Thursday, September 21, 2017

Water, Water Everywhere, But….

Biology concepts – symbiosis, mutualism, water storage

“Gobi” means desert in Ural-Altaic,
so when you say, “Gobi Desert,” you
are really being redundant.
Sometimes the places with the most water are the most lifeless areas. Everyone thinks of sand and heat, but Lawrence of Arabia wouldn’t even recognize most biological deserts.

The term biological desert is misleading, since places like the Gobi Desert in Asia support over 600 species of plants and hundreds of animal species, vertebrate and invertebrate. Death Valley in the USA has over 100 plants species; it could hardly be called dead! A biological desert has less to do with the climate and more to do with the adaptability of organisms to adverse conditions of oxygen, salt, water, light, or too often - pollution.

Take for instance the South Pacific Gyre. This area of about 34 million square kilometers (10 million sq. miles) has very little life in the pelagic zone (the below the surface waters to just above the sea floor). In the last posts we learned why water and salts are crucial for life, and the extreme evolutionary adaptations that have occurred in many organisms in order to conserve body water and maintain safe salt levels. But here we are in the ocean – water everywhere, salt everywhere, but almost nothing lives in the gyre.

The north and south Pacific gyres represent
a huge portion of the Earth’s surface, and
these are relatively life free areas, the largest
deserts on Earth.
The reason for this paucity of life has more to do with nutrients than with water or salt. Because the current moves counter-clockwise, the center of the gyre is isolated from the upwelling of nutrients from the ocean floor, and the winds can’t help to churn the waters. Even if they could, it would help little. The waters of the gyre are rigidly layered due to salt and temperature differences (stratification, I Am Your Density), so nutrients find it difficult to travel to the surface from below. Adding to the problem, there is little landmass in the South Pacific, so windblown organic material and terrestrial runoff are limited. Nutrients are coming from neither above nor from below.

With limited nutrients, there is a ceiling to the amount of primary productivity of phytoplankton (phyto = plant, and planktos = wandering in Greek) that can take place. Fewer producers means that few primary consumers can be supported, and so forth up the food chain. Little life on the surface means few nutrients drop to the ocean floor (waste and dead organisms), and so on.

The ocean gyres have little upwelling if nutrients
and therefore little plankton production. The bad
news - with global climate change, the gyre-related
low productivity zones are growing in size.
Strangely enough, the lack of producers in the gyre has benefited humans in at least one aspect. The chlorophyll of the producers changes the color of the ocean, and this affects the trapping of heat and the wind currents. With a loss of living things in the North pacific gyre, a 2010 study states that typhoon formation has decreased in this region by more than 70%............Don’t get too excited, global surveying also says that the biological deserts of the gyres are growing much faster than global warming models would predict. As they grow, global productivity will be reduced, and that can’t be good for any of us.

We don’t make things any easier by letting chemicals run into the oceans either. Man made dead zones from increased nitrogen and phosphorous. These nutrients are needed for growing phytoplankton, but you can have too much of a good thing. The overgrowth of phytoplankton and algae in these areas, along with the decomposers they support, deplete O2. The result is that there is no oxygen left for succession organisms, so larger animals cannot live there (neither can the plankton or algae after a while).

Man made dead zones correspond to areas of
runoff from sprayed fields. For instance, the estuary
of the Mississippi River in the Gulf of Mexico forms
the second largest man made dead zone in the world
each summer. Not to be outdone, the Baltic Sea dead
zone in Northern Europe is the largest, and it is
present all year round.
So the gyres are “almost dead” zones, and some polluted estuaries are considered dead zones. What about a body of water with dead in its name, the Dead Sea? At 423 meters (1388 feet) below sea level, the Dead Sea is officially the lowest body of water on Earth. Water flows into it, but not out of it, so all the salts and minerals just accumulate.

The temperature of the desert surrounding the Dead Sea is warm enough that evaporation plays a factor in increasing the salinity and mineral content of the remaining water. Only certain types of bacteria and algae can survive in the 33.7% saline waters (~8.6 x the salinity of the Mediterranean Sea).

 Dunaliella salina algae are particularly abundant in the Dead Sea after the rainy season. These green algae produce antioxidant carotenoids to protect themselves from the intense sun exposure of the Jordan Rift Valley as well as huge amounts of glycerol (a three carbon carbohydrate) to counteract the osmotic pressure which would otherwise move all the freshwater out of the algal cells.

The algae is a good food source for halophilic (salt-loving) bacteria. However, during dry years, both the alga and bacteria are present in much lower numbers. But isn’t just the high salt that prevents larger plants and animals from living in the Dead Sea. The minerals that accumulate, such as magnesium chloride, calcium chloride, magnesium bromide, and calcium sulfate, are toxic to animals that drink the water. Fish from the freshwater feeders of the Dead Sea sometimes swim into the mineral-laden waters and are killed almost instantly.

