Does Life Come In XXXS?

Biology concepts – characteristics of life, archaea, bacteria, mycoplasma, synthetic biology, symbiosis, parasitism, nanobacteria, genome

As part of this blog, we have talked about some pretty small life. Wolffia globosa is the smallest flowering plant, only 0.6 mm long. We also talked about archaea, a different kingdom than bacteria, but still on the smallish side of life. The tardigrade is the toughest animal, but is also one of the smallest, at 100 µm (0.00394 inch).

The organism on the top is T. dieteri, and arthropod, just

as is any crab or spider. The size is deceiving. The pictures

on the bottom are to scale and are copepods, also

arthropods. The organism on the top is a parasite of the

organisms on the bottom. The small blue line? That would

be the scaled size of T. dieteri. So…. it’s SMALL.

The question for today is – is there a minimum size for life? Candidates might include bacteria or archaea; heck there’s an arthropod, Tantulacus dieteri, that's only 85 µm long! As long as we can keep finding smaller and smaller cells, we know that the minimum size for life is that small or smaller. So we keep looking – you’d be surprised how important it is to keep looking for smaller life.

Here’s one thing we should be able to agree on, viruses don’t get to play in our game. Viruses are very small, but they're not life! We’ve talked about this before - the seven characteristics of life (see this post). Viruses need a host in order to replicate, they don’t manage homeostasis, and they aren’t cells, so they aren’t life.

So how small has actual life become? Let’s assume that since tardigrades and T. dieteri are over 50 µm, huge when compared to some bacteria, our current minimum for life is probably a bacterium or archaea.

Let’s go straight to the genus of smallest bacteria we know about – the mycoplasma (from mykes = fungus, and plasma = formed). They were first described in 1898, but the observer didn’t have a clue what he was looking at; hence the fungal part of the name.

Mycoplasma don’t have the traditional cell wall of many bacteria, so they look different and this might be why they were mistaken for fungi. Whatever the scientists thought of them, they were confusing enough to be roundly ignored for 50 years. Rediscovered in the 1950’s-1960’s, this time they were thought to be L-forms of bacteria. L-forms are organisms that for some reason have lost their cell wall.

There are stable forms of L-bacteria; they can live divide and live on without their cell wall. There are also unstable L-forms as well; those that may revert to walled bacteria at any moment. Are mycoplasma simply bacteria that have lost a cell wall? Nope. They didn’t have a cell wall to lose. They have no cell wall genes, so if they had a cell wall, it was millions of years ago, before they became their own genus.

The difference between some free living cells. You can

probably see the E. coli in bright green, but you may have

to squint to see the mycoplasma above it. It’s pink. Really,

it’s there. Compare these sizes to those of the arthropods

above. 1 mm is equal to 1000 µm.

Mycoplasma is really, really, SMALLLLLLL.

Mycoplasma are generally described in the range of 0.2-0.8 µm in diameter. But this is a little misleading, because they are often not spherical. Even without a cell wall, they can take interesting three-dimensional forms and maintain them. Mycoplasma pneumoniae, which causes a form of …..….. anyone?…….. right, pneumonia, is pear shaped, so its 0.25 µm diameter is actually the measurement on its short side.

So mycoplasma are small, but they still have to play by the rules. They contain DNA and salts and proteins and ribosomes and other things that take up room. A single ribosome is about 50 nm in diameter (0.05 µm or 0.00000005 m), so there must be a certain volume required for the cell to function – a minimum size for life.

Which of the mycoplasma species is the smallest? Mycoplasma genitalium is considered to be the smallest mycoplasma known, and the smallest form of free-living organism - my gosh – you can fit about 400 M. pneumoniae inside one E. coli! As such, it is the current minimum size for life that we have. M. genitalium is 200 nm (0.2 µm) x 600 nm (0.6 µm), so they’re pretty dawg on small. Let’s put it this way, there are 25,400,000 nm in one inch – mucho small.

