Wednesday, June 26, 2013

The Colors of Alien Plants

Biology concepts – photosynthesis, chlorophyll, pigmentation, astrobiology, exoplanet, dormancy

The King Crimson Norway Maple in our front yard is
at least 50 ft. tall. It isn’t a rare tree, but I like it a lot.
In fact, it is invasive and native only in Asia. You can’t
plant on in several eastern states in the US, they are
taking over in some deciduous forests.
There is a large King Crimson Norway Maple (Acer platanoides 'King Crimson) in our front yard. Healthy and round, it is a fine showpiece. We are also blessed with a 15-foot tall burning bush (Euonymus alata 'Compacta') not more than thirty feet from the maple. The burning bush straddles the property line with our neighbor, so when it needs work, its theirs, and when it is beautiful in autumn, it’s ours. Together, they make our landscaping come alive with color and provide ample shade.

They’re autotrophs (auto = self, and troph = feed) as are most plants. They make their own carbohydrates from sunlight, carbon dioxide, and water. Our sun radiates light energy that can be captured and transduced to chemical energy, but not all stars are the same and not all plants are green, so…..

Question of the Day: How can starlight support non-green plants and could it might it be different elsewhere?

Chlorophyll is one of several plant pigments, and chlorophyll itself comes in several flavors, but the primary plant chlorophylls are a and b. The “a” version is the major pigment for photosynthesis, absorbing light at the two ends of the visible spectrum – blues and reds (see picture). Green and yellow light get reflected, and this is what we see. Chlorophyll probably evolved to use red and blue because blue is high energy and red is abundant.

Chlorophyll b is an accessory pigment that plants use in smaller amounts. The “b” version absorbs light from near the same wavelengths as chlorophyll a, but they pass the energy on to the “a” version for use in photosynthesis. The two chlorophylls differ at only one of their 55 carbon atoms.

Green light is higher energy than red light, but less abundant
in our atmosphere. Blue light is much higher energy, so it
can power a lot of photosynthesis even if it isn’t that abundant.
Therefore, is it surprising that our plants appear green,
chorophylls absorb and use red light because it is abundant, and
blue light because it is high energy. Green isn’t worth bothering
with and is reflected.
There are also chlorophylls c, d, and f. Chlorophyll c is also an accessory pigment which transfers energy to chlorophyll a, but it is very different structurally. Chlorophyll c is found only in some marine algae, and actually comes in three similar structures; c1, c2, and c3.

Chlorophyll f absorbs in the near infrared (NIR, not visible but close to red) range. Discovered in 2010 as the major chlorophyll in stromatolites of Australia, it is the first new chlorophyll identified in the last 60 years. However, its usefulness in photosynthesis has not yet been confirmed.

Chlorophyll d, on the other hand, is found to be the primary chlorophyll in cyanobacteria. A recent study showed that this chlorophyll absorbs NIR light as well. Though lower energy than red light, but the 2012 paper shows that the cyanobacteria are just as efficient at photosynthesis as plants with chlorophyll a. This works out well since in water, the higher energy wavelengths are absorbed near the surface and the only light that penetrates to the cyanobacteria is the NIR.

This is important for the science of astrobiology, predicting what life might look like on other planets and trying to identify which planets might hold life. Knowing that low energy light can still power photosynthesis tells us that we should not discount the planets around red dwarf stars. These stars have light of different wavelengths than our sun. Autotrophs from planets around red dwarfs may use NIR chlorophylls exclusively; therefore they might reflect all light and appear almost white.

On the other hand, light from different stars might drive evolution of different chlorophylls, so plants on other planets might not be green at all, but could reflect just lower energy light and appear red, or reflect just higher energy waves and be blue – blue plants, cool!

Current possible habitable exoplanets have been numbered and are
under investigation. Scientists look for planets in the habitable zone,
meaning they are of a temperature to have liquid water. They also look
for rocky planets that are about the same size as Earth to provide the
same amount of gravity. They also look for planets around stars with the
same kind of light as our sun – maybe they shouldn’t limit it to stars like
ours. Those with “Kepler” in their name come from the orbiting Kepler
telescope, which is now in danger of never working again.
Based on the light reflected from exoplanets (planets outside our solar system), a 2007 study in the journal, Astrobiology, says we might be able to predict the color of their possible plants and the wavelengths they might use. Furthermore, a study in 2012 stated that in binary systems that have two stars, each giving off different wavelengths of light, might force the evolution of dual photosynthetic mechanisms, leading to perhaps alternating plant colors, depending on which sun is shining.

Chlorophylls provide energy through photosynthesis, but they also have a cost. The old saying, “It takes money to make money” applies to plants as well. It takes energy to make chlorophyll, so it only pays to make chlorophyll when there is ample sunlight to put through photosynthesis. When the daylight get shorter on Earth, the profit margin for producing chlorophyll goes down, so the plant just stops making it.

This is when we start to see the other pigments, those that might play a role on other planets. Other major pigments are the yellow, orange or red carotenoids and the flavonoids. When the plant reduces chlorophyll production, the green color is then a lower percentage of the total pigment in the leaf and the other colors can show through. This gives the bright colors of fall foliage.

But these same pigments can make it seem that a green plant is a non-green plant. Plants that produce large amounts of purple, brown, or maroon pigments have leaves that are so dark that they appear black. Purple, black, and red plants have chlorophyll aplenty, it’s just that the color is masked by other pigments.

