Tag Archives: purple

Sewage sludge dry.

Purple bacteria turn sewage into hydrogen fuel

Purple bacteria are poised to turn your toilet into a source of energy and useable organic material.

Desiccation cracks sludge.

Dried sewage sludge.
Image credits: Hannes Grobe.

Household sewage and industrial wastewater are very rich in organic compounds, and organic compounds can be very useful. But there’s a catch: we don’t know of any efficient way to extract them from the eww goo yet. So these resource-laden liquids get treated, and the material they contain is handled as a contaminant.

New research plans to address this problem — and by using an environmentally-friendly and cost-efficient solution to boot.

The future is purple (and bacterial)

“One of the most important problems of current wastewater treatment plants is high carbon emissions,” says co-author Dr. Daniel Puyol of King Juan Carlos University, Spain.

“Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint.”

The study is the first effort to apply purple phototrophic bacteria — phototrophic means they absorb photons, i.e. light, as they’re feeding — together with electrical stimulation for organic waste recovery. The team showed that this approach can recover up to 100% of the carbon in any type of organic waste, supplying hydrogen gas in return — which is very nice, as hydrogen gas can be used to create power cells or energy directly.

Although green is the poster-color for photosynthesis, it’s far from the only one. Chlorophyll’s role is to absorb energy from light — we perceive this absorption as color. Green chlorophyll, for example, absorbs the wavelengths we perceive as red (which sits opposite green on the color wheel). If you’ve ever toyed around with the color-correction feature in graphical software (a la Photoshop, for example), you know that taking out the reds in a picture will make it look green. The same principle applies here.

Plants are generally green because red wavelengths carry the most energy — and plants need energy to create organic molecules. But the substance comes in all sorts of colors in a variety of different organisms. Phototrophic bacteria also capture energy from sunlight, but they use a different range of pigment — from orange, reds, and browns, to shades of purple — for the job. However, the color itself isn’t important here.

“Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism,” explains Puyol.

These bacteria use organic molecules and nitrogen gas in lieu of CO2 and water as food. This supplies all the carbon, electrons, and nitrogen they need for photosynthesis. The end result is that they tend to grow faster than other phototrophic bacteria or algae and generate hydrogen gas, proteins, and a biodegradable type of polyester as waste.

But what really sealed the deal for the team is that they can decide which of these waste products the bacteria churn out. Depending on environmental conditions such as light intensity, temperature, and the nutrients available, one of these products will predominate in the material they excrete.

The team doubled-down on this property by flooding the bacteria’s environment with electricity.

“Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements,” says co-author Professor Abraham Esteve-Núñez of University of Alcalá, Spain.

“But what is unique about our approach is the use of an external electric current to optimize the productive output of purple bacteria.”

This concept — a “bioelectrochemical system” — works because all of the purple bacteria’s metabolic pathways use electrons as energy carriers. They use up electrons when capturing light, for example. On the other hand, turning nitrogen into ammonia releases electrons, which the bacteria need to dissipate. By applying an electrical current to the bacteria (i.e. by pumping electrons into their environment) or by taking electrons out, the team can cause the bacteria to switch from one process to the other. It also helps improve the overall efficiency of both processes (see Le Chatelier’s principle).

The team included an analysis of the optimum conditions for hydrogen production in the paper (it relies on a mixture of purple bacteria species). They also tested the effect of a negative current (electrons supplied by metal electrodes in the growth medium) on the metabolic behavior of the bacteria.

Their first key finding was that the nutrient blend that fed the highest rate of hydrogen production also minimized the production of CO2 — this would allow the bacteria to recover biofuel from wastewater with a low carbon footprint, the team explains. The negative current experiment proved that these bacteria can use cathode electrons to perform photosynthesis.

Even more striking were the results using electrodes, which demonstrated for the first time that purple bacteria are capable of using electrons from a negative electrode, or “cathode“, to capture CO2 via photosynthesis.