It is important to note that M. genitalium is free living, but does need some help. It uses cholesterol in its membrane but doesn’t make it itself. It picks it up from the cells that it lives near……wait for it….. your genital epithelium.

One of the human diseases that is becoming more

convincingly associated with M. genitalium is pelvic

inflammatory disease (PID). Resulting when many

different sexually transmitted diseases go untreated,

PID can cause permanent damage to the reproductive

organs of women. It is important to get treatment early.

The inflammation of PID may be associated with the

fallopian tubes or ovary, and will cause a chronic pain

in the lower abdomen, bleeding and pain on urination.

M. genitalium is a cause of non-gonococcal urethritis (inflammation of the urethra). A late 2013 review states that 1-3% of the general population is infected with M. genitalium, more than with gonorrhea. It is linked to pelvic inflammatory disease, and the review cites studies showing that people infected with this mycoplasma are more at risk for HIV and have more dual infections. It’s a sexually transmitted organism, just another reason for proper restraint. But even though it's helped out by your genital epithelium, it can live on its own and divide outside a host, so it's considered a free-living organism.

The idea of free-living is important because M. genitalium also has a very small genome (amount of DNA in one cell, including the list of all its genes). M. genitalium has about 580 kbp of DNA where kbp = kilobase pairs. Remember that DNA is doubled stranded (usually) so each base is paired with another. Knowing this, we count them as a unit. In all, M. genitalium has just 520 or so genes; it can make about that many proteins.

Genome size could be another way of determining the minimum size of life - what's the minimum number of genes or number of base pairs of DNA for an organism to still meet all seven characteristics of life? As of summer 2014, no organism smaller in size than M. genitalium has been described, but there have been some other organisms discovered with smaller genomes.

Nanoarchaeum equitans was thought to have the smallest gene for a while, with only 491 kbp of DNA. It is an archaea that lives on the edge of hydrothermal vents at the bottom of the ocean. But it is an obligate symbiont with another archaea; it can’t survive without its partner, so can you say it has the minimal genome? It relies on another organism’s DNA.

On the left is the leafhopper in which N. deltocepahlinicola makes

his home. Well inside its cells that is. The leafhopper survives on

phloem and xylem; high in carbs but little protein. The bacterium

makes the amino acids the leafhopper can’t in exchange for energy

in the form of ATP. On the right is a colored photomicrograph of

the abdomen. The red is one type of endosymbiont bacteria,

the green is N. deltocephalinicola.

This is also true of Carsonella ruddii (159 kbp, 182 genes) and Nasuia deltocephalinicola. They are bacteria that must live inside insect cells, like those of grasshoppers. N. deltocephalinicola has the smallest known genome (112 kbp, 137 genes), but it doesn’t even make ATP, it steals it from the arthropod cells. This could hardly be considered free living, and so it can’t be considered the minimal genome for life. And even at that, their cell sizes are still bigger than M. genitalium.

So why is it important to find the minimal size and minimal genome for life? So we can use the information. J. Craig Venter (of the human genome project) wanted to develop a synthetic form of life (synthetic biology); a bacterium that could be developed to provide hydrogen for energy or eat waste to reduce pollution. Others say we need to know so that we can better recognize life on other planets, or life that may have come here from other planets (astrobiology).

Being J. Craig Venter, develop a synthetic form of life is exactly what he and his research institute did. It’s interesting that Venter was one of the scientists that first sequenced the entire M. genitalium genome in 1995. Some 15 years later, Venter’s JCVI-syn1.0 (2010) was the first synthetic life, housing 1000 kbp and 500 or so genes. The genome was based on that of another mycoplasma, M. mycoides. They modified the genome, and introduced it into a cell membrane that had been evacuated of all its constituents. The resulting cell was capable of growing, dividing, you know…. living.

If M. genitalium represents our current estimate for the minimum size of life, it’s only because we’re thinking of life as we know it. Perhaps we have already found life that is smaller, and the minimum size is actually much smaller than M. genitalium.