Carotenoids are a diverse group of pigments, but yellows and oranges seem to predominate. Carrots get their color from carotene, one type of carotenoid. Xanthophyll is another, which reflects yellow light wavelengths. While chlorophylls absorb red and blue light, carotenoids absorb the blue wavelengths, as well as green light, reflecting only the lower energy yellow, orange, and red light.

Retinal is the major pigment used in our vision. Transduction
of light energy into chemical energy and a nerve impulse is
powered by a cis- to trans- conversion of part of the molecule.
Is it any wonder that this ability to capture light energy can
also be applied to photosynthesis.
By absorbing the green light that would usually be bounced back from chlorophyll, they can prevent us from seeing them as green. Additionally, non-green plant pigments can contribute to photosynthesis, serving as accessory pigments to chlorophyll.

Carotenoids absorb light energy, and while they can’t convert this directly to chemical energy through photosynthesis on Earth, they can transfer this energy to chlorophyll, which then carries it through photosytems I and II of photosynthesis.

In addition, some archaea use retinal (another pigment) to extract energy from the green wavelengths of light. So, why aren’t plants truly black? Wouldn’t it be most efficient to absorb all wavelengths of light for photosynthesis and reflect nothing, thereby appear black to us. Wouldn’t this be the most efficient use of the sun’s energy?

The answer is easy – evolution doesn’t work to maximum efficiency. Natural selection is random and works with what it is given – nothing in nature is engineered by decision to maximize efficiency. But that doesn’t mean there can’t be black plants around other stars, having undergone completely different evolutionary paths.

Even if something used carotenoids, retinal, xanthins and chlorophylls, could it extract energy by absorbing all light waves that strike the plant? Um, no. No plant comes close to absorbing all the light that it can use, and no plant is made of only pigment molecules. There will always be reflections from other molecules.

Plus, if all light was absorbed, can you imagine how hot the plant would get? Imagine a blacktop parking lot being alive; you can fry an egg on an asphalt surface during the summer!

Purple heart (left) and black pepper pearl (right) have lots of pigments
that make them colored purple and almost black. However, they have
chlorophyll too, it is just masked by the other colors. They do
photosynthesis just like other plants, but they certainly look
Carotenoids are longer lived than chlorophyll. When autumn comes around, the plant breaks down chlorophyll so that the components can be reused, but the carotenoids stick around much longer. Therefore, the yellows and oranges are not masked by the greens, and the leaves change colors.

Anthocyanins of the flavonoid class are another set of plant pigments. These colors are also more stable than chlorophylls. Our King Crimson Maple makes a lot of red anthocyanin pigments that absorb the green light coming in to the leaf and perhaps a lot of the green light reflected by the chlorophyll. Therefore, as the amount of the anthocyanins in a leaf increases, the green color is masked by the red.  

Plants can use anthocyanins as “sunscreen” because in addition to absorbing green light, they also absorb ultraviolet light. Even though plants and animals need oxygen, they can also be damaged by the production of oxygen radicals (highly reactive compounds) produced by ultraviolet light energy striking oxygen-containing molecules and breaking them apart. Ultraviolet light can especially damage DNA, so anthocyanins can protect cells from mutations that might lead to inefficient activity or even cancer. It might be that on other planets, anthocyanins could be photosynthetic and plants live on UV light.

Sunscreen protects our skin from damage, just as red pigments protect the plant leaves. Even more, eating plants high in anthocyanins, like red grapes, blackberries, and blueberries, can transfer those antioxidant molecules to us for protection of our tissues and blood…. but don’t eat your Norway Maple.

On the left is our King Crimson. The yellow arrow shows the darker leaves
that get more sunshine. The green arrow shows the shaded leaves that
make much less red pigment because they don’t need the protection. On
the right is the burning bush in the open, so it has more carotenoids that
show up in the Fall.
When fall comes, or it is time for the fruits to ripen, plants start to produce even more anthocyanins (as in green apples turning red), because as other compounds in the plant breakdown more oxygen radicals will be produced. Therefore, the plant needs more protection.

Returning to our maple and our fire bush, it would seem that the maple leaves are dark red, almost purple, because of the high anthocyanin pigment concentration relative to the chlorophyll concentration (red + green = almost purple). But not all of them are purple (see picture). Other examples of this, the purple heart plant and the oxalis regnelli, remain purple all through their growing cycle. 

Our burning bush is deep red in autumn because it is not shaded at all, so it produces more anthocyanin to protect its leaves in the summer. If it were shaded part of the time, it might be more pink. If the leaves need protection, they make more anthocyanin, and if not, they don’t.  Don't ask me about shade on other planets.

Next week, your Fourth of July ice cream may have a side effect - ever wonder how "brain freeze" works?