“Recordings from our bioelectrochemical system showed a clear interaction between the purple bacteria and the electrodes: negative polarization of the electrode caused a detectable consumption of electrons, associated with a reduction in carbon dioxide production,” says Esteve-Núñez.

“This indicates that the purple bacteria were using electrons from the cathode to capture more carbon from organic compounds via photosynthesis, so less is released as CO2.”

The paper “Biological and Bioelectrochemical Systems for Hydrogen Production and Carbon Fixation Using Purple Phototrophic Bacteria” has been published in the journal Frontiers in Energy Research.

The color purple is unlike all others, in a physical sense

Our ability to perceive color is nothing short of a technical miracle — biologically speaking. But there is one color we can see that isn’t quite like the rest. This color, purple, is known as a non-spectral color. Unlike all its peers it doesn’t correspond to a single type of electromagnetic radiation, and must always be born out of a mix of two others.

A violet rectangle over a purple background. Image credits Daily Rectangle / Flickr.

Most of you here probably know that our perception of color comes down to physics. Light is a type of radiation that our eyes can perceive, and it spans a certain range of the electromagnetic spectrum. Individual colors are like building blocks in white light: they are subdivisions of the visible spectrum. For us to perceive an object as being of a certain color, it needs to absorb some of the subdivisions in the light that falls on it (or all of them, for black). The parts it reflects (doesn’t absorb) are what gives it its color.

But not so for purple, because it is a…

Non-spectral color

First off, purple is not the same as violet, even though people tend to treat them as interchangeable terms. This is quite understandable as, superficially, the two do look very similar. On closer inspection, purple is more ‘reddish’, while violet is more ‘blueish’ (as you can see in the image above), but that’s still not much to go on.

Why they’re actually two different things entirely only becomes apparent when we’re looking at the spectrum of visible light.

Image via Reddit.

Each color corresponds to photons vibrating with a particular intensity (which produces their wavelength). Humans typically can see light ranging from 350 to 750 nanometers (nm). Below that we have ultraviolet (UV) radiation, which we can’t see but is strong enough to cause radiation burns on the beach, DNA damage, and other frightful things. Above the visible spectrum, we have infrared (IR), a type of electromagnetic radiation that carries heat, and which armies and law enforcement use in fancy cameras; your remote and several other devices also use IR beams to carry information over short distances.

The numbers above aren’t really extremely important for our purposes here; they describe the exact colors used for flairs on a subreddit I follow, and the wavelengths noted there will shift slightly depending on the hue you’re dealing with. I left the numbers there, however, because it makes it easier to showcase the relationship between light’s physical properties and our perception of it.

What we perceive as violet is, quite handily, the bit of the visible spectrum right next to that of UV rays. This sits on the left side of the chart above and is the most energetic part of light that our eyes can see (low wavelength means high vibration rates, which mean higher energy levels). On the right-hand side, we have red, with high wavelength / low energy levels.

Going through the spectrum above, you can find violet, but not purple. You may also be noticing that while we talk of ultraviolet radiation, we’re not mentioning ultrapurple rays — because that’s not a thing. Purple, for better or worse, doesn’t make an appearance on the spectrum. Unlike red or blue or green, there is no wavelength that, alone, will make you perceive the color purple. This is what being a ‘non-spectral’ color means, and why purple is so special among all the colors we can perceive.

More than the sum of its parts

If you look at orange, which is a combination of yellow and red, you can see that its wavelength is roughly the average of those of its constituent colors. It works with pretty much every color combination, such as blue-yellow (for green) or red-green (for more orange).

Now, the real kicker with purple, which we know we can get by mixing in red with blue, is that by averaging the wavelengths of its two parent colors, you’d get something in the green-yellow transition area. Which is a decidedly not-purple color.

That’s all nice and good, but why are we able to perceive purple, then? Well, the short of it is “because brain”. Although purple isn’t a spectral color in the makeup of light, it is a color that can exist naturally and in the visible spectrum, so our brains evolved the ability to perceive it; that’s the ‘why’. Now let’s move on to ‘how’. It all starts with cells in our eyes called ‘cones’

CIE colour matching functions (CMFs) Xbar (blue), Ybar (green) and Zbar (red). Image via Reddit.