This is a photomicrograph of a meteorite from Mars. The

small spheres (like the ones the arrows point to, are

supposedly nanobacteria. Proof of life on Mars,

contamination from Earth nanobacteria, or just mineral

spheres that look a little like incredibly tiny bacteria?

The answer is C.

Something termed a nanobe and something else called a nanobacterium were described 20-30 years ago. Nanobes were first found in the rocks that came up during oil drilling in Australia, while nanobacteria were also found in surface rocks.  The size of both (about 1/20 size of M. genitalium) negates their use of ribosomes and DNA. They stain for DNA, but this may be artifact, the artificial result of other things picking up the stain.

But nanobes/nanobacteria have their proponents. Some scientists say that since no DNA has been exhibited, they are a completely different form of life, so size restriction (big enough to hold ribosomes) doesn’t apply. Nanobacteria are also claimed to be important in human disease, as these structures are found in many calcifications of diseased tissues.

On the other hand, nanobacteria are probably just mineral formations. A 2013 study showed that they form spontaneously from many different biological fluid samples, and their appearance in diseased tissues is more a sign of disease than a cause of it. We’ll just have to keep looking for something smaller.

Next week, another question tackled and dissected - think pink.

Manhart LE (2013). Mycoplasma genitalium: An emergent sexually transmitted disease? Infectious disease clinics of North America, 27 (4), 779-92 PMID: 24275270

Wu CY, Young L, Young D, Martel J, & Young JD (2013). Bions: a family of biomimetic mineralo-organic complexes derived from biological fluids. PloS one, 8 (9) PMID: 24086546

Gibson DG, Glass JI, Lartigue C, Noskov VN, Chuang RY, Algire MA, Benders GA, Montague MG, Ma L, Moodie MM, Merryman C, Vashee S, Krishnakumar R, Assad-Garcia N, Andrews-Pfannkoch C, Denisova EA, Young L, Qi ZQ, Segall-Shapiro TH, Calvey CH, Parmar PP, Hutchison CA 3rd, Smith HO, & Venter JC (2010). Creation of a bacterial cell controlled by a chemically synthesized genome. Science (New York, N.Y.), 329 (5987), 52-6 PMID: 20488990

Let's Get Loud

Biology concepts – vocalizations, mechanical sounds, sonar, decibels, stridulation

Today it seems that truth is more complex than ever.

van Goethe was a German statesman and a very successful

writer. He wrote novels, scientific treatises, lyric poems, as

well as dramas. Born in 1749, one might say that his quote

was true for his day; it was a simpler time. But think how

simple our time will seem to those who live a hundred

years from now – unless we’ve found our way back

to the Stone Age.

I have worked for years in science, and I’m supposed to be a big boy and realize that things are complicated. But I still get frustrated when I can’t get a simple answer. It seems nothing's simple, every answer has a caveat – heck, I make a blog of the exceptions to answers!

This week’s question is no exception – What living thing makes the loudest sound? Notice I said living thing, because I didn’t want to exclude anyone from the contest. Who knows, maybe some bacterium living in Wyoming makes a heck of a racket, it’s just that nobody is around to hear it. Or maybe a redwood falling down is the loudest. See, nothing’s ever simple.

This leads us to a second question – one which we have to answer first. What’s a sound? We are surrounded by air, and air has mass and density – it’s stuff. You can push stuff around. When you push the air, it moves away, which creates a wave, because the air you move then moves the air next to it, and so on.

A sound wave is generated when a force creates a vibration, and that vibration moves air, and that vibration is then propagated through the air. The air moves in the same direction the vibration was moving, and this makes it a longitudinal wave (see animation below).

A sound wave is a longitudinal wave, where the source moves

in the same direction as the wave. There is a compression of

the medium (air or water) and then a rarefaction with fewer

molecules as the compression moves on. The wavelength is

the distance between compressions and the frequency (how

high or low the sound is) is 1/wavelength.