The Dead Sea has receded a mile in the past twenty
years, and environmentalists warn it could be
completely gone by 2050. As it recedes, it leaves
salt on the rocks after the water evaporates.
The exception to this is the recently discovered freshwater springs that also feed the Dead Sea. Along the sea bottom near these vents lives a multitude of Archaea (often called extremophiles) that used to be classified as bacteria, but are now known to be a different kingdom of life. Spreading along the seafloor, mats of Archaea form biofilms, previously unknown in the Dead Sea.

The Great Salt Lake in Utah is similar to the Dead Sea biologically, but the lower salinity (some places are 5% salt, while others are 25 %; a railroad causeway has separated it into a more saline north arm and less saline south arm) allows more types of organisms to thrive in the water. Still no fish, but more types of algae, as well as some brine shrimp and brine flies.

Surprisingly, there is abundant flora and fauna around both the Great Salt Lake and the Dead Sea. The Jordan Rift Valley boasts camels, leopards, and ibexes, as well as fig trees and the rose of Jericho. In the western hemisphere, the Great Salt Lake has millions of shore birds, mostly fed by the 100 billion brine flies that hatch each summer. It is just the exception that here you have to move away from the water to find the life.

The above two examples indicate areas that have a lot of water, but too much salt for it to be useful. There is another place on Earth that has plenty of H2O, but not enough liquid water to support much life – does that make sense?

Antarctica. It is hard to believe that with all that ice, miles thick in some places, there is not enough free water to keep plants and animals alive, but in many parts of the continent, that is the case.

McMurdo Station is the largest community on
Antarctica, if you don’t count the penguins. It is
located near the McMurdo Dry  Valleys, the driest
places on Earth. This is due to the katabatic winds.
Cold air is more dense, and is pulled downhill. The
wind can reach speeds of 200 mph, and as it warms,
it evaporates all the moisture on the ground and in the air.
Some areas of Antarctica do support a little life; two vascular plants exist on the frozen continent, hair grass (Deschampsia antarctica) and the pearlwort (Colobanthus quitensis). These plants only grow on the west coast peninsula.

In the McMurdo Dry Valleys, east of McMurdo Station and the Ross ice sheet, almost nothing grows. There are hypersaline lakes here that put the Dead Sea to shame, including the Don Juan Pond that is 18x the salinity of the ocean.  

There are no vertebrate animals in the valleys; microbes make up all the biology there. In all of Antarctica, only 67 species of insect are found, and most of these live as parasites on penguins.

The exception is the wingless midge (Belgica antarctica). At an average of 6 mm long, this fly is the largest purely terrestrial and year-round animal on the entire continent (penguins only live on the continent for part of the year).  This flightless fly relative lives in algae mats, on rocks, and in the mud… just about anywhere it wants to. There are no competitors on Antarctica; this walking fly reigns supreme!

Belagica is well adapted to life in Antarctica. It is
black to absorb heat, and it is wingless so it won’t
be blown out to sea by the strong winds. It has a
short egg laying time and adult life span so that it
can complete its life cycle in the highly variable
summer season.
Other adaptations allow B. antarctica to thrive in this harsh environment. While the vast majority of plants and animals die with a relatively low level of dehydration (5-25%), these midges can survive a 70% water loss event - I suspect they can’t expectorate! In the winter…… WINTER? Isn’t it always winter there? Well, no; there is a colder season....the midge can react to winter by dehydrating and then coming back to life in the spring.  Something like having a piece of beef jerky moo after you start salivating on it. Amazing.

Recent evidence shows just how adapted B. antarctica is for the dehydration. The midge has one genetic response to thermal stress, whether it be hot or cold. They turn off some pathways and increase glucose metabolism pathways.

But in dehydration, it has different responses to different patterns of dessication. If it is a rapid dehydration, glucose metabolism pathways are up regulated, but if it is slow and steady, a whole different set of pathways are upregulated, including those for different osmoprotectant molecules (trehelose and proline).

The dry valley temperatures (-10˚C to -51˚C) could easily cause havoc with the midge’s protein function, including the pathways that protect it from dehydration stress. Heat shock proteins help to stabilize protein function in temperature extremes, usually they are expressed (transcribed from DNA and translated from mRNA) for short periods of time, only when there is an abnormal event. But Belgica’s heat shock proteins are expressed all the time. This is a huge energy investment, and an investment that few animals are willing to make. But in areas with too much salt or too little water, sacrifices must be made.

Next time we will talk about one of the greatest exceptions in biology, an organism that can live in the Atacama Desert, the Jordan Rift Valley, the Great Salt Lake, and even at Antarctica. It's not a bacteria, not a fungus, not a plant, not an animal – this is one heck of an exception.

Teets NM, Kawarasaki Y, Lee RE Jr, Denlinger DL. (2012). Expression of genes involved in energy mobilization and osmoprotectant synthesis during thermal and dehydration stress in the Antarctic midge, Belgica antarctica. J Comp Physiol B DOI: 10.1007/s00360-012-0707-2  

For more information or classroom activities on biological deserts, life in the Dead Sea, and life on Antarctica, see:

Biological deserts and gyres –

Life around the Dead Sea –

Life in Antarctica -

Wednesday, September 14, 2016

I Am Your Density -- Life On Ice

Biology concepts – density of water, latent heat, stratification

Ernest Rutherford showed that atoms were
mostly space by shooting alpha particles at
a sheet of gold foil. Only a few particles struck
something solid, most just passed straight
through – because the atom is mostly the
absence of matter.
It is amazing to know that atoms are mostly empty space. Atoms make up everything around us, including the stuff that hurts when it hits me in the head, but even those things are mostly empty space... or maybe its my head that's empty.