Behrendt, L., Schrameyer, V., Qvortrup, K., Lundin, L., Sorensen, S., Larkum, A., & Kuhl, M. (2012). Biofilm Growth and Near-Infrared Radiation-Driven Photosynthesis of the Chlorophyll d-Containing Cyanobacterium Acaryochloris marina Applied and Environmental Microbiology, 78 (11), 3896-3904 DOI: 10.1128/AEM.00397-12

O'Malley-James, J., Raven, J., Cockell, C., & Greaves, J. (2012). Life and Light: Exotic Photosynthesis in Binary and Multiple-Star Systems Astrobiology, 12 (2), 115-124 DOI: 10.1089/ast.2011.0678  

Kiang, N., Segura, A., Tinetti, G., Govindjee, ., Blankenship, R., Cohen, M., Siefert, J., Crisp, D., & Meadows, V. (2007). Spectral Signatures of Photosynthesis. II. Coevolution with Other Stars And The Atmosphere on Extrasolar Worlds Astrobiology, 7 (1), 252-274 DOI: 10.1089/ast.2006.0108

Wednesday, June 19, 2013

The Roots Of Our Animal Family Tree

Biology concepts – porifera, last common ancestor, placozoa, cladogram, lower metazoan, bilaterians

Bonobo apes (Pan paniscus) are very closely related to
chimpanzees. They have longer legs than common
chimpanzees (Pan troglodytes) and are also
distinguished by having pink lips. I think this makes
them look significantly more human-like. Also like
humans, the families seem to be run by the mothers.
Humans are descended from primates; we share 99% of our DNA with chimpanzees and Bonobos (pygmy chimps). But what do we find as we go farther back along the line of mammals, and then from animals in general?

Question of the Day:  What ancestor gave rise to all the animals and is it still around today?

This is a much tougher question than it would seem at first glance. When I was studying biology for the first time, I thought that since humans descended from apes, and we see apes, then apes must have diverged from some other animal type that we recognize – something like apes descended from rodents.

But evolution doesn’t have to work this way; not every group of animals has evolved directly from some other group of animals. At some point, mammals had a last common ancestor with some other group of animals, and before that, those ancestors had a last common ancestor with some older group, and so on until the last common ancestor was the organism that gave rise to the first animal.

So did me need a mammal to give rise to all mammals? It is much like - which came first, the chicken or the egg? We all know that dinosaurs were laying eggs millions of years before chickens, but try thinking of it like this – which came first, the chicken or the chicken egg?

In terms of evolution, there was some bird like animal that was almost what we would agree was a chicken genetically; let’s say it was missing just one mutation or rearrangement of genes that prevented it from being called a chicken. So this non-chicken lays an egg. The embryo inside just so happens to contain the very mutation or change that will let us call it a chicken. Is it a chicken – yes.  In a chicken egg – no. The chicken came first.

The chicken or the egg question is much more interesting
than most people realize. Consider what you call a chicken
egg – is it an egg from a chicken, or an egg that houses a
chicken? If you think it is an egg that develops around an
embryonic chicken, then the egg came first, as opposed to
the explanation in the text. I love discussion about what
words mean, they make us thinkers.
Our discussion of the non-chicken egg description makes it easier to imagine that there was some organism that, while not an animal, was mother to all animals. The question still remains as to how that animal might have looked or behaved – but that won’t keep us from looking at some possibilities.

It would be a nice feather in your cap if you were the person to discover evidence of the first animal. In 2012 there was one article that displayed fossils of track marks from possibly 585 million years ago – pushing back the previously accepted date of animals by 30 million years.

Yet there was another 2012 paper showing Namibian fossils that could be 760 million years old – pushing the start date of animals back more than 200 million years! The truth may be somewhere in between, or might be even earlier. However, fossils of the first animals, if they exist, would only give limited information. Can we look further?

The 760 million year date is in line with what some geneticists estimate for the first animal. By looking at genes that all animals have in common and the rates at which those genes change over time, scientists can backtrack to see when they might have emerged.

What if we look at today’s animals, and which may best represent the first animal. Are we talking about primitive animals? What does it mean to be a primitive animal? If an animal species was closely related to the last common ancestor of all animals, it would be easy to say that it was a primitive animal – it lived long ago when animals were new, and it had a lot in common with the first, most primitive animal.

But do not confuse a species or genus with an individual animal. We have animals today whose ancestors were very closely related to the first animal, but that doesn’t mean that these individuals are primitive – they could have undergone extensive evolution through the millennia. Quite a number of adaptations could have taken place that increase the complexity of the animals biochemistry and/or behaviors.

The last common ancestor is sometimes called the most recent common 
ancestor (MRCA). They both mean the same thing. This chart 
pinpoints the MRCA for all life on Earth. That does not mean that it 
gave rise to all life. There could have been several parallel lines that all died
off. Same for the animals – there could be whole animal
phylums we know nothing about.
On the other hand, we can look at organs and systems as a measure of complexity or primitiveness. All animals are classified as metazoans (meta = changing, zoa =  animal). Some are termed lower metazoans, because they do not have complex structures like spinal chords (chordates) or bilateral symmetry (bilaterians).

Organization makes animals more complex as well. Cells of different types can form tissues that have specific functions. Tissues can organize into organs and organs join together to form systems. Animals without these characteristics are termed “lower” or “simple” or “primitive.”

Likewise, animals that can’t perform behaviors that other animals can are supposedly more primitive. If one species can move while another can’t, then the sessile (non-moving) animal is more primitive. Nervous systems are supposedly a big feature of more complex animals.

These ideas can lead to great discussions relating cells to life. What does it mean for one culture to be more primitive than another. Does a lack of cell phones make you primitive? Amazonian cultures had been using certain medicines for thousands of years before we arrived and stole their pharmacology. Now who looks primitive?