So now we have a sound wave, but do we have a sound? It’s like the old question, if a tree falls and nobody’s there to hear it, does it make a sound? If you define a sound as a sound wave, then yes it does. Does that mean that every sound wave is a sound? There are sounds waves that dogs hear and we don’t because the frequency is too high – are they sound? There are waves that are too low or too soft for anything to hear, are they still sounds?

If you define sound as something your brain recognizes, then a sound doesn’t occur until the sound wave is transduced (changed in form) by a your ear to an electrical chemical impulse and that impulse is interpreted by the auditory cortex of your brain. See, nothing has a simple answer.

So, does a lonely falling tree make a sound? Scott McFarland of the University of Oregon installed microphones all over Crater Lake National Park 100 miles southeast of Eugene. In the remotest parts of the preservation land, he has heard everything from buzzing mosquito wings to, yes, falling trees. Unfortunately, he has also heard human intrusion.

Even from the most isolated areas, about 20% of every recording included airplane noise. He also heard cars, people, and detonations. It seems that no place is really naturally quiet anymore. But there were those falling trees -does that mean they do make a sound? Nope, it was a sound wave captured by recording equipment, transduced to digital information, stored, changed back to a sound wave by a speaker, and then to a neural impulse by our inner ear. It still isn’t interpreted as a sound until it reaches a brain, any brain. If a rabbit or bull moose is nearby, their ears would transduce the sound wave and they would “hear” something, even if they don’t think to themselves, “Hey, a tree just fell.” So again, it’s not so simple, maybe something with a functioning ear is in range.

The old tree falls in a forest question started out as a philosophical

question. It was meant as a thought experiment to students to

discuss the nature of what is real versus what is observable. However,

when it existed as a philosophical question, no mention of sound was

made, but was concerned with whether the tree existed at all if no one

was there to perceive it. It wasn’t until 1883 that the sound reference

was made, and then it was posed as more of a scientific question,

as we approach it in today’s post.

The human hearing sense is pretty sensitive. The pressure needed to generate a sound wave that a human could hear is about one billionth the value of atmospheric pressure. But this sound wave would be just barely audible (depending on the frequency), so it would be soft.

This brings us to a very short discussion of what it means to be loud or soft. We often measure loudness in decibels (db). A decibel is one tenth (deci) of a bel (named for Alexander Graham Bell), which is a unit of power or intensity.

Every 10 db increase represents a 10 fold change in intensity, so the scale is logarithmic. In acoustics (from Greek akoustos = hearing, and ic= pertaining to) this means that decibel is a measure of sound pressure, compared to a reference pressure (20 micropascals – we’ll get back to this).

Howler monkeys are the largest of the New World monkeys. But in

one way they are like old world primates. Howlers are the only new

world monkeys with color vision in both males and females. They all

see like we see, so they can discriminate different shades of green, red,

and blue. In most new world monkeys, the color receptor gene is on

the X chromosome, so females may get two types and be color vision

function, but all males get only one, and see only black and white.

A fairly recent gene duplication in howlers have given it color

vision again.

Examples of things that are loud (high sound pressure) would be a jet taking off 25 meters away (150 db), a clap of very nearby thunder (120 db), or a Harley Davidson 25 feet away (70 db). Note the distances; sound waves dissipate in power as they travel, the transference of energy along the wave goes in all directions and is not 100% efficient. This is why you can’t hear your brother playing with your Rock’em Sock’em Robots when you’re down the hall.

So who’s the loudest? The howler monkey (genus Alouatta, 15 species) has a great claim to being the loudest living thing. Used for communication over tremendous distances, the howl of this primate reaches 128 db from several feet away (hear it here). Howlers have an enlarged hyoid bone that is U-shaped. This creates an air sac on their throat that they can use to make their howl resonate.

The howl is really more of a growl for males but is higher pitched for females, and they can be heard more than 3.5 miles away. The question is why do they do it. A 2014 study concluded that the black howler monkey (Alouatta pigra) use their calls for several reasons, but most relate to defense.

