Occasional Paper: Purple Sun Yeast

by Doug Muir on January 19, 2024

An interesting paper: researchers inserted a gene for photosynthesis* into ordinary brewer’s yeast. It worked! The yeast began to photosynthesize, tapping energy from the sun.

I’m not generally alarmist about this sort of thing. But this is… maybe very slightly alarming.


1) Baker’s yeast is a single-celled fungus. A fungus, like a mushroom. You remember back in grade school, learning that fungi don’t photosynthesize? This breaks that.

2) If you can do this with a single-celled fungus, you can probably do it with a wide range of other organisms.  That probably includes other eukaryotes — complex “higher” organisms — likely including animals.  Animals have no evolutionary history of photosynthesis and aren’t designed for it, but the same is true for yeast.  So… no reason this shouldn’t be possible.  A photosynthesizing cat?  Sure, why not.

3) The precise point where I went from “Huh” to “Holy crap”: the altered yeast showed *enhanced fitness*. Given sunlight, it outperformed both normal yeast and altered yeast with no sunlight.

Gene alteration has been around for many years now, and our biologist friends have come up with some crazy stuff already.  Remember the goats that secreted spider-silk in their milk?  That was years ago, ancient history.  They’re way past that.  So in principle this isn’t a big deal.

But silk-goats aren’t *adaptive*.  They don’t show superior fitness.  Released into the wild, they wouldn’t make a bit of difference. This… might.

Here’s the paper if you want the details.

Anyway.  Weekend, woo!

*The gene was to produce rhodopsin, which is sort of like the cheapass version of chlorophyll.  Rhodopsin is purple instead of green**, and it’s mostly used by bacteria.  On one hand, it’s less efficient than chlorophyll at turning light into chemical energy. Other hand, chlorophyll needs a lot of complicated molecular machinery to work — basically it needs to be wrapped inside a chloroplast, which is a complex little dingus that requires more genes to build.  Rhodopsin doesn’t — drop it into a cell, shine light on it, and it’ll start providing a steady modest trickle of energy. Power drill versus screwdriver, type of thing.

**So a hypothetical rhodopsin-photosynthesizing cat would be, not green, but purple.




NomadUK 01.19.24 at 1:39 pm


David in Tokyo 01.19.24 at 2:20 pm

It’s not the rhodopsin you need to worry about, it’s the “gain of function” researchers who insist that if they don’t make a new killer virus, we won’t be ready for it when nature gets around to it. Sure, but that puts us into “what could possibly go wrong” territory, something fierce and without a paddle. And without even knowing what the paddle might look like.

Lots of people are working on and/or thinking about how to prepare for the next virus without the insanity of actually creating it. (Not that we’re dealing with the current one particularly well: we’ve decided that it doesn’t matter if it’s just old folks who die, and it doesn’t matter if 3% of the people who catch covid never actually recover.)


P.D. Magnus 01.19.24 at 2:45 pm

Apart from the fitness of the spider-goats, the consequences of a lab leak would be very different. A stray goat can be chased and caught. A stray yeast, not so much.


Phil H 01.19.24 at 4:39 pm

On the one hand, yes, yikes. On the other hand, it seems like pretty much every advance that has been made throughout history has an attendant downside/risk factor. And it’s not obvious to me that we have any way to balance up these two sides and decide what’s really too risky. Or rather, we have basic safety precautions at labs already. Does the historic-new-wow nature of the advance demand anything beyond normal safety precautions? I can’t work out how to even think about what new risk prevention framework we could adopt.


marcel proust 01.19.24 at 5:01 pm

I am trying to imagine what this would do to bread (dough), beer or wine. What would the differential affect on ales vs. lagers be? On reds vs white wines? Whole grain vs white breads? Would the yeast “decide” not to bother with fermentation because photosynthesis is more productive for the same effort? Would they do both (turning all breads purple?*). Would all bread rising, beer brewing and wine making have to be done in pitch black conditions worthy of a dark room?

*Cue old shaggy dog joke about a purple beer that was so strong and so awful, it was the beer that either made Phil Walkie maim us, or made Mel Famy walk us.


Casey 01.19.24 at 5:20 pm

The paper is paywalled, but are the yeast actually deriving ATP from light, or are they merely minimizing photo-damage?


Thomas P 01.19.24 at 5:37 pm

Then there are GMO intentionally designed to cause extinction. Spreading infertility throughout a population:


Ebenezer Scrooge 01.19.24 at 5:52 pm

I don’t understand. We have rhodopsin in our eyeballs. A lot depends on what the rhodopsin is connected to.