The chart on the left is a very rough and imperfect approximation for how the cone cells on our retinas respond to different parts of the visible spectrum. There’s three lines because there are three types of cone cells lining our retinas. While reality is a tad more complicated, for now, keep in mind that each type of cone cell responds to a certain color (red, green, or blue).

How high each line peaks shows how strong a signal it sends to our brain for individual wavelengths. Although we only come equipped with receptors for these three colors, our brain uses this raw data to mix hues together and produce the perception of other colors such as yellow, or white, and so on.

The more observant among you have noticed that cone cells that respond to the color red also produce a signal for parts of the visible spectrum corresponding to blue. And purple is a mix of red and blue. Coincidence?! No; obviously.

The thing is, while every color you perceive looks real, they’re pretty much all just hallucinations of your brain. When light on the leftmost side of the spectrum (as seen in the chart above) hits your eye, signals are sent to your brain corresponding only to the color red. Move more towards the middle, however, and you see that both red and green are present. But the end perception is that of yellow, or green.

What happens is that your brain constantly runs a little algorithm that estimates what color things you’re seeing are. If a single type of signal is received, you perceive the color corresponding to it. If a mix of signals is received, however, we perceive a different color or hue based on the ratio between signals. If both green and red signals are received, but there’s more of the red than the green, our brains will tell us “it’s yellow”. If the signal for green is stronger than that for red, we see green (or shades of green). The same mechanism takes place for all possible 9 combinations of these colors.

That bit to the right of the chart, where both red and blue signals are sent to the brain, is where the color purple is born. There’s no radiation wavelength that carries purple like there is with violet or orange. The sensation of purple is created by our brains, sure, but the reason why it needs to be created in the first place is due to this quirk of how the cone cells in our eyes work. From the chart above you can see that cells responding to green pigments also show some absorption in the area corresponding to purple, but for some reason, our brains simply don’t bother with it.

From my own hobbies (painting) I can tell you that mixing violet with green produces blue, but mixing purple with green results in brown. Pigments and colored light don’t necessarily work the same way, this is all anecdotal, and I have no clue whether that’s why green signals get ignored in purple — but I still found it an interesting tidbit. Make of it what you will.

In conclusion, what makes purple a non-spectral color is that there isn’t a single wavelength that ‘carries’ it — it is always the product of two colors of light interacting.

Are there any others like it?

Definitely! Black and white are prime examples. Since there’s not a single wavelength for white (it’s a combination of all wavelengths) or black (no wavelengths), they are by definition non-spectral colors. The same story with gray. These are usually known as non-colors, grayscale colors, or achromatic hues.

Furthermore, colors produced by mixing grayscale with another color are also considered non-spectral (since one component can’t be produced by a single wavelength, the final color can’t be produced by a single one either). Pink is most often given as an example, as is brown, since these can be produced using non-spectral colors (white and/or purple for pink, gray/black for brown).

Metallic paints also, technically, are non-spectral colors. A large part of the visual effect of metallic paints is given off by how they interact with and scatter light. A certain wavelength produces a single color; the shininess we perceive in metallic pigments can’t be reproduced using a single wavelength, as this is given off by tiny variations in surface reflecting light in different directions. The metal itself may well be a solid color, but our final perception of it is not. A gray line painted on canvas doesn’t look like a bar of steel any more than a yellowish one can pass off as a bar of gold. As such, metallic colors are also non-spectral colors.

Ancient shred of Israeli fabric reveals the secrets of “royal purple”

Some fashion choices can be uninspired, but the great have a potential to echo through the ages. Archeologists working in Israel’s Timna Valley have uncovered an ancient example of the latter — scraps of purple cloth from biblical times.

A wool textile fragment decorated with pink-purple threads discovered at the site. Image credits Dafna Gazit, courtesy of the Israel Antiquities Authority.