The lesser water boatman (Micronecta scholtzi) is only 2 mm

long, but packs a big auditory punch. It’s a freshwater (aquatic

rather than marine) insect. Many marine animals have loud

calls, but here is an example of one that lives in slow moving

streams and ponds. You can hear it when standing on the bank.

This means the sound is so loud it can traverse the water/air

boundary, which usually stops most sounds.

They howl most often in defense of feeding areas. The volume of the howl makes the monkey seem bigger than he/she is. They also use it to defend infants or mates from males that are not part of the group. It calls attention and help comes a runnin’.

If we look at smaller land animals, some of them are loud too. Cicadas can produce stridulations (see this post) that reach 100 db from a foot away. But the king of the small animals would be the lesser water boatman.

This small freshwater insect (Micronecta scholtzi) can put out a stridulation of 105 db, even though it’s entire body is only 2 mm (0.078 in) long! A 2011 paper in PLoS One broke down the song of the male into three different parts, each with its own peak intensity. The loudest part could be heard from a riverside, even though the insect is underwater. By comparing peak db to size, the male outcalls every other organism we will discuss. 

But it’s only the male that calls so loud. Why? Because he’s looking for a mate, and it’s his penis rubbing against his abdomen that makes the stridulation. This sort of eliminates the females from participating in the contest.

But back to the bigger animals. Bats are extremely loud, but their calls are of such high frequency that we can't hear them. Parrots can call to each other in the range of 100 db, but we can go bigger. In fact, the biggest animal may have the biggest voice as well.

The blue whale is the heaviest animal to yet live on Earth.

Being this large, it still has two natural enemies – man, who

hunted it to near extinction, and the orca. Orcas coordinate

their attacks on the blue behemoths, often trying to separate

babies from their mothers. Nearly 25% of sighted blue whales

have scars from either orcas or monumentally stupid octopus.

The blue whale (Balaenoptera musculus) is the largest living animal and the heaviest animal ever to live on Earth (yet). The whale’s song can reach 188 db! Some of the frequencies are too low for us to hear, but the higher pitched (higher energy) sounds can be detected 800 km (497 miles) way (hear it here).

Interestingly, a 2009 study shows that blue whale songs are getting lower in frequency. Since the whaling ban of 1966, male blue whale songs have been using lower and lower tones. The authors suggest several reasons for this. Males may not need to call out so loudly to find females because the ban has resulted in higher numbers of whales. But it is also possible that more man made noise in the ocean is forcing them to use lower frequencies.

The sperm whale is right there too. It doesn’t really vocalize, but it makes clicks to echolocate each other and prey. The clicks are made by forcing air through two lips (folds of tissue) in front of their blowhole. These clicks come in several varieties, but the “usual” click can reach 230 db. This makes them the kings of sound, if you don’t limit your choices to sounds made with the mouth.

Sounds in water are louder than in air, because the density is higher and the transmission is more efficient. So decibels in water are relative to 1 millipascal instead of the 20 micropascals in air. If you want to compare directly, you need to subtract 61.5 db from the water sounds. This still makes the sperm whale clicks 170 db and the blue whale song about 127 db, just about the howler monkey level. So the sperm whale wins our contest.

The different pistol or mantis shrimp are not very large, but

they have second largest sound to size ratio in the natural

world. Only one claw is the pistol. The right image is an

enlargement of the claw. S= socket, pl = plunger, D= dactyl,

p = propus. The muscles of the dactyl close it so fast, that the

water displaced in the socket by the plunger shoots out at

100 km/hr. This produces the cavitation bubble.

But for interesting and functional loud noise, I like to go with the pistol shrimp (family Alpheidae, hundreds of species). A special engineering twist allows them to cock one pincher like a gun. When they release it, the sound wave travels so forcefully and fast that the water around it turns to vapor and a cavitation bubble is produced (see video). The temperature in the immediate vicinity reaches 4000˚C and prey within 1.8 meters is stunned or killed! The sound, from several inches away, is 218 db (before subtracting the 61.5 db to match air measurements) when the bubble collapses.  