When atoms fit together to form molecules and molecules fit together to form solids and liquids there is also space. How massive the molecules are and how much space is between them determines a substance’s density.

Density (mass per unit volume) has a big impact on biology, and we have been talking about water for a few weeks, so let’s talk about the density of water. Simply put, without water’s unique density properties, life as we know it on Earth would not be possible.

Pure liquid water has a density of 1 g/cm3 (or 1 g/ml). This is 800x times the density of air, so moving around in water is much harder and requires more energy than moving around on a land. Try running in the pool – we just aren’t built for moving in water.

Gram for gram, fish have more muscle than
any other vertebrate animal. Notice how the
muscle fibers are arranged in different
directions to provide forward movement as
the skeleton changes orientation.
But fish have adapted streamlined shapes and big muscles in order to move through water a little easier. The skeleton of a fish is the most complex of all vertebrates. The skull anchors the waving of the vertebral column and the attached muscles. The muscle fibers (myomeres) are arranged so that the muscles can contract in several different directions as the swaying motion passes down the fish body. In all, a fish is about 80% muscle. If you are a marine fish, you’d better be even stronger, since ocean water is slightly more dense (between 1.02 and 1.03 g/cm3, depending on the salinity).

But here is the amazing part - when water freezes, its density goes down. Most substances are denser as solids than as liquids, but water is the exception. As ice crystals form, the water molecules arrange themselves in a very particular order, and this order places slightly more space between them as compared to when they are in liquid form. More space means less mass per unit volume, ie. lower density (0.92 g/ml)….. and this is a key to life on Earth.

Water will form ice crystals in a definite structure,
with more space between the molecules than when
in liquid form. Snow crystals form from water vapor,
not liquid water, and retain a more hexagonal lattice
shape that may stack on one another.
Imagine for a moment that ice was denser than water. Then as the winter came, the winds would blow, the surface water in the pond behind your house would start to cool down, but the deeper water would be a little warmer (remember that water has a high specific heat, it likes to retain its heat. As the surface water arranged itself into a crystal form, ie. turned to ice, it would sink. The warmer water would then be pushed up higher and exposed to the colder temperatures, freeze, and fall to the bottom. Eventually the pond would fill with ice, and be completely frozen.

Few animals or plants could survive in a solid block of ice, so life would cease to exist in the pond. What is more, when spring came, the sun’s energy and warmer temperatures would have to penetrate to bottom of the pond in order to melt all the ice, and this would take longer than a spring summer and fall to occur. Most bodies of water would stay somewhat frozen all year long.

Our food webs (who eats who) depend so much on the growth in water, and half of the Earth’s oxygen’s production oxygen depends so much on phytoplankton, the one celled plants that float on the water’s surface and release oxygen as a by product of photosynthesis. So we couldn't survive for long with completely frozen bodies of water. What is more, frozen lakes and bays would eliminate huge heat sinks that normally keep the surface of the earth warm, so we would plunge into another ice age.

Can you imagine if the massive number of aquatic organisms died as a result of their environment being frozen year round? The animals that feed on them would then die, and the animals that feed on them would die, etc. Eventually the animals on the land that feed on the amphibians and fish would die, and so on.  What’s more, we humans would be looking for more warm clothing while we gasped for enough oxygen to survive! Relax, we are all just fine, and it is because ice floats. Surface water freezes, trapping heat below and keeping the aquatic organisms comfy and cozy until spring.

The North American wood frog can freeze
solid in a long Arctic winter, but once it thaws,
it has work to do. It must find a find a mate and
then fertilize the eggs. The fertilized eggs have
to develop from to tadpoles and then to adults
during the short warm period. Then they can
freeze next winter.
You might have noticed that above I mentioned that MOST organisms can’t survive being frozen, but there is an exception. The wood frog (Rana sylvatica) winters in shallow burrows that are not protected from the cold. To survive, the frog actually freezes solid!

Nucleating proteins in the frog’s blood act as point for ice to form as soon as the frost touches the amphibian’s porous skin. Since the frog is still above 0˚C at this point, the freezing is slower, and the frog can control it. As the liquids freeze, the water is pulled out of the frog’s cells.

It replaces the water with huge amounts of glucose and sugar alcohols, that keep the cells from forming ice crystals (they are sharp and would puncture the cells causing permanent damage and death). Eventually, the frog is 65% frozen and the internal organs are surrounded by a pool of ice until spring, when it takes about 10 hours for the frog to thaw and hop away. Scientists are now using this process to freeze and thaw rat hearts and livers without damage, in hopes to use to the process in human organs for transplant.