Being rich and powerful isn’t as fun if you don’t flaunt it to everyone. Judging by the fancy hats and other items that ancient nobles and royalty wore, our ancestors likely agreed. Still, while the modern world gave us brave new ways to show off our status, our forefathers had to resort to simpler means, such as wearing clothes dyed with expensive pigments.

Excavation works at an Iron-Age copper production site in the Timna Valley yielded a scrap of such refined clothing. The patch of ancient woolen fabric still bears tassels and fibers dyed with purple, a ‘royal’ color at the time due to its price. Purple dye is often mentioned in the Bible, the team notes, and analysis of the cloth revealed it hails from approximately 3000 years ago, around the time of kings David and Solomon, two important kings in Jewish and Christian history.

This is the first time we’ve found remnants of purple cloth from this time, the team adds.

This shirt? King material.

“This is a very exciting and important discovery,” explains Dr. Naama Sukenik, curator of organic finds at the Israel Antiquities Authority. “In antiquity, purple attire was associated with the nobility, with priests, and of course with royalty. The gorgeous shade of the purple, the fact that it does not fade, and the difficulty in producing the dye, [made it] often cost more than gold.”

Finding the material here of all places is a two-fold surprise: first, this was an industrial area. The Timna Valley site is still littered with slag produced by bellowing furnaces in which copper was smelted. It’s not exactly a place for fine clothes, even if you own some. Furthermore, the closest source for the dye (made in minute quantities from individual mollusks) is the Mediterranean sea which is over 300 km away.

Still — important people need to get around, and they have the money to afford luxurious, far-away dyes. What’s more exciting about the discovery is that it represents the first actual piece of dyed purple cloth we’ve found from the Iron age in the whole Southern Levant.

“Until the current discovery, we had only encountered mollusk-shell waste and potsherds with patches of dye, which provided evidence of the purple industry in the Iron Age,” adds Dr. Naama Sukenik. “Now, for the first time, we have direct evidence of the dyed fabrics themselves, preserved for some 3000 years.”

Excavations at the Timna site have been ongoing for a few years now. The very dry climate of the area means organic material such as textiles could remain well preserved even after thousands of years, giving us a unique opportunity to peer into the lives of our ancestors. This is why the team is confident that the discovery of this strip of cloth was only possible here. Prof. Erez Ben-Yosef from Tel Aviv University’s Archaeology Department, the paper’s corresponding author, explains that “the state of preservation at Timna is exceptional and it is paralleled only by that at much later sites”.

“In recent years, we have been excavating a new site inside Timna known as Slaves’ Hill. The name may be misleading since far from being slaves, the laborers were highly skilled metalworkers. Timna was a production center for copper, the Iron Age equivalent of modern-day oil. Copper smelting required advanced metallurgical understanding that was a guarded secret, and those who held this knowledge were the “Hi-Tech’ experts of the time,” he adds.

“Slaves’ Hill is the largest copper-smelting site in the valley and it is filled with piles of industrial waste such as slag from the smelting furnaces. One of these heaps yielded three scraps of colored cloth. The color immediately attracted our attention, but we found it hard to believe that we had found true purple from such an ancient period.”

The Banded Dye-Murex and Spiny Dye-Murex (Bolinus brandaris) are two species of mollusks endemic to the Mediterranean. They’re also the source of ancient purple dye. Pigments were produced starting from a gland within their bodies which was then processed in a complex series of chemical steps that could take several days to produce dye. If the materials were left exposed to light, an azure color (‘tekhelet’) would be produced; if not, purple (‘argaman’) was the end result.

Both colors, the authors note, are mentioned in ancient sources and often mentioned together. They often held religious or symbolic value (such as showcasing wealth and power). In the Bible, the Temple priests, kings David and Solomon, and Jesus of Nazareth are described as having worn clothing colored with purple.