All this being said, can we learn anything by looking at extant (living) species as representatives of what early animals might have looked like or how they might have behaved? Yes, I think we can. You can’t know where you are going if you don’t know where you’ve been.

Sponges might be a good place to start. Sponges are so primitive that most non-scientists don’t even think of them as animals. Most have no body symmetry, they appear to be sessile, and they have a very few cell types, none of which are organized into tissues or organs or systems.

Sponges have been around for about 760 million years, if we are to accept the Nambian fossils as well-dated and representative of the earliest sponges. This would put them in the front seat of the animal bus. But are they really that primitive?

This is the harp sponge (Chondrocladia lyra) that was discovered off 
the coast of Oregon, Washington state and Canada. It lives in the 
trenches below 3000 m and represents just one of the carnivorous
sponges. You want weirder, loo up a picture of the ping pong tree sponge!
Sponges generally have three different regions, the outer layer, the more acellular mesohyl, and the inner surface. Choanocytes line the inner surface and have a single flagellum that help the cell to harvest floating food in the filtered water. The outer layer is made up of pinacocytes that filter the water and digest food particles too large to be filtered. So far, interesting but not amazing.

But the mesohyl of this “primitive” animal has some cool stuff.  There are motile cells that secrete collagen protein. There are muscle cells that help the sponge contract and relax. There are “grey” cells that act as an immune system. And there are other motile cells that are totipotent stem cells and can become any cell type with in the sponge. Still sound primitive?

If you want a nice example of just how complex sponges can be, meet the harp sponge (Chondrocladia lyra). It is definitely a member of the phylum porifera (Latin for “bearing pores”), but it does have elegant symmetry. Described in late 2012, the harp sponge is also one of about 24 different carnivorous sponges.

The harp sponge uses sharp spikes on the vertical growths to harpoon and hold fish and crustaceans, which it then wraps them on a membrane and digests whole. This is in contrast to most sponges that filter microscopic food particles from the water by passing the water through its body from the outside and then up and out of its chimney (see video).

Trichoplax adharens may represent the most basal animal alive. Made
up of just a few thousands cells of only four different types, it has the
smallest genome of any known animal. The cutaway drawing on the
right shows that there are layers of cells, so it does have some
organization, just no tissues or nervous system.
The harp sponge is like other sponges in that it can reproduce asexually through fractured off pieces, by gemmules that are like clonal spores, or by budding. They can also reproduce sexually, but in the harp sponge the spermatophores are not simply released form the sponge body. They gather in the bulb portion at the top of the vertical shafts and are released all at once. The oocytes are found in the middle bulges. Sponges can reproduce four different ways while we only have one. We can, and will, spend more time on the exceptions that are sponges.

In recent years, less emphasis has been placed on sponges as a basal form of animal and more attention has been given to the placozoans (placo = flat, and zoa = animal). Only one species of placozoan is known (Trichoplax adharens) has been described, mostly because they have never been observed in their natural habitat (ocean, we think) and have only been seen on the walls of laboratory and zoological aquariums.

Placozoans have only four different cell types, no symmetry, two layers of cells, and no nervous system. Even by sponge standards, this is awfully primitive. The 2009 study of Schierwater et al. has given the best proof that T. adharens is the most basal of the lower metazoans, based on comparisons of thousands of genetic loci.  This agreed with several earlier studies, but Dr. Schierwater’s group went much further.

The cladogram on the left dates from 2009, showing that a more primitive
animal gave rise to both the lower metazonas and separately to the more
complex animals. The tree on the right is from 2013 symposium write up
in Integrative and Comparative Biology (doi:10.1093/icb/ict008), and
represents a consensus of the genetics data and opinions. They seem to
think that sponges diverged early than all the rest of the animals. Needless
to say, opinions vary.
Their cladogram evidence seems to indicate that sponges, cnidaria, ctentophora (comb jellies), and placozoans diverged as a single group and in parallel with bilaterian animals. Together, these data mean that as a group, the lower metazoans diverged from the more complex metazoans even before the emergence of sponges or placazoans (see cladogram). Complex animals did not evolve from sponges, jellies or placozoans at all – they came from some different ancestor.

So, this evidence suggests that there was something out there that was an ancestor of both the lower metazoans and the bilaterians, but was itself neither of them – an animal whose ancestor wasn’t an animal. Will we recognize it when we see it? It leads to another question. What will it have to have to be considered the first animal and not the last non-animal – just what makes an animal an animal?

Next week - how do stars determine the color of plants, and what colors might alien plants be?

Dohrmann, M., & Worheide, G. (2013). Novel Scenarios of Early Animal Evolution--Is It Time to Rewrite Textbooks? Integrative and Comparative Biology DOI: 10.1093/icb/ict008

Schierwater, B., Eitel, M., Jakob, W., Osigus, H., Hadrys, H., Dellaporta, S., Kolokotronis, S., & DeSalle, R. (2009). Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis PLoS Biology, 7 (1) DOI: 10.1371/journal.pbio.1000020

Wednesday, June 12, 2013

A Big Plant In A Little Package

Biology concepts – angiosperm, utricle, fruit, flower, phytoremediation, monoecious, dioecious, stalk, stamen, pistil, acaulescent

Eucalyptus regnans is the tallest flowering plant in
the world. It grows in southeastern Australia and
Tasmania. As a eucalypt, it is food for koala bears,
but I can’t imagine a small koala climbing a monster
like this for food. There are over 600 species of
eucalyptus leaf that koalas can feed on, most of them
being closer to the ground than these leaves.
Some of the most massive living organisms are flowering trees. The eucalyptus tree (Eucalyptus regnans) is the tallest/largest angiosperm in the world. Specimens can reach 380 ft (116 m), and can gain up to 200 ft (61 m) of this height in just their first 50 years. Eucalypts are also economically important, from wood to essential oils used in medicines. If this tree is the biggest flowering plant, what do we find on the other end of the scale? Could a small plant be just as important?