A 2006 study describes that the hoods of the carapace (shell) that partially cover the eyes of the pistol shrimp are to protect themselves from their own explosive snap. The study suggests the hoods evolved first, and that allowed for the development of stronger and stronger snaps. If this continues, the pistol shrimp may start taking humans out!

Next week, another question about the extremes of life.

Van Belle S, Estrada A, & Garber PA (2014). The function of loud calls in black howler monkeys (Alouatta pigra): Food, mate, or infant defense? American journal of primatology PMID: 24865565

Sueur J, Mackie D, & Windmill JF (2011). So small, so loud: extremely high sound pressure level from a pygmy aquatic insect (Corixidae, Micronectinae). PloS one, 6 (6) PMID: 21698252

McDonald, M., Hildebrand, J., & Mesnick, S. (2009). Worldwide decline in tonal frequencies of blue whale songs Endangered Species Research, 9, 13-21 DOI: 10.3354/esr00217

Anker A, Ahyong ST, Noël PY, & Palmer AR (2006). Morphological phylogeny of alpheid shrimps: parallel preadaptation and the origin of a key morphological innovation, the snapping claw. Evolution; international journal of organic evolution, 60 (12), 2507-28 PMID: 17263113

East To West And Back Again

Biological concepts – carbohydrates, heliotropism, monoecious, dioecious

I’m trying to think of a situation where quantity is better than

quality. Perhaps some could argue that since quality is subjective,

one person’s quality would be another person’s attempt for

quantity. In friends and experiences, I go with quality. You can

travel to every place on Earth, but if you don’t come back

changed, there was no quality. You can have many

acquaintances, but you really need only one true friend.

When it comes to the number of economically important plants, the Americas have not got many to show off. But what the two continents lack in number they make up for in quality. We have talked before about the biology of corn from North America and how it has been important for the development of molecular medicine.

Potatoes, cocoa beans, peanuts, and vanilla are also from the New World and deserve posts of their own. We’ll hear about vanilla later this summer. But one plant from the Americas has been important for food, oil, and decoration – the sunflower.

If we are going to talk about sunflowers, one question immediately comes to mind. Do sunflowers really turn to follow the sun?  The answer is more complicated than it would first seem, and the answer is just part of the amazing biology of this plant.

First things first – the sunflower (genus Helianthus, about 50 species), as named in Carolus Linnaeus in 1752, does not refer to their tendency to follow the sun. Instead, he called them sunflowers because, ”Who could see this plant….without admiring the handsome flower modeled after the sun’s shape.”

Analysis of nearly fossilized human waste from the caves of Arizona (4000 BCE) show that sunflowers were an important part of the Native American diet. Sunflowers were tough, so they could grow in the Great Plains and other environments that got little rain and lots of sun. They could also grow in temperate environments. Basically, all of North America was there home.

The buffalo would trample huge swathes of land in their migrations, and the torn up ground was perfect for germination of the sunflower seeds. Slowly, this rapacious weed became a cultivated crop. Hybrids were grown, crossing prairie species with forest species and such. In modern science, the sunflower has been used extensively to study genetics of hybrids, much of this work being done at Indiana University in Bloomington, IN – my alma mater, thank you very much.

Number two - the sunflower isn’t a flower, it’s an inflorescence. This is a scientific word for a group of flowers bunched together on the same stem. We talked long ago about the Philodendron selloum inflorescence that controls it’s own temperature and gets hot to attract pollinating beetles.

Sunflowers actually have two types of flowers, the rays and the

discs. The ray florets have a longer petal, they are yellow because

bees see yellow best. The rays are fertile, and have very small

stamens and pistils that provide pollen and ovules. The disc florets,

when male, may have a sterility gene, and this makes sunflowers

very good for studying hybridizations. They also have a naturally

occurring restorer gene, so that they can again make

functional pollen.