Doug Muir 01.19.24 at 6:17 pm

Casey @5 asks, “The paper is paywalled, but are the yeast actually deriving ATP from light, or are they merely minimizing photo-damage?”

— My understanding is, neither. The rhodopsin isn’t creating ATP (that would require a lot more than one single gene). But it isn’t just passively soaking up photons like melanin, either. It’s acting as a proton pump, deacidifying the cytoplasm — meaning it is actually transferring energy from light to the chemistry of the cell.

Apparently acidification is associated with starvation and stress. So the rhodopsin is making the yeast less damaged by those things, and thus more robust.

Presumably this is why the paper calls it a “facultative” heterotroph — facultative being a biologist’s term for when an organism /can/ do something (but doesn’t usually). The rhodopsin isn’t feeding the yeast under normal circumstances, but it’s providing an emergency energy boost when the yeast is stressed or hungry.

So it’s not “the yeast is now getting a lot of energy from sunlight”. More like, “the yeast now has access to coffee”.

Doug M.


Doug Muir 01.19.24 at 6:22 pm

“We have rhodopsin in our eyeballs. ”

We sure do! But we don’t use it for photosynthesis. It’s the visual pigment in the rod cells of our retinas, and [simplification] when light hits that rhodopsin, it releases energy, which gets converted into neurosignals, which we perceive as vision. [/simplification]

Fun trivia: rhodopsin exposed to bright light stops working for a while. This is why full night vision takes a while to kick in — even after your eyes are fully dilated, the rhodopsin needs some time to reset.

Doug M.


Neville Morley 01.19.24 at 6:53 pm

At the level of extreme pedantry, but speaking as both a brewer and a baker, baker’s yeast and brewer’s yeast are not the same thing – the latter indeed comes in multiple species. Yes, you can brew beer with baker’s yeast, and bake bread with brewer’s yeast, but neither is as good at the task as the stuff we’ve been selectively breeding for millennia.

Less pedantically, I am likewise most interested in the implications for brewing/baking – and so would like a more specifics explanation of ‘outperformed’. Generally, we aim to use small amount of yeast that then multiples and then starts producing carbon dioxide and alcohol. I’m not clear if this new yeast would multiply extra-rapidly in sunlight, hence allowing more rapid fermentation and/or use of smaller quantities, or not bother switching to the ‘digest sugar for energy, produce carbon dioxide and alcohol as by-products’ stage, which is why we actually use them. And the development of chemical processes to reduce fermentation time immediately reminds me of of the Chorleywood method, which produces very cheap, fluffy and utterly tasteless and unhealthy bread…


Oscar Charlie 01.19.24 at 8:54 pm

This seems a bit like the thing with the GMO salmon that got everyone’s dander up a few years ago. They knocked out a gene that caused the fish’s growth hormones to turn off seasonally, so it would grow continuously and thereby get to commercial size in half the time. People spread all kinds of alarm about frankenfish escaping and interbreeding with wild fish to make giant mutants that would take over. The developers also did something to make the fish infertile, and of course that wound some people up as well – “infertility might spread!!!”

The catch, as here, is that if a simple gene alteration would allow the resulting organism to outcompete in the wild, it would have already happen over the millennia of ordinary evolution. For the salmon, a mutation knocking out the seasonal variation gene almost surely happens all the time, but those fish aren’t found because it’s evidently disfavored by selection. Otherwise why did the seasonal-variation phenotype take over in the first place?

With the yeast, the gene that they inserted was from another fungus, so this is already something in the “fungo-sphere”. They say in the abstract “this system has apparently spread widely via horizontal gene transfer”. In this case it’s adaptive (maybe) in the context of a specific controlled lab environment. But in the wild? There’s an energy cost to making any extra thing, and so it starts out with a strike against it. And who knows what other harms a proton pump without all the ancillary machinery to make it useful could cause.


Lee Arnold 01.19.24 at 9:59 pm

Pretty soon YOU may be able to photosynthesize.

And regenerate limbs (starfish). And live to 300 (sharks).


KT2 01.20.24 at 4:17 am

Why not go to the end point – breed mushrooms which fruit salmon flesh. Or pork. Or soy protein. Or antibiotics?

And yes, as Doug says… “3) …“Holy crap”: the altered yeast showed enhanced fitness.”

We need databases such as this;
“… about 315 million groups of genes from microbes living in the Arctic, Indian, Southern, Atlantic and Pacific oceans and the Mediterranean sea2.”