Question of the Day – What are the world’s smallest flowering plant, fruit, and seed?

To be an angiosperm, a plant must produce a fruit of some kind and have enclosed seeds (angio = vessel, and sperm = seed, so seeds in a vessel). It will have a pistil and/or stamen, and if fertilized, the embryo becomes a seed and a fruit formed from the ovary (and perhaps other parts).

There is no size requirement to be an angiosperm, it just has to be big enough to carry the requisite anatomical features. Flowering plants run the gamut of sizes, from huge trees to small Australian violets (Viola hederacea) at only 1.5 in. (3.8 cm). But even this tiny violet, with its 0.25 in. (6 mm) flowers is huge compared to the smallest of the flowering plants.

Imagine a thimble filled with plants. How many plants? How about 5000! Not seeds mind you, but fully mature plants. This is easy for the watermeal plant (Wolffia globosa), the world’s smallest flowering plant. When you pick up a single plant (if you can), you can hardly see it on your finger. The entire plant is only 0.6-0.8 mm long, about the same size as a grain of salt, and weighs only 150 micrograms (0.00015 grams).

It takes a determined plant to fit itself into such a small volume. Decisions must be made about what is necessary and what can be lost. In evolutionary terms, this is called reduction. W. globosa is a greatly reduced plant. It has no roots, no leaves, no petals, and no stem to speak, although developmental studies show that it's mass is part stem, part leaf.

I bet we have all seen a pond or wetland that looks like the one
on the left. I had always assumed that the green covering was
algae or leaves or pollen that had fallen from trees. Now I know
that it might just as well be hundreds of a millions of individual
plants. On the right is a picture that gives you some scale, every
green speck there is an individual, mature, reproducing plant.
Watermeal is a floating plant, which is a good way to acquire water when you don’t have roots. I always assumed that the green dots that covered the surface of still waters were dropped leaves or small seeds, but it is very likely that I was looking at millions of individual plants.

W. globosa is also one of the fastest growing plants in the world. It can double its biomass in just 30 hours. We think of bamboo as a fast growing plant, and it is, but doubling time for a bamboo plant can is measured in days or weeks, not in hours.

The water hyacinth (Eichhornia crassipes) was supposedly the fastest growing angiosperm in the world, with a biomass doubling time of six days under the best growing conditions. I think this was probably before they started looking at watermeal in more depth. It could be easy to overlook.

The fast reproduction and growing time for W. globosa means that it can completely cover a pond in a matter of days. This reduces the amount of sunlight for underwater plants, and crowds out the photosynthetic phytoplankton. Dissolved oxygen will become depleted. This could lead to a fish kill that would decimate the entire pond. Watermeal is so small that it is easily transferred to other bodies of water on the feathers and feet of ducks, so it is invasive. “Reduced” apparently doesn’t apply to survival capability.

Wolffia gets away with being leafless because it has chloroplasts in the cells of its body. I don’t really know what to call the body of watermeal. It isn’t a stem, since a stem connects different structures of a plant to one another. In the case of W. globosa, there is nothing to connect to anything else. 

There are other plants that don’t show a stem above ground, but they have a connection for leaves to the root, and these are called acaulescent (a = without, and caulis = stem) plants. For example, many succulents have thick leaves that come straight out of the ground.

The common dandelion (Taraxacum officinale) is an
acaulescent plant. The stem is the unelongated nodule to
which tall the leaves attach and is usually found
underground. The flowers sit atop a hollow stalk which
is not a stem, and the yellow blooms are not individuals,
but actually hundreds of individual flowers.
You probably have an acaulescent plant growing in your yard right now, the dandelion. But wait, dandelions have stems with the flowers, right? But those aren’t stems, the stem (also called a crown) is underground. The flowers sit atop a stalk, part of the inflorescence (the total flower structure). Stalks connect parts of the plant to the stem, like petioles for instance. The petiole is the part of a leaf that connects the leaf blades to the stem; think celery stalks - those are petioles.

W. globosa does have a flower, but you wouldn’t recognize it. The flower is held in a small cavity on the top of the football shaped plant (see picture). The flower is also reduced, having only one pistil and one stamen. Pistil is the name for the complete female structure, including the stigma, the style, and the ovary. The stamen is the name given to the complete male structure, including the filament and the pollen-producing anther.

These are the only parts of a flower that are necessary; many flowers just have one or the other (male or female flowers). Plants that have both types of flowers on a single individual are called monoecious, while plants with just one type or the other are dioecious. So even though the flower of the watermeal has no petals and is only 0.2 mm in diameter, it is functionally more complete than flowers that are thousands of times its size.