In the sunflower, there are two types of flowers, the ray florets around the edge that every one thinks are the only flower petals, and the disc florets, which everyone assumes are the seeds. The ray florets are sterile and therefore for show only; they attract the pollinators.

The ray florets are usually bright yellow, but the disc florets are different colors in different species. They can be yellow, maroon, or even red. The red varieties all stem from a single mutation, but that isn’t the weird part. The disc florets start out male, and produce stamens and pollen, but then turn female as they mature, with the stigma pushing its way up through the middle.

This makes the flowers “perfect” and the sunflower monecious, meaning that have both male and female structures on one plant, but it also makes them smart, as the different timing reduces the chances of self-pollination (pollen and stigma aren’t around at the same time).  For more discussion of monoecious (meaning “one house – male and female flowers on same plant, maybe even as the same flower as with the sunflower) and dioecious plants (male and female flowers on separate plants), see this post.

But even in this, the sunflower can be an exception. The florets mature from the outside discs to the inside discs over time. So while the inner ones may still be male, the outer ones may have become female. In times when pollinators are more rare, if a disc floret remains unpollinated, its stigma may bend down enough to touch the pollen of the still male florets more towards the center of the inflorescence! This is rare, but does occur in species that are annuals.

An achene is a type of fruit that has a hard shell and the seed is

inside. Strawberries are accessory fruits, where the accessory

organs from many achenes join together. The achenes are the

little pieces on the outside. The papery husk (exocarp) of the

sunflower achene is  made from the ovary wall and protects

the seed until it is ready to germinate, like being stuck in dry,

hard, cold ground, or in the belly of a bird.

And third, the disc florets each produce a single fruit (achene), which we call (incorrectly by the way) a sunflower seed. Inside the achene shell is the sunflower seed that we eat. A single sunflower inflorescence can have as many as two thousand disc florets, so that’s a lot of fruit. In species that have more than one inflorescence, each inflorescence will have many fewer than two thousand. Flowers are energetically very costly to produce. Incidentally, almost all the wild varieties have more than one inflorescence, the domesticated versions are bred to have one.

Now for the answer to today’s question – do sunflowers follow the sun? Well, yes and no. Young sunflower plants, including the very small, juvenile flowers, have the capacity to grow very quickly. This means lots of cell growth, and the need for lots of sunlight (to produce ATP and carbohydrates by photosynthesis).

The ability to follow the source of sunlight, called heliotropism (helio = sun, and tropic = loving) requires lots of cell growth. The flower stalks don’t turn so much as they grow in a different direction. As long as the cell growth is rapid enough and the stalk is small enough to respond to changes in cell size, the plant can appear to turn.

Heliotropism is seen in many plants; they need the sun for their

very lives, so it isn’t surprising that their biology would evolve to

maximize sun exposure. The reason the cartoon uses grass – that’s

the plant in which heliotropism was first studied. What scientist

discovered this marvel of nature? Charles Darwin.

The sunlight causes destruction of a plant hormone group called auxins, so they build up in the cells of the shady side. Auxins like indole acetic acid (IAA) promote cell growth and division, so there is much more growth (longer cells and more cells) on the shady side. The uneven growth pattern makes one side longer than the other and forces the stalk to turn (see picture).

So, immature flowers will face east in the morning and west in the afternoon. But that is only part of the answer. By morning, they’re facing east again. How does that happen? A current review (2014) suggests that there may be a diurnal rhythm of several plant hormones, or a natural easterly face that is altered by light signaling. The actual mechanism for the daily turning waits to be identified.

But even this is only half the story. As the stalk gets larger and the heavy inflorescence matures, there can’t be enough cell division or hormone action for the plant to move this massive flower. The mature flowers face east all the time. But why east? Maybe they just can’t bring themselves to move one morning, and since they start out facing east, they stay that way when they give up.