Which fortunately also: “… act as a baseline measurement for marine microbial diversity, so that scientists can track the effects of activities such as burning fossil fuels or deep-sea mining, he adds.”

Will the… “fungi represent more than half of the gene groups identified in the ‘twilight zone’, a region between 200 and 1,000 metres beneath the ocean surface.”…
– gain a new rhodopsin weighted goal to move up the water column towards sunlight? Or evolve a more complex light sensor? Or attract food?

With this database one could find novel antibiotics or a dangerous pathogen. And the delay in knowing the unintended consequences of difference between useful and dangerous needs serious monitoring and control. Although if I were a twenty something biologist, I’d be cranking up the Crisper CAS9 + robot… + Pick-a-function. Baselines and delay in proving safety be damned.

“Largest genetic database of marine microbes could aid drug discovery”

“Genes and proteins derived from marine microbes have endless potential applications,” says Duarte. “We can probe for new antibiotics, we can find new enzymes for food production,” he says. “If they know what they’re searching for, researchers can use our platform to find the needle in the haystack that can address a specific problem.”

Oscar Charlie said “This seems a bit like the thing with the GMO salmon”.

Yes it does. The CSIRO way back in 1990’s “knocked out a gene that caused the fish’s growth hormones to turn off seasonally, so it would grow continuously” and to asuage “everyone’s dander up” brigade, they also inserted 2x ‘trigger death if sexual hormones detected’ genes. How has it gone? The CSIRO has turned bleeding edge genomics into a platform to act as a (proton) money pump “that will transform commercial production producing even greater genetic and economic gains.” The CSIRO under Larry Marshall don’t even bother nentioning feeding humans anymore. Just ever greater genetic and economic gains.

“Developing a genomic selection platform for the Tasmanian salmon industry”
“A platform to enable the implementation of genomic selection
“Due to rapid and successful platform development, genomic selection was implemented for all production traits for the first time in 2016, a single year after the programme began in 2015, with full deployment achieved in 2018.

“Substantial economic impact is expected to continue through production efficiencies and productivity gains. The genomic selection platform is enabling selection in previously unselected commercial populations that will transform commercial production producing even greater genetic and economic gains. The platform continues to evolve, expanding to tackle new traits important to the industry, including summer growth.”

And we have a fungus map now too, enabling automated selection and testing of novelty.
“Megaphylogeny resolves global patterns of mushroom evolution”
Authors 50+
Figure 1
“Phylogenetic relationships and diversification across 5,284 mushroom-forming fungi…”

Purple equity will exhibit swarming behaviour.


Adam Roberts 01.20.24 at 8:07 am

I wrote a novel set in a future in which people photosynthesise through their hair. The tech was developed ith utopian dreams of freeing the world from reliance on food, but in the event what happens is that the poor, since they can now survive on much less money, are further impoverished, working for sub-subsistence wages, whilst the the rich concentrate much more wealth to themselves. More, since a punishment head-shave leaves a person liable to starve unless they can source some actual food to eat whilst the hair grows back, which the very poor cannot do, it’s easier to keep them in line. The rich enjoy the benefits of photosynthesising hair whilst also feasting on the most delicious food. Not using CT to spam for sales, so I won’t link to it, but I’ll say that I wanted to call the book Beggars Banquet, but my publisher publishes a very famous, very high-selling crime writer who had issued a collection of short-stories with that title, so I wasn’t allowed, and had to go with the much inferior By Light Alone.


Thomas P 01.20.24 at 11:23 am

Doug, your description makes me wonder if something like this might be how photosynthesis evolved in the first place. One small step at a time.


Peter Erwin 01.20.24 at 1:48 pm

The (non-paywalled) preprint version is here:


You remember back in grade school, learning that fungi don’t photosynthesize? This breaks that.

Apparently not, since the rhodopsin gene they inserted came from another fungus. (Obviously, “what you learn in grade school” can often turn out to be simplified and outdated.)


Peter Erwin 01.20.24 at 2:15 pm

marcel proust @ 5
I am trying to imagine what this would do to bread (dough), beer or wine.

Probably nothing. I have to assume that most fermentation is done in extremely dark conditions (not to mention the fact that any light leaking in would only affect yeast at the very surface). The paper notes that the modified yeast were slightly less fit than unmodified yeast in dark conditions (by about 1% relative to unmodified yeast, which is about the same size as the increased fitness when light is supplied), so it would probably be outcompeted by unmodified yeast.