But freezing and thawing a whole organism is harder than using a glucose bath to freeze individual organs. Research from early 2013 shows the energy that R. sylvatica must spend to accomplish this feat. In response to cooling near the freezing point, the wood frog increases its metabolism to prepare for freezing. But this increase in metabolism is nothing compared to the increase the frog undergoes when freezing is first detected in its tissues. Carbon dioxide (a sign of metabolism) is increased by 5.8 fold during freezing, as to the period just before freezing. This increase is needed to mobilize glucose into the tissues as the cryoprotectant.

The same thing happens when R. sylvatica thaws, metabolism increases to exactly the same degree as during freezing. But in this instance, the increased cellular activity is necessary for re-establishment of homeostasis and for tissue repair (no anti-freezing strategy is perfect). We have a long way to go to mimic the wood frog's entire preservation strategy, especially since the frog may go through these increases as many as twenty times each winter!

The wood frog takes advantage of freezing in order to survive. Humans can also take advantage of freezing water (other than keeping your drink cold); in fact, your orange juice may depend on it. Freezing of oranges or grapes ruins them for the same reason it kills animals, it causes frostbite. Ice crystals stab through the cell membrane and cell contents spill out. This isn’t conducive to continued function.

To prevent oranges and grapes from freezing, farmers will spray them with water when their frost warning systems sound the alarm. Does that make sense, spraying with water to keep something from freezing? It has to do with a property of freezing called latent heat. This is an amount of energy taken up or given off when a substance changes phase (solid to liquid to gas). The energy goes to changing the arrangement of molecules with no change in temperature.

Oranges can be protected from freezing by
spraying them with water which then freezes!
In a controversial use of genetic modification,
bacteria that do not permit ice crystal formation
can be sprayed on the oranges to compete with
the normal bacteria there. These "ice-minus"
Pseudomonas syringae can reduce frost damage
on oranges, but have not been used commercially.
As water surrounding the orange or grape changes from liquid to solid, the formation of crystals gives off heat (539.4 gram-calories per gram of water frozen).  The latent heat of the freezing mist is enough to keep the fruit above 0˚C. This technique doesn’t work if the temperature falls much below 0˚C or stays at 0˚C for an extended time, but it does work well enough to save millions of dollars per year in freezing damage.

Thermal changes have more to do with differences in water density than salt concentration does, so seasonal changes can alter density in both freshwater and salt water. Even if the changes are not enough to form ice or boil the water, differences in temperature can result in different layers of water within a freshwater body or an ocean.

Both salt water and freshwater are affected by the sunlight that strikes their surfaces. As water warms, it’s density decreases, and the nutrients in the water stay close to the surface. This supplies phytoplankton and algae with lots of food, and blooms can occur.

As winter approaches, the surface water cools and becomes more dense (down to 4˚C). The dense water drops to the bottom and taking nutrients down to the benthic organisms. When all the water reaches 4˚C, the surface can begin to freeze.

In the spring, the process is reversed, and the temperature layers (stratification) can churn again. In salt water, the differences in salinity are added to the differences in density to bring complex stratifications, both in salt content and temperature.

Stratification shows how temperature can set up
layers of water of different density (least dense is
the epilimnion). In the winter, the water is churned,
and then churned again in spring. These churnings
based on changing density move the nutrients around
so everyone gets fed.
Different organisms thrive in different temperature and salinity layers. In order to stay put, some floating organisms (planktonic) and swimming organisms (nektonic) can adjust their buoyancies. Fish can use swim bladders, which are air filled cavities to help them stay buoyant. The size of the bladder is regulated by the CO2 and O2 in the blood that can remain dissolved or leave the blood as a gas.

Bladderwort plants also use air filled cavities to keep part of themselves afloat. Sharks, on the other hand, produce large amounts of oil in their livers to reduce their density; oil is less dense than water, just look at your salad dressing layers.

Plankton can also slightly adjust their densities, but floating is easier for very small things. To them, water is thick, the polar charges have a larger effect on their small bodies. It would be like us trying to swim in molasses. They still have to adapt to seasonal changes in density, but they do it in more subtle (and harder to explain) ways.

Just because there is water around, it doesn’t mean that life will be easy. Next week we will look at a continent-sized exception to idea of water availability.

Sinclair, B., Stinziano, J., Williams, C., MacMillan, H., Marshall, K., & Storey, K. (2012). Real-time measurement of metabolic rate during freezing and thawing of the wood frog, Rana sylvatica: implications for overwinter energy use Journal of Experimental Biology, 216 (2), 292-302 DOI: 10.1242/jeb.076331

For more information, classroom activities and laboratories on the density of water, latent heat, North American wood frog, or stratification, see:

Density of water –

latent heat –

North American wood frog –

stratification –

Wednesday, September 7, 2016

Do You Drink Like A Fish?

Biology concepts – fish osmoregulation, shark osmoregulation, semelparity, iteroparity

The irony of fish drinking is not lost on this café in
the Hotel Portofino at Universal Orlando. What I
really like is the eye patch.
You’d think that fish would never be thirsty; if he needs a drink, he just opens his mouth. But some fish don’t drink a drop! Wouldn’t that be similar to some birds never breathing? Ridiculous.