Maybe, but I would imagine there’s a more biologically reason than surrender. The 2014 review cites a study that hypothesizes that facing east protects pollen from the mature florets from sun damage. Final answer, sunflowers follow the sun until it’s time to make little sunflowers, then they settle down and face the rising sun.

So young sunflowers turn with the sun, but how about another question – Why? It’s an inflorescence, not the most efficient photosynthesizer (more about this soon), so why would that structure turn to keep facing the sun? It seems like it would keep the flower in one place and turn the leaves to the sun. Hmmmm.

Now that we’ve answered the question of the day and raised another, let’s talk about the sunflower and world history. But for some unfortunate biology, you might eat sunflower roots like French fries.

The Jerusalem artichoke tuber (top) looks a little like ginger root,

but it is sweeter and not so fibrous. See the text for why you almost

grew up eating McDonald’s sunchoke fries instead of potato fries.

One species of sunflower, Helioanthus tuberosus, has an edible tuber root that is often called a Jerusalem artichoke. Since the sunflower is from North America, you know that the Jerusalem part of the name is wrong. And it’s not an artichoke either.             How it got its name

Around 1600, the Jerusalem artichoke became a popular foodstuff. Easily grown and propagated, the sunflower tuber was a great source of carbohydrates and protein. It was easy to prepare, lasted a long time in storage, and didn’t taste like dirt or wood. Cultivation of the Jerusalem artichoke took off, and it became the primary food for many poor people and a delicacy for the rich.

The South American potato filled the same role, so who would win out as the food of the day? The Jerusalem artichoke (also called a sunchoke) had one big drawback, and it lost the battle. The potato won out, and 250 years later the great potato famine changed the immigration/emigration and ethnic patterns of the world.

What was this thing that cost H. tuberosus the war? It gives you gas. Among the many carbohydrate molecules produced by the Jerusalem artichoke is inulin. This polymer of six carbon sugars is one of those sugars that humans can’t digest, like cellulose. But our gut bacteria can.

Inulin is a branched chain of six carbon sugars. They come in several

varieties and together are called fructans. The “n” means there can be

any number of these units in the chain. They are a good source of

natural fructose, and chicory (right) is the most commercial source of f

ructans. Chicory has been used as a coffee substitute, a salad green

(endive and radicchio are types of chicory) and even in brewing beer.

In breaking down inulin, bacteria produce fructose monomers. They use these monomers as an energy source, and in doing so, produce carbon dioxide. In Central Europe, where the potato vs. Jerusalem artichoke battle was taking place, about 30-40% of the population have a genetic predilection for poor fructose absorption. This means more fructose stays in the gut….more bacteria food. This means much more carbon dioxide and …. flatulence. 

In U.S. finer restaurants and gastropubs, the sunchoke is making a comeback, mostly because Americans can usually absorb fructose just fine. And the fructose helps diabetics too. Many diabetics use the high fructose:glucose ration to even out their glycemic indices.

What’s more, a 2014 study found that mice fed a high fructose diet over time do develop type II diabetes and/or fatty liver. Preceding the disease development, many specific genes change their expression patterns. If their diet was supplemented with extract from Jerusalem artichoke, many of the genes showed normal expression, and the diseases did not develop. Not bad for a sun chasing flower.

Next week, another question to investigate - what/who makes the loudest noise in life?

Vandenbrink JP, Brown EA, Harmer SL, & Blackman BK (2014). Turning heads: The biology of solar tracking in sunflower. Plant science : an international journal of experimental plant biology, 224C, 20-26 PMID: 24908502

Chang WC, Jia H, Aw W, Saito K, Hasegawa S, & Kato H (2014). Beneficial effects of soluble dietary Jerusalem artichoke (Helianthus tuberosus) in the prevention of the onset of type 2 diabetes and non-alcoholic fatty liver disease in high-fructose diet-fed rats. The British journal of nutrition, 1-9 PMID: 24968200