic medium– they can potentially lose important ions to that solution also– Therefore, the strategy among most freshwater organisms is to:

a) cover as much of the body as possible with an impermeable coat, b) leave all water exchange to a relatively small number of cells. c) These cells will maintain the water balance, and the remaining cells are bathed in an isotonic solution.

– Cells can maintain osmotic balance by using ATP to pump Cl- ions into the cell actively. • These use the same proteins found in the salt glands of marine turtles, they just run in reverse. • The inside of the cell becomes negatively charged, and other ions, such as Na+ come in because of this.

– Water that flows into the body of a freshwater organism moves into the blood and excreted as a dilute urine.

– Freshwater organisms, because of this active manipulation of their ionic balance, are called osmoregulators and are frequently tolerant of a wider range of osmotic concentrations, in other words, they are euryhaline.

• Finally, organisms in hypersaline environments such as the Great Salt Lake face problems similar to those of marine fish with hypotonic body fluids. – They must actively pump chloride and other ions out of the body, and obtain

water by drinking

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Osmotic Regulation• Chloride cells are used by both marine and freshwater organisms to

pump ions.1. In fish (both freshwater and marine), they are located on the gills (Fig 3). 2. Because respiratory structures must have permeable surfaces for gas exchange, they

are also a common place to put chloride cells on a body which is otherwise impervious to water flow

3. Another popular place is in the gut and kidneys4. in both places ion concentrations are manipulated to get water to flow where the

organism wants it to5. Amphibians in freshwater locate these cells on their skin to absorb ions from the

water6. Chloride cells are also more common on organisms in habitats with changing salinity,

such as temporary pools or tide pools7. Aquatic mayfly larvae, which must use chloride calls on the gills (mayfly gills may be

more osmoregulatory then respiratory in function) to pump ions into the body, increase the number of chloride cells as they move into more dilute water and decrease the number as the ionic concentration of the water increases.

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Osmoregulation and the Transition Between Habitats

As a final note, let's consider the evolutionary history of how different groups of organisms have come to colonize all the available habitats in terms of osmoregulatory adaptations.

1. Life originated in the oceans2. early organisms were probably

a) isotonicb) stenohalinec) osmoconformers

3. Terrestrial arthropods, including insects, probably arose from marine arthropods – had developed water-saving adaptations in tidepools and other marginal marine habitats.

4. Some of these organisms, i.e. insects, were later able to move into freshwater. 5. Terrestrial vertebrates, on the other hand, no doubt arose from fish which had

invaded freshwater. The freshwater fish had developed internal fluids more dilute than the oceans as a means of

minimizing their osmotic regulatory needs in dilute freshwater, the first terrestrial vertebrates, the amphibians, retained these relatively dilute body fluids. Vertebrate specialization on land required increased ability to deal with lack of water, these water-conserving methods were useful when certain groups - marine turtles, crocodiles,

birds, and mammals - returned to the sea.

6. The main point here is that it was the osmoregulating groups that were able to colonize land, and this flexibility later allowed members of these groups to move into nearly all conceivable habitats (Fig. 4).

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The End

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