Fish are good examples of the problems of maintaining proper water and salt concentrations. Some fish live in freshwater, and some in saltwater. These are opposite sides of the same coin when dealing with osmoregulation.

Freshwater fish live in a hypotonic (low salt) environment. The flesh of the fish contains more salt than does the water. Diffusion and osmosis work to equalize salt concentrations in different compartments. Therefore, water will move from the lake or river into the fish’s tissues in order to balance the salt concentrations by osmosis. Salt will not move out of the tissues, since there are molecular mechanisms that work to keep the inside.

Like the kangaroo rat, freshwater fish don’t drink. They do take in water when they eat and move water across their gills, but they don’t take in water just for the water. Even without drinking specifically, freshwater fish take in way more water than they need. Anywhere freshwater contacts a fish cell, water will move inward; this includes the gills, the mouth and gut, and the skin.

In a situation like this, kidney-mediated concentration of urine would be counterproductive; why retain water when water is exactly what you have too much of? Therefore, freshwater fish excrete large amounts of urine. Their kidneys have large glomeruli, which move lots of water into the collecting tubules for excretion.

Saltwater and freshwater fish have different ways of
dealing with salt and water loss and conservation.
Freshwater fish must conserve salt, while saltwater
fish must conserve water. The kidneys play a role,
but so do the chloride cells in the gills.
But if the freshwater fish aren’t drinking, how do they get their salt, which is present in low concentrations in the water? You’d think they would have to be drinking all the time just to collect enough salt.  To get around this, they conserve the salt they ingest through the food they eat. They also take in salts through their gill chloride cells, actively pumping sodium and chloride out of the freshwater and into cells that have a lot of mitochondria (to provide energy to pump the salts). The relatively short collecting tubules of the freshwater fish kidney allow for reuptake of a lot of salt, while excluding almost all the water.

Marine (saltwater) fish have the opposite problem. Their tissues are of much lower salt than they surrounding hypertonic ocean, so osmosis wants to dry them out, sending water out of their bodies. The amount of available drinking water is extremely low - can you imagine dying of dehydration while surrounded by water. Just ask anyone who has survived a shipwreck and prolonged float in the ocean; drinking seawater can be lethal.

However, marine fish must drink all the time in order to keep enough water in their body. Retaining water would be an essential function of marine fish kidneys. They are all fish, but their kidneys work in exactly opposite ways.  Marine kidneys have small or absent glomeruli, so little water is taken out of the blood, but long collecting tubules in order to excrete as much salt as possible.

Drinking a lot of saltwater leaves marine fish with way too much salt; more than their kidneys can get rid of. To aid in salt excretion, they also have chloride cells in their gills. In the opposite fashion of the specialized gill cells of freshwater fish, the chloride cells of saltwater fish actively sequester salts from the blood, and then pump the sodium and chloride out into the seawater.

Sharks have unique ways of maintaining
salt and water. I have no idea of their
mechanisms for pepper regulation.
But sharks are an exception among marine fish. They have a different way to combat high salt concentrations. Remember that osmosis means that water moves from areas of low solute (high water concentration) to areas of high solute (lower water concentration). For many marine fish, this would mean a constant loss of body water to the ocean and quick death by dehydration; much like pouring salt on a slug.

To overcome this movement, sharks produce and retain a huge amount of a chemical called urea; it is one of the soluble wastes that animals normally get rid of. This molecule doesn’t affect the electrical potential that salts create, but increases the solute concentration in the shark’s tissues at levels higher than in the seawater, so water (without the salt) will diffuse into the shark’s body. This is its source of fresh water.

Therefore, sharks are osmoconformers; they maintain an osmotic balance with their environment. If the shark becomes too salty and salt needs to be excreted, it has a salt gland, much like that of birds and reptiles, but the shark’s gland is located in it anus, not near its eyes or nose – that’s a big difference! Taken together, there is no force for movement of water in or out of the shark’s tissues, and the shark remains shark-shaped instead of shriveling or swelling up.

Here is a bullshark caught in the Potomac River.
And you thought that sharks in Washington D.C.
were just in the federal buildings.
An exception to this rule for sharks is the bull shark; it can live in both saltwater and freshwater. Most sharks put into in freshwater would absorb too much water and die of water toxicity. However, the bull shark’s kidneys can adjust to the salinity of the water within a short period of time. Their kidneys will remove less salt and more urea from their blood and tissues and into their urine. They move from being osmoconformers to osmoregulators.

A shark that can live in freshwater; this can present a real problem. There have been many bull shark attacks in rivers and estuaries (video), where people don’t expect to encounter sharks. It is suggested that this behavior and physiology is an adaptation that gives the bull shark a protected nursery for their young, away from predators.

Most fish are stenohaline (Greek, steno = narrow and haline = salt), which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than to that which they are adapted. However, there are exceptions- like the bull shark mentioned just a second ago.