The white bar in this picture is 0.25 mm long – see how small Wolffia is! 
This isn’t even W. globosa, the smallest species, but one of its 
bigger cousins, W. australiana. The flower sits in the little pit on 
the top, and it has a two lobed anther (A1 and A2), as well as a 
pistil (Pi). The MF is the mother frond, and the DF is the clonal 
daughter bud. Remember that wolffia species float, so the ventral bulge
(VB) keeps them right side up. Other labels (DL is where the anther splits to 
release pollen, and S is the stomata for gas exchange).
The world’s smallest flower also produces the world’s smallest fruit. The watermeal fruit is called a utricle, meaning it is thin walled and bladder-like, so it floats. The terminology for fruits is expansive. It would take a few posts to wade through it all – maybe later. The globosa fruit is 0.4 mm long, half the size of the plant and 2x as big as the flower. Even though the fruit is the smallest on Earth, it is the world’s largest fruit relative to the size of the plant that produces it.

W. globosa is an exception in that it is an angiosperm that most often reproduces through asexual means. Watermeal usually buds, much like yeast or coral polyps. The bud grows from the end of the mature watermeal and can be as large as the parent plant. This is why W. globosa is fast growing; by the time the bud separates, it is a mature plant.

In the rare cases that it is pollinated, just one seed is produced. You would think that it would be the world’s smallest seed, but it isn’t - not by a long shot. The W. globosa seed is 0.3 mm, between half and ¾ the size of the fruit, but some orchids have much smaller seeds.

The coral root orchid (Corallorhiza maculata) has seeds that measure just 0.085 mm each – there are bacteria larger than that! There are many similarities between the coral root and W. globosa. Coral root doesn’t have leaves or roots to speak of, just like watermeal. It is parasitic and gathers its nutrition from the soil fungi. The main difference is that while wolffia produces only one seed, the orchid has thousands, easily dispersed by the wind, since it takes 375,000 of the to equal one ounce (28 g).
The coral root orchids on the left is Corallorhiza maculata. It grows
in North America. It has no leaves, and no conventional roots, just
suckers that invade and parasitize the fungal mats that live just
below ground. It gains all its nutrients this way, it does not make
chlorophyll. On the right are the fruits of the coral root. While the
capsule is large and has many seeds, each small white structure
houses only one seed, the smallest seeds in the world.
You can’t even see the seed itself, it is so small.

Could this speck of a watermeal plant impact humankind? You betcha. Soybeans are supposed to be the world’s superfood, being about 40% protein, but wolffia has the same amount of dietary protein as soybeans, with more of the essential amino acids that humans must gain from our food. And watermeal produces protein 50x faster than soybeans.

W. globosa is already used a vegetable in southeast Asia, but with its high protein and carbohydrate concentrations, small size, easy growing conditions and rapid maturation reproduction, it may be much more. There are scientists who are proposing that watermeal form the basis of the astronaut on trips to Mars and beyond.

Don’t think phytoremediation is important? This is a
picture of itai-itai disease due to cadmium poisoning.
Itai-itai translates as “it hurts-it hurts.” The cadmium
poisoning leads to osteomalacia, a softening of the
bones, so that the body can’t support its own weight.
Watermeal can take cadmium out of the environment.
A big deal for a little plant.
More important, watermeal may save us before we head to the stars. Humans have been wildly successful at poisoning the earth’s soil and water, so much so that this might be the reason we will have to visit other planets. W. globosa has a talent for pulling poisons out of water and sequestering them for disposal. Watermeal has long been known as a good accumulator of arsenic, W. globosa can tolerate such high levels of arsenic.  Watermeal conjugates (chelates) arsenic with several proteins called phytochelatins, thereby thereby reducing the arsenic toxicity of freshwater. A 2012 study has identified just how watermeal survives the arsenic. It has an enzyme that converts the arsenic to a nontoxic form and also stimulates the production of the phytochelatins.

This ability to remove arsenic from water is matched by W. globosa’s talent for binding up cadmium as well. Cadmium is toxic to the human body, and is released from natural sources as well as from discarded or weathered paints and batteries. A 2013 study shows that watermeal is great at sequestering cadmium. It’s ability to remove cadmium from the environment is amazing since it is not affected by levels of arsenic in the plant – it can detoxify water sources of potentially many poisons. Since it can grow so fast and takes such little room, this gives it a great potential for phytoremediation (phyto = plant, and remedium = restore balance).  

Xie, W., Huang, Q., Li, G., Rensing, C., & Zhu, Y. (2013).CADMIUM ACCUMULATION IN THE ROOTLESS MACROPHYTE AND ITS POTENTIAL FOR PHYTOREMEDIATIONInternational Journal of Phytoremediation, 15 (4), 385-397 DOI: 10.1080/15226514.2012.702809  

Zhang, X., Uroic, M., Xie, W., Zhu, Y., Chen, B., McGrath, S., Feldmann, J., & Zhao, F. (2012). Phytochelatins play a key role in arsenic accumulation and tolerance in the aquatic macrophyte Wolffia globosa Environmental Pollution, 165, 18-24 DOI: 10.1016/j.envpol.2012.02.009

Wednesday, June 5, 2013

The Living Earth – Rocks and All

Biology concepts – Gaia Hypothesis, photosynthesis, biogeology, plate tectonics, biological weathering, oxygen crisis

The physical form of Earth definitely influences how life evolves on Earth. You can't argue that ice ages and the birth of shallow seas in the middle of continents changed what life forms survived and thrived. But what about the other possibility?