The paper also mentions that the yeast were grown in a medium containing (externally supplied) retinal pigment, so you’d have to add that to get any advantage. (The full rhodopsin metabolism apparently requires another four genes to synthesize retinal.)


Alex SL 01.21.24 at 12:06 am

Biologist here. I can’t currently get past the paywall, so I am only going off the discussion above and the abstract of the paper, but my immediate thought is that even if this works for yeast, it would only work for yeast because it is a fairly simple, single-cell organism that could integrate the enzyme into its membranes and then do something when hit by light. That doesn’t mean it would do anything useful whatsoever if the same was done to, say, a cat. Hardly any of the cat’s skin would be exposed to significant amounts of light, and even if were (naked cat), only a minuscule fraction of its cells would be exposed, i.e., the upper skin layer. That strictly limits what would maximally happen.

There is a reason why larger photosynthetic organisms, even mosses, invent some variation of leaf and consist mostly of very thin surfaces. Animals just aren’t built to get a lot of benefit from sunlight. Consider also that there are a few animals that take up chloroplasts, e.g. certain sea slugs, and as the article points out, horizontal transfer of chloroplasts and photosynthesis genes is actually very common in evolutionary history; if it would confer a major advantage to animals, then why aren’t we already photosynthetic, what with animals taking up photosynthesis genes four hundred million years ago hypothetically having outcompeted those that didn’t?

All seems like a very special case nothingburger.

Doug M,

If your interpretation is correct, it is even nothinger of a burger, but I also don’t quite understand how the cell could gain any energy without producing ATP. That is what energy is in a cell.


Gareth Richard Samuel Wilson 01.21.24 at 3:00 am

Spider Robinson wrote about a symbiote that would photosynthesize and perfectly recycle wastes, allowing humans to live in space without supplies or technology. He had a character describe the humans in symbiosis as perfect Communists – there’s nothing you can sell them. I always thought that was an enormous failure of imagination from a professional writer.


J-D 01.21.24 at 7:07 am

Presumably this is why the paper calls it a “facultative” heterotroph — facultative being a biologist’s term for when an organism /can/ do something (but doesn’t usually).

I don’t think that’s right; I don’t think the paper is saying the organism is (or has been made) a facultative heterotroph; I think that’s a misreading of the terminology.

Heterotrophs derive carbon from organic sources (other organisms, or the tissue of other organisms, or the remains or waste of other organisms). Autotrophs derive carbon from inorganic sources (carbon dioxide dissolved in air or water).

Phototrophs derive energy from light (by photosynthesis). Chemotrophs derive energy from chemical sources (for example, as in the case of human beings, by eating food).

Plants (in general) and some microorganisms are photoautotrophs. Animals and fungi (in general) and some microorganisms are chemoheterotrophs. There are also microorganisms which are photoheterotrophs, and others which are chemoautotrophs.

Yeast, not being able to photosynthesise, are obligate chemotrophs (‘obligate’ is a converse of ‘facultative’: they can only obtain their energy from chemical sources). The experiment converted them into facultative phototrophs. The paper (judging by the linked pre-print) uses exactly this description at some points; at other points it uses the more specific description ‘facultative photoheterotrophs’. But this doesn’t mean (as far as I can tell) that they have become facultative heterotrophs; they are still obligate heterotrophs (at least, there’s nothing to indicate to the contrary). The ‘facultative’ in ‘facultative photoheterotrophs’ relates only to the ‘photo-‘ part, not to the ‘hetero-‘ part.

(I have said nothing about the distinction between organotrophs and lithotrophs, partly because the paper doesn’t seem to mention it, but mostly because I’ve never been able to understand it properly.)



Robert J Berger 01.21.24 at 7:27 am

I read another Science Fiction story where genetic engineering gave people the ability to photosynthesize that was different than the one by Adam Roberts wrote (mentioned above). It had the opposite impact from Roberts story. It did in fact end wage slavery and allowed people to live without having to worry about feeding themselves. Unfortunately I can’t remember the title or author. The story had two stories in one. It primarily took place in the future created by this which was pretty much post singularity with genetic engineering fully embraced. People would modify themselves through out their lives into all different kinds of animals and more. Also people can be backed up and restarted in new bodies if they die.

The main character of the story is telling the story of how she “kills” her now ex-lover in a way that he doesn’t get to backup himself before he spurns her or something like that. But the more interesting second story is she is also doing a dissertation on the how the ability for people to photosynthesize was created and brought into the world. That story is full of intrigue, politics and a spy thriller. It was a great story that had a lot of great ideas and that I think about a lot.