Some salmon species are born in freshwater, then move to saltwater for several years, and then return to freshwater to spawn. Other fish, like some eels, are born in a marine environment, move to freshwater, and then go back out to sea to reproduce. If freshwater and saltwater fish kidneys work opposite of one another, how can there be fish that can do both?

Salmon returning upstream to spawn have many obstacles
to overcome. Their spawning grounds are usually a thousand
feet or more above sea level so they must leap up many
waterfalls. Oh, there are hungry bears too.
Salmon are famous for migrating to and from the sea. Almost all the species are semelparous (in Latin, semel = once and parous = breeding); this means that they return to their freshwater streams to spawn only once, and the trip and the reproduction kills them. The one exception is the Atlantic Salmon (Salmo salar). This species is spawned in, and returns to, the calm streams along the Atlantic coast several times in its life to spawn. This reproductive strategy is call iteroparity (itero = repeated). Iteroparous species lay fewer eggs at a time, the advantage is that survival chance is increased by repeated spawning – one bad year doesn’t destroy a big proportion of the population.

The migratory species of salmon are osmoregulators, as are most freshwater fish; their physiology demands a certain salinity level, and use energy to produce that level in their tissues. However, they can also adapt to various salinity levels. As such, these salmon as well as bull sharks are known as euryhaline (eu = good, haline = salt). Their physiology changes with the salt concentration.

While in freshwater the salmon will not drink, and will produce copious amounts of urine to get rid of the excess water it absorbs through osmosis.  But when it migrates to the ocean, it drinks all the time, and its kidneys work hard to remove the excess salts.

Chloride cells in euryhaline fish can sequester or
excrete salt, based on the hormone signals they receive.
This helps some fish move from aquatic to marine
environments and back again.
But the gills are the key to survival in the both the freshwater and saltwater environments. Energy consuming reactions will transport both Na+ and Cl- against their gradients, so they pump Na+ and Cl- into the fish’s tissues in freshwater and out of the fish’s tissues in saltwater. It is an adaptation of the marine fish’s chloride cells to work in both directions. This switch, as well as the kidney’s change in urine concentration, takes time. Therefore, salmon will spend days or weeks in intermediate zones, or estuaries, before going out to the ocean, and before returning to the rivers.

These are difficult lifestyle choices for salmon, the trips and the spawning kills them. So what is the advantage? The movement to oceans provides the growing salmon with readily available sources of food, so competition is reduced. The return to where they were spawned is just a good bet; if the stream was good enough to spawn them, then it is still probably a good place to lay eggs. Finally, working so hard to get to the spawning ground just a single time allows for selection of strong individuals, allows for huge numbers of eggs to be laid (the chance that some survive goes up), and the death and decomposition of the adults provides nutrients for the hatched fry (baby salmon). But these are human interpretations, I bet there are other advantages and disadvantages. However,  one thing is for sure, the balance sheet for these species comes out in favor of these adaptations – if it did not, nature would adapt further.

The eggs that don’t hatch and the carcasses of the mated
Adults create nutrient rich waters for the fry to develop in
before they head out to sea.
How about one more exception for today? Some individuals in semelparous species of salmon (Chinook, Coho, Pink, Steelhead, etc.) will not die after spawning, and will return again to the ocean. These individuals are often females, and are often smaller than average. These gals reverse their salt and water conservation strategies several times in their lives, making them prize winners for osmoregulatory exceptionality.

Next week, let’s tackle how the properties of hard water affect all life on Earth.

Sakamoto T, Ogawa S, Nishiyama Y, Akada C, Takahashi H, Watanabe T, Minakata H, & Sakamoto H (2015). Osmotic/ionic status of body fluids in the euryhaline cephalopod suggest possible parallel evolution of osmoregulation. Scientific reports, 5 PMID: 26403952

Cozzi RR, Robertson GN, Spieker M, Claus LN, Zaparilla GM, Garrow KL, & Marshall WS (2015). Paracellular pathway remodeling enhances sodium secretion by teleost fish in hypersaline environments. The Journal of experimental biology, 218 (Pt 8), 1259-69 PMID: 25750413

For more information and classroom activities on osmoregulation in fish and sharks, chloride cells, and reproduction strategies, see:

Osmoregulation in fish –

Chloride cells –

Osmoregulation in sharks –

semelparity and iteroparity –

Wednesday, August 31, 2016

Don’t Eat The Yellow Snow

Biology concepts – osmoregulation, tonicity, phytohormones, avian kidney, pinnieds, cetaceans

African elephants are larger than asian elephants, but their
urine production is similar. A 2007 study found that
African elephants can differentiate family members
based on their urine. It is similar to marking territory,
but they use urine to keep track of family members who
may be out of sight.
The asian elephant can urinate as much as 55 liters/day. That's about 3/4 of the volume of the average size bathtub! By comparison, the vaunted racehorse can only manage about 6 liters/day. Maybe we should rethink that old saying.

We know from the posts of the last few weeks that both salts and water are necessary for life, and that they work together to keep their concentrations within safe limits; a process called osmoregulation. You suspect correctly that kidneys and urination is involved, but what about plants – they don’t use the restroom.