The presence of the dye in the cloth was established using a high-performance liquid chromatography device, which found unique molecules known only in certain species of mollusks. In archaeology in general, explains lead author Dr. Naama Sukenik of the Israel Antiquities Authority, cloth is typically dyed with plant-based pigments, as these were much cheaper, simpler to produce, and readily available while animal-based pigments were more “prestigious”.

As part of the research, the team also recreated the dye using mollusks from Italy (where they are enjoyed as food). Although it took ‘thousands of mollusks’, they managed to successfully recreate the color — having the ancient equivalent to check against helped a lot. Among some of the findings is a ‘double-dyeing’ method “in which two species of mollusk were used in a sophisticated way, to enrich the dye,” says Dr. Sukenik.

“The practical work took us back thousands of years,” adds co-author Prof. Zohar Amar, “and it has allowed us to better understand obscure historical sources associated with the precious colors of azure and purple.”

During its day, Timna was part of the Kingdom of Edom, which bordered the Kingdom of Israel to the south.

The paper “Early evidence of royal purple dyed textile from Timna Valley (Israel)” has been published in the journal PLOS ONE.

One of the oldest known New Testament copies could have been written in pee-based ink

Restoration experts have identified the materials that went into making the purple dye of the Codex Purpureus Rossanensis, one of the oldest known New Testament manuscripts, and they aren’t exactly ecclesial: the ink was made from a combination of lichens and fermented urine.

The debate over exactly how the ancient bookmakers, most likely hailing from today’s Syria, crafted the amazing book using the simple tools and limited resources available to them 1,500 years ago has been ever since the manuscript was found.

The beginning of the gospel of Mark in the codex.
Image via wikimedia

“Even though early medieval illuminated manuscripts have been deeply studied from the historical standpoint, they have been rarely fully described in their material composition,” lab director Marina Bicchieri, from the Central Institute for Restoration and Conservation of Archival and Library Heritage (ICRCPAL) in Rome, told Discovery News.

The strikingly beautiful book is usually housed in the Museum of the Diocese in Rossano, a town in southern Italy. The work is 188 pages long, containing the gospels of Matthew and Mark written down in gold and silver ink. Its exact history is unknown, but it’s believed that Italian monks brought the manuscript from Syria. It was re-discovered in 1879 in the Cathedral of Rossano, and since then the debate over how it was written rages on.

Sadly, much of the book has been lost over time, and the book is extremely fragile. Most of it was destroyed in a fire inside the cathedral, and Bicchieri’s team also had to deal with the damage left by earlier restoration efforts. These conducted by an unnamed team around 1917 and irreversibly modified some of the pages.

“Most likely, what we have today represents half of the original book,” museum officials suggest.

The discovery of the purple ink’s materials was made during the book’s restoration by the ICRCPAL. Aiming not to further damage the work, the team only mended a few of its stitches to keep it from falling apart, then used X-rays to examine the composition of the inks in the codex. They compared their findings with dyes recreated in the lab using recipes found in the Stockholm papyrus – a Greek ink recipe book that’s been dated to around 300 AD.

The team reports that the purple dye, thought to have been made out of Murex (a species of sea snail,) was actually produced with orcein, a dye extracted from the lichen Roccella Tinctoria, and sodium carbonate. The latter was obtained from natron — a salt-like material used to mummify bodies in ancient Egypt, Lorenzi explains. But, to bring out the best shade of purple, the dye-making process seems to have involved using fermented urine to mix the compounds.

The pages are dyed with the purple ink.

The pages are dyed with the purple ink.


“Fibre optics reflectance spectra (FORS) showed a perfect match between the purple parchment of the codex and a dye obtained with orcein and an addition of sodium carbonate,” Bicchieri told Rosella Lorenzi at Discovery News.

To you and me this might seem pretty….gross. But in the day it was actually a very practical choice, as urine was the only readily available source of ammonia available 1,500 years ago.

The team is still preparing their findings for publication, and have yet to pass the test of peer-review — but once they do, they could finally end the century long debate around the purple ink.