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 –