For many animals, the kidney is the major organ of osmoregulation. The average adult human voids 1-2 liters of urine each day, but an uncontrolled diabetic with polyuria (poly=much and uria=urine) might expel 5-6 liters. Maybe we should bet on diabetics at the racetrack.

We get rid of water and soluble wastes via our kidneys. The kidneys filter the blood; nearly 800 liters of the red stuff each day. The basic filtering unit of the kidney is the nephron, who we met previously (Sorry, I Don’t Drink), made up of the Bowman’s capsule and sets of tubules.

Solutions of different tonicity have similar effects on plant
and animal cells, but plant cells can handle it better because
they have a rigid cell wall.
If the body is low on water, more water is reabsorbed in the tubules. Likewise, if the body has too much salt, few of the salt ions are reabsorbed in the tubules. In this way, our kidneys are basically concentrating our wastes in a small amount of water for excretion from the body. The amount of water depends on many factors, including the need to keep the cells at the right level of tonicity (concentration of salt relative to outside the cells).

Solutions can be hypertonic, meaning they have more salt than the cytoplasm, and water will flow out of cells by osmosis. Solutions can also be hypotonic, with less salt than in the cells (water will flow in to the cells) or isotonic, with the same osmotic pressure inside the cells as outside.

We all know that we don’t urinate the same amount all the time – drink more, go more. However, you don’t urinate the same amount you drink; your urine is concentrated by your kidneys in order to conserve water. Therefore, there must be some control mechanism. The answer is hormones. A hormone (“to set in motion” in Greek) is a small protein that is released from one cell and then acts as a chemical signal on other cells, either through the bloodstream (endocrine hormones) or through a duct (exocrine hormones) to the bloodstream or directly to other cells.

The angiotensin system. 1. The body senses that water
is low. 2. The kidney releases renin. 3. Renin  and
angiotensin converting enzyme produce angiotensin II
from angiotensin I in the lung. 4. Angiotensin II stimulates
aldosterone in the adrenal glands. 5. Aldosterone causes
more water and salt to be reabsorbed in the Loop of Henle;
this increases the blood volume and solves the problem.
Aldosterone is produced by the adrenal glands and acts on the distal collecting tubules of the kidneys. This endocrine hormone acts to conserve sodium and water and secrete postassium, thereby reducing urine volume but increasing the loss of potassium.  Aldosterone is released in response to angiotensin levels in the plasma, which in turn are controlled by sodium and water levels in the blood.

Arginine vasopressin (AVP, also called antidiuretic hormone) is another endocrine hormone that reduces the amount of water to be lost in the urine. This hormone is produced in the pituitary gland of the brain and also works to conserve water. By reducing the amount of water lost, the blood volume (which is mostly water) is increased, so blood pressure increases. This is why people are given intravenous fluids when they have lost a lot of blood.

The exceptions to this mechanism of kidney function are the mammals that live in hypertonic (saltwater) environments, like whales and dolphins (cetaceans) and seals or walruses (pinnipeds, latin for feather- or fin-footed). It is hard to study the urination in these animals in their native environment; they urinate in the ocean. Are you going to measure their individual contribution to the ocean – I think not.

Water wants to flow out of the cells and into the sea (hypertonic as compared to the cells), trying to balance the salt concentrations in both places. Therefore, the marine mammals must conserve freshwater or they become dehydrated. Both pinnipeds and cetaceans have large kidneys with enough renal tubule length to produce very concentrated urine, thereby conserving water. However, it appears cetaceans don’t really take advantage of this. Instead, they make a lot of metabolic water (Gimme Some Dihydromonoxide) and can keep from dehydrating by using the water they produce through cellular respiration.

Here is an inside view of a seal kidney. It’s huge! The many
lobules provides much tubular area to take up freshwater
and concentrate the urine.
Pinnipeds don’t drink water saltwater to any degree at all, they get their freshwater from their diet and their metabolic water.  Scientists use to think this was also true for cetaceans, but recent studies show that they do drink a small bit of seawater – not enough meet their water needs, but also not more than their kidney’s can handle.

Don’t think that marine (saltwater) mammals have it so bad. If they were to abandon the seas for freshwater sources, they would just trade one problem for another. Freshwater mammals have too much of a good thing, they run the risk of losing too much salt by being in so much salt poor (hypotonic) water all the time. This is why the kidney is so amazing, it can adapt functionally and anatomically to get rid of too much water or too much salt, depending on where you are. That is not to say the kidney is the only anatomic mechanism needed to maintain osmolarity within a tight range. Many organisms need more than kidneys, and have developed completely different mechanisms of osmoregulation.

Bird kidneys may be small, but they represent an evolutionary
intermediate, Some parts have short loops, like most mammals,
and some have long loops, like pinnipeds and cetaceans. However,
most of the kidney has reptile-like nephrons with long loops.
Birds share some water conserving and salt regulating apparatus with mammals. Avian (bird) kidneys have about 75% of their nephrons with reptilian structure, and 25% mammalian nephrons, containing a Loop of Henle. Therefore, avian kidneys are not as good at removing water and regulating salts as mammals are. Mammal urine can be concentrated 20-50x as compared to blood (the Kangaroo rat can produce a 9000x concentration), but birds can only manifest about a 2-3 fold concentration.