Question of the Day – Does life have an effect on the physical form of Earth?

This is a small town in Kansas about to be inundated by a dust storm. 
The dust would reach miles high and would deposit hundreds 
of tons of Kansas topsoil in the Atlantic Ocean. Unfortunately, these 
were nearly daily occurrences. Every morning people would 
have to dig their houses out from under the dust that blew in overnight.
Obvious examples include humans. We have had a recent effect on Earth, probably coinciding with the Industrial Revolution. Our activities, to a greater or lesser degree, have changed the climate. Reasonable people can disagree on how much, but could we really have an effect on something as big as the physical Earth?

What about the Dustbowl? In the 1930’s, the bottom fell out of the wheat market and farmers in Oklahoma, Texas, and Kansas abandoned their land. Farmland that isn't farmed is no longer held in place by roots. The farmers had plowed under the grasslands that had kept the soil in place for thousands of years, but a couple of years of drought and a surplus of corn and wheat led to a national disaster. By 1938, 5 inches of topsoil had been lost from more than 26 million acres of farmland.

Other life has also had an effect on the physical Earth. Organic rich sedimentary rocks are formed when living things die, decay, and under the influences of pressure heat, and time come to form specific products. The rocks and other end products are considered organic rich only if they are greater than 3% organic material. You might have heard of these end products - coal and oil shale.

These are interesting examples, but allow me to relate a newly written story. It's still a hypothesis, but there is a lot of data that supports this fresh idea of the massive effects which life has had on the planet. How massive you ask? Let me give you this hint – it doesn’t get much bigger.

The earliest life on Earth appeared more than 3.8 billion years ago. Oxygen was very scarce in the atmosphere, so these organisms did not use oxygen as an electron acceptor in the production of cellular energy. The earliest bacteria necessarily used those things that it had at hand, things like methane or other chemicals.

The early Earth atmosphere was affected greatly by erupting volcanoes. 
These would spew hydrogen gas, carbon dioxide and water into 
the air. Some helium and hydrogen would be lost to space since they 
are so light, but some oxygen would combine with the hydrogen 
to form more water. Notice that no free oxygen is indicated.
About 3.8 billion years ago, photosynthetic bacteria evolved to make use of sunlight and carbon dioxide. These photosynthetic bacteria didn’t look a lot like the cyanobacteria of today, using much more iron (Fe2+) and sulfate (SO42-) than is made use of in modern organisms.

But these photosynthetic bacteria did release oxygen – something that had not happened on Earth previously. The oxygen that was produced didn’t just float around in the atmosphere; there were other compounds that were ready to react with the O2.

One thing that the Earth had in abundance 3.8 billion years ago was hydrogen. But hydrogen is light. A thin atmosphere could be made thinner by losing hydrogen to outer space, so combining hydrogen and oxygen to produce water was a fortunate way to keep some of this hydrogen on the planet and to increase the water supply. We will see just how important this was in a few paragraphs.

Other things early Earth had in great supply were iron and basalt. Basalt is the rock formed by the eruption of volcanoes and interaction of the magma with the atmosphere. The oxygen produced via photosynthesis (and much of the oxygen already present) quickly reacted with iron in the earth and lava and with the basalt.

The basalt and oxygen underwent a much longer process in the earth’s crust, changing from igneous rock to metamorphic rock (meta = change and morph = form). Basalt or clay + minerals + terrestrial O2 + water, form granite. Is it a coincidence that the only place in the universe we have seen granite is right here on Earth? Sounds like photosynthetic life had an influence on the kinds of rocks located on this particular planet. But wait, it gets bigger.

Plate tectonics theory explains how continental crust and
oceanic crust floats on top of the mantle. When two plates
crash into one another one plate can slide (subduct) under
the other. Since the granite of the continental crust is lighter,
it usually ends up on top. Or, the two plates can pile up to
form mountains (like the Himalayas), but remember that
the Smokey Mountains used to be much taller than the
Himalayas, so weathering also plays a role in what we see.
Granite is dense, but the addition of oxygen makes it less dense than basalt. Because of the density difference, basalt tends to sink underneath granite; granite sort of floats on basalt. When Earth’s plates of crust meet, the basalt tends to subduct under the granite. Also, basalt morphs into a much more dense rock called eclogite – but granite doesn’t.

What were the results of this density difference? Floating granite formed continents, while heavier basalt and eclogite formed ocean floors. Yes, photosynthetic bacteria influenced the formation of the continents! Photosynthesis had effects on water, on terrestrial oxygen, on granite formation and density – and therefore they effected the physical form of Earth.

This theory was proposed by Minik Rosing in his 2006 paper. I actually read about it in a thriller paperback called The Last Good Man, by Anders Klarland and Jacob Weinreich. Intrigued by the author’s discussion, I checked on it and learned a whole bunch. Strange, but a murder mystery was the beginnings of this post.

I did another literature search, looking for papers that talked about the photosynthesis-granite theory and paper. I couldn’t find any. This made me wonder if the hypothesis had been refuted, or if the scientific community just didn’t buy it. I contacted a couple of well-known geologists and asked about the state of the theory.

I laid out the idea as I understood it and asked if I was getting the point and if the point was worth getting. They both agreed that the theory is alive and well and has been well accepted. Amazing – almost nothing this revolutionary is well received at first. And it shouldn’t be easy; every step forward should be scrutinized and tested to make sure it isn’t a step backward.