Alison 01.21.24 at 10:29 am

By Light Alone made me think about the dangers of Universal Basic Income. The food-making hair seemed like a kind of UBI for calories? Without radical reform these band-aids don’t fix things.

The Green Leopard Plague by Walter Jon Williams is another SF story about human photosynthesis. I remember I felt that animal metabolism (particularly warm-blooded) would require so much energy that you would need to be attached to some kind of massive sail covered in chloroplasts. But I don’t really know if that’s true.


Alex SL 01.21.24 at 9:21 pm

Hair cannot make food, because hair is dead, it isn’t living cells.

It is possible that the -troph terms are muddled up a bit. Autotroph means getting one’s energy without having to break down complex organic substances, and heterotroph means getting one’s energy from breaking down complex organic substances (that have been produced by autotrophs). Among autotrophs, most are photosynthetic, but very few species of bacteria are chemoautotroph, often living in extreme habitats underground where there is no sunlight. They grow extremely slowly by conducting a simple inorganic chemical reaction to get energy that they then use to build their own complex organic substances like others do with photosynthesis. Among heterotrophs there is the added complexity that most use oxygen to dismantle complex organic substances for energy gain, but others can facultatively grow under anoxic conditions by doing a much less efficient reaction called fermentation. That may be where the ‘facultative’ comes from.

It seems clear that the GMO yeast is still a heterotroph, it merely grows a bit better with light. It is not autotroph, and certainly not chemoautotroph, and I’d wager the only thing it is facultative about is oxygen.

(Still haven’t seen the full paper, so I am starting to wonder if its authors may themselves be confused about terminology. I am saying this as a phylogeneticist and systematist who is frequently faced with manuscripts from otherwise perfectly capable, PhD-wielding colleagues who are confused about terms like incomplete lineage sorting or think that outgroup rooting on one outgroup species demonstrates that the ingroup is monophyletic. But it is a Cell journal, so I would hope they’d have strict peer review.)


KT2 01.21.24 at 10:43 pm

Justification and plea to synthetic biologists to create more purple sun yeast style innovations; “we aim to demonstrate why a search for novelty, rather than optimizing function alone, is a more productive strategy for achieving continuous innovation. ”

“Open-endedness in synthetic biology: A route to continual innovation for biological design”
19 Jan 2024

“Castle et al. (14) go a step further and consider the evolutionary landscape as something to be sculpted as part of the design process such that resultant biological designs are more robust or more evolvable when deployed: the engineering of evolution itself.

“Starting from an evolutionary mindset, we provide a perspective on biological design that recognizes the crucial role that open-endedness and novelty play in biological evolution and their potential for integration into design workflows for synthetic biology (Fig. 1). Building on efforts by artificial life and evolutionary biology communities, we aim to demonstrate why a search for novelty, rather than optimizing function alone, is a more productive strategy for achieving continuous innovation. We want to persuade biological engineers that new approaches are required and to embrace creativity when designing the engineered biological systems of the future. Because of the breadth of topics covered, Box 1 provides a glossary of terms used throughout.”

Via “Embracing idiosyncrasies over optimization: The path to innovation in biotechnological design”


Anthony Burnetti 01.22.24 at 2:48 pm

Hey! Anthony Burnetti, an author of the paper here. The preprint from 9 months ago contains most of the important data, the extra analysis in the final version is mostly doing some extra control experiments to make sure we were not fooling ourselves. There is a temporary publicly available link for the final version too, at https://authors.elsevier.com/a/1iPzr_LsQSPs8m

We yanked this gene from U. maydis, a corn smut fungus. There is a family of fungal proton-pumping vacuolar rhodopsins that is almost exclusively found in fungal plant pathogens, and almost certainly came from bacteria in the distant past from natural horizontal transfer. We brought it into this yeast (S. cerevisiae, same species as brewing and beer yeast, though not a strain you would want to use for either), and as we say, it just worked. It is pumping protons across the vacuole membrane where ordinarily the yeast are using lots of energy to pump protons, but this can substitute for that process. In THEORY you could run this so hard that you start making ATP by running the normal cellular pumps in reverse but we don’t think we are actually reaching a high enough voltage across this particular membrane. Another group just recently (https://microbialcellfactories.biomedcentral.com/articles/10.1186/s12934-023-02273-1) did something similar and published additional measurements that doing this boosts ATP levels but again that might be from reducing necessary consumption.