Therefore, birds have another mechanism to get rid of salt and maintain an osmotic potential within its limits. The salt gland is found in birds and reptiles. In many birds it is located near the eyes or nostrils (in crocodiles, salt is excreted through their tongues – everything tastes salty to them).  The salt gland removes Na+ and K+ from the blood, allowing birds and reptiles to consume saltwater or animals that live in saltwater.

Some organisms have it easier, like amphibians. With semi-permeable skin, they just leak salt out through their entire skin surface. Other organisms aren’t so lucky, like plants.

Plants must also regulate salt concentration, but they don’t have a familiar excretory system; in fact, they don’t have a specific osmoregulatory system. Water is lost via transpiration (Sorry, I Don’t Drink), and adjustments can be made to alter the amount of evaporation that occurs. Unfortunately, transpiration of water is linked to moving nutrients such as salts up the plant from the roots to the leaves. Therefore, shutting down transpiration will also shut down movement of nutrients. 

Plants in high temperature, low humidity, high wind environments have the highest rates of transpiration and are in danger of losing too much water. Once again, hormones are the answer. Plants do have hormones (phytohormones), so they probably have to deal with teenager issues just like human parents. Abscisic acid is an important hormone which shuts off transpiration. This phytohormone closes the stomata (stoma = mouth in Greek) on the upper sides of leaves, from which water evaporates and gases are exchanged. Abscisic acid also promotes water absorption from roots and root growth.

Some plants are cryptophytes by surviving unfavorable
seasons either underground (geophytes), hide their
seeds in the marshy mud (helophytes) or underwater
(hydrophytes). Hydrophytes in general are plants that
have their roots in water or water-logged soil.
Many xerophytes (plants that live in hot, dry places) have adapted to resolve these issues. They have leaf modifications to reduce water loss; needle-shaped leaves, sunken stomata, and waxy cuticles to cover the leaves. On the other hand, in hydrophytes (plants that live completely or almost completely in water), salts and water can be absorbed in the entire plant, not just the roots.

In terms of cations (Na+, K+), plants have a problem. They use potassium as their primary intracellular cation, but dirt is usually potassium-poor. Therefore, plants have K+ transporters to actively take up this ion. Unfortunately, the transporters don’t discriminate very well between K+ and Na+, so often times too much Na+ is taken up into plants.

Red mangroves have impermeable roots that help keep
out salt, and can also secrete some salt from there leaves,
but their most visible mechanism is the yellow salt leaves.

Excess Na+ can be toxic to cells, so measures must be taken to deal with these ions. Glycophytes are plants that are salt-sensitive, and include many of the plants that we cultivate. Therefore, soil salinity is an important factor in agriculture and gardening. Much research and breeding continues to an effort to produce crops that are better at differentiating uptake of K+ and Na+. Halophytes (halo=salt, phyte=loving), on the other hand, will allow the uptake of the excess ions, and then sequester them in vacuoles to prevent cellular damage.

Some plants live in extremely high salt environments. One example, the red mangrove tree, is a facultative halophyte. Facultative is a fancy way of saying “optionally.” These trees live in estuaries, where the river meets the sea. The water is quite salty there, and the mangroves are rooted in the water, so excess salt could be a problem. To deal with the toxicity of the excess Na+, the mangrove will store the salts in selected leaves, called the “kidney leaves.” When a toxic level is reached, the leaves turn yellow and just drop off. The tree must constantly invest energy in producing new leaves, so there is a cost to this way of life, but it seems to work for them.

If plants that live in or near seawater have adaptive mechanisms to maintain proper salt concentrations, then how about fish? We'll look at the osmoregulatory tricks by these organisms next week.

Ben Hamed-Laouti I, Arbelet-Bonnin D, De Bont L, Biligui B, Gakière B, Abdelly C, Ben Hamed K, & Bouteau F (2016). Comparison of NaCl-induced programmed cell death in the obligate halophyte Cakile maritima and the glycophyte Arabidospis thaliana. Plant science : an international journal of experimental plant biology, 247, 49-59 PMID: 27095399

Peña-Villalobos I, Valdés-Ferranty F, & Sabat P (2013). Osmoregulatory and metabolic costs of salt excretion in the Rufous-collared sparrow Zonotrichia capensis. Comparative biochemistry and physiology. Part A, Molecular & integrative physiology, 164 (2), 314-8 PMID: 23103672

Takei Y (2015). From aquatic to terrestrial life: evolution of the mechanisms for water acquisition. Zoological science, 32 (1), 1-7 PMID: 25660690

For more information, classroom activities or laboratories about osmoregulation, tonicity, abscisic acid, avian kidney, pinnipeds, cetaceans, see:

Osmoregulation –

tonicity and osmotic pressure –

abscisic acid –

avian kidney –

pinnipeds –

cetaceans –