So photosynthetic bacteria are responsible for the continents. Does the story end there? Nope, there’s more.

Basalt crystals are large and hexagonal. Columns of basalt
crystals form in nature, as shown on top. In Micronesia,
an island city called Nan Madol was constructed in the 12th
and 13th centuries, completely from basalt columns. The city
was abandoned by 1628 (bottom picture), but before that
it was used as a residence for the nobility of the civilization.
After stable continents were formed and plate tectonics was working, photosynthesis continued and expanded. About 2.6 billion years ago, most of the rock became saturated with oxygen, and the excess was then released to the atmosphere over the next 200 million years. This wasn’t necessarily an easy thing with which to deal.

Most forms of life on Earth at this time were unable to deal with higher concentrations of oxygen. What makes oxygen so great for cellular respiration is that it easily takes electrons from other atoms, or can combine easily to share electrons. This is also what makes it dangerous. It can damage other molecules are hinder their function by combing with them or altering their structure. This is oxidative damage.

The increased oxygen in the environment starting killing most of the forms of life. This was such an important happening that biologists gave it a name – the Oxygen Crisis, the Great Oxygenation Event, the Oxygen Catastrophe. The Great Oxidation, or the Big Bad Breath (OK, I made that one up).  Only the organisms that evolved a mechanism to deal with oxidative damage continued to survive and change. Eventually, some came to use the oxygen in their metabolism.

So life has affected the physical form of the earth, and of course life has affected the later forms of life. But there is even more to this story, including how both processes are at work at the same time.

Before the oxygen crisis, iron-rich hematite recorded seasonal
changes in ocean waters. The iron oxide precipitated when
warm, oxygen-bearing water from the surface mixed with cold,
iron-rich water beneath it. When there was sufficient oxygen
in the rocks, the excess oxygen could be released to the
atmosphere. Later, the iron from these formations could be
used by surviving organisms to employ more complex
Over time, granite and other surface rocks were weathered by physical means (water, wind, chemicals, and temperature), and by biological means. Some things grow inside rocks and expand them or dissolve them, like lichens, and other things grow in cracks and physically break down rock. The color of plankton can affect temperature changes and wind or thunderstorm formation. Life can affect the climate and the weathering of rocks in many ways.

Just recently a study indicated that trees in the rainforest can sense when they are receiving too much sunlight and are getting too hot. They will then release chemicals that promote cloud formation by acting as seeds for vapor to form into water droplets. The clouds reduce the amount of sunlight reaching the trees and cool them down. Smart guys those trees.

So early life built up Earth’s continents, and then proceeded to help tear them back down. There is a balance between the weathering of rock and the formation of rock. And this, in and of itself, has also affected life.

When granite weathers, it releases some of the minerals it contains, heavy minerals that were brought up from the mantle. These heavy metals and minerals are able to act in chemical reactions, including the banded iron forms that were made during the initially increase in oxygen formation. Some organisms managed to find ways to use them as they were spread over the ground and the surface of the shallow seas.
In Greek mythology, Gaia was the Earth goddess who gave birth
to Heaven and ocean (the two children above). When James
Lovelock came up with the idea of Earth as a single organism,
his neighbor, William Golding (author of The Lord of the Flies),
suggested the name Gaia.

A new study indicates that the weathering of granite and the release of minerals was a crucial event in the development of life on Earth. Carbon release and heavy metal release, especially iron, apparently stimulated and increase in complexity of life forms on Earth. What was the most crucial increase in complexity brought about by weathering? The evolution of eukaryotes about 2.0-1.6 billion years ago!

In the 1970’s, the Gaia Hypothesis was introduced, stating that Earth and life were inextricably linked. The hypothesis was adopted by some non-scientific types who started to talk about the earth as a living organism, with a soul and the ability to die. Too touchy feely for me, but these new studies definitely bring us back to the hypothesis as it was presented by Lynn Margulis and James Lovelock. Earth and life – life and Earth, just one system after all.

Next week, life on a smaller scale. Can a plant as small as a grain of salt really be considered a whole plant? 

Parnell, J., Hole, M., Boyce, A., Spinks, S., & Bowden, S. (2012). Heavy metal, sex and granites: Crustal differentiation and bioavailability in the mid-Proterozoic Geology, 40 (8), 751-754 DOI: 10.1130/G33116.1  

Paasonen, P., Asmi, A., Petäjä, T., Kajos, M., Äijälä, M., Junninen, H., Holst, T., Abbatt, J., Arneth, A., Birmili, W., van der Gon, H., Hamed, A., Hoffer, A., Laakso, L., Laaksonen, A., Richard Leaitch, W., Plass-Dülmer, C., Pryor, S., Räisänen, P., Swietlicki, E., Wiedensohler, A., Worsnop, D., Kerminen, V., & Kulmala, M. (2013). Warming-induced increase in aerosol number concentration likely to moderate climate change Nature Geoscience DOI: 10.1038/NGEO1800  

Rosing, M., Bird, D., Sleep, N., Glassley, W., & Albarede, F. (2006). The rise of continents—An essay on the geologic consequences of photosynthesis Palaeogeography, Palaeoclimatology, Palaeoecology, 232 (2-4), 99-113 DOI: 10.1016/j.palaeo.2006.01.007