On its own rhodopsin only pumps protons and charges up membranes, and can’t do the fancy electron reactions you would need to fix biomass from CO2 like chlorophyll does. Other people have just recently rube-goldberged together complex systems that can use this as an energy source to fix carbon in bacteria (https://www.nature.com/articles/s41467-023-43524-4), but they were bacteria that could already fix carbon, they just changed their energy source. Lots of people are doing interesting things with rhodopsins lately! We only added the one gene, and we fed the yeast the pigment that makes the gene work. We could add more genes to make the pigment too but that would require more fiddling and when we tried the first time they were making SO MUCH PIGMENT that they were bright yellow and growing half as fast from shunting a large fraction of sugar into pigment, so we just started feeding it to them rather than fiddling and tuning the production pathway to not run in overdrive.

So far, the cells are about 1% less fit than their non-phototroph ancestors in the dark, and about 1% more fit in the light (we illuminated them pretty brightly). Most wild yeast are not growing much when in direct sunlight or even in particular bright environs, and I wonder if this could be the reason that the only fungi that we reliably find these genes in are pathogens that live on plant surfaces. The differences are more pronounced when they are growing on glycerol, an inferior carbon source, compared to when they grown on sugar, an easier carbon source. The cells are acting like they are less quiescent in the light during starvation, and as mentioned earlier other people have measured increased ATP levels in illuminated cells with a similar modification. It’s a super exciting field that several different groups are making advances in! We are actively trying to make this system more efficient, both trying even more rhodopsins from further-afield genomes and messing with targeting them to mitochondrial membranes where they might be better plugged into energy metabolism (though this is WAY harder than we thought it would be).

We started the whole project first and foremost because we in the Ratcliff lab are actually interested in studying the evolution of multicellularity – how do microbes evolve from single cells to multicells and learn to cooperate? Near as we can tell, the big question any time you go from a microbe to a macroscopic multicell is how do you get energy to your interior now that oxygen has a hard time diffusing in. We think that one of several ways you can deal with this is to be a phototroph – light can often penetrate deeper than the oxygen can diffuse into a metabolizing tissue, and you get an extra energy source to your innards. We have a model system of simple multicellular yeast (“Snowflake yeast”) that we evolve for increased size and complexity in the lab and we want to try to get around their limitation by oxygen in different ways to understand what was going on on Earth >600 million years ago when a lot of your favorite multicellular lineages originated. We have done other modifications too, like giving them myoglobins from animals, and this is all coming together in a project to understand major innovations at the dawn of multicellularity and why they occurred when they did in the history of Earth. But then, optimizing the phototrophic yeast comes along as a whole new interesting project! We have a whole range of ways we are now trying to improve it and take it in new directions.

Regarding terminology – a photoheterotroph is an organism that has to use something other than CO2 as its carbon source and breaks down organic matter, but uses light for energy. Facultative because it does not HAVE to do so, but can when given the opportunity.


Alex SL 01.22.24 at 8:57 pm

Thanks for the comprehensive explanation. As I thought, it isn’t photosynthetic. Note that when I wrote ‘nothingburger’, I did not mean the scientific insights gained from the experiment but merely that this yeast or anything like it is unlikely to go Frankenstein’s Monster.


J-D 01.22.24 at 10:34 pm

Regarding terminology – a photoheterotroph is an organism that has to use something other than CO2 as its carbon source and breaks down organic matter, but uses light for energy. Facultative because it does not HAVE to do so, but can when given the opportunity.

In this context there is ambiguity in the words ‘to do so’. If, in this context, ‘to do so’ means ‘to use light for energy’, and the organism does not have to use light for energy but can when given the opportunity, then it’s a facultative phototroph. If, in this context, ‘to do so’ means ‘to use something other than carbon dioxide as its carbon source’, and the organism does not have to use something other than carbon dioxide as its carbon source but can when given the opportunity, then it’s a facultative heterotroph. It is possible for both descriptions to be true, but they don’t both have the same meaning. If an organism is described as a facultative photoheterotroph, it’s not clear whether that means that it’s a facultative phototroph and a facultative heterotroph, or that it’s a facultative phototroph and an obligate heterotroph, or that it’s an obligate phototroph and a facultative heterotroph.


ef 01.24.24 at 5:25 pm

… I wonder if anyone has checked in with the lichenologists* about this one – or whether rhodopsin is present in the purple lichens humans have been using as dyestuffs for millenia (or, rather, whether rhodopsin is why dyer’s lichens are known and used as such.

*given that their entire ~thing~ as far as I understand it is an example of said enhanced-fitness-due-to-combo-of-funghi-and-bacterial-genes-for-photosythesis.

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