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The Phys.org article Scientists discover first organism with chlorophyll genes that doesn't photosynthesize says

"For the first time scientists have found an organism that can produce chlorophyll but does not engage in photosynthesis.

It is referring to the new paper in Nature A widespread coral-infecting apicomplexan with chlorophyll biosynthesis genes (paywalled).

"This is the second most abundant cohabitant of coral on the planet and it hasn't been seen until now," says Patrick Keeling, a University of British Columbia botanist and senior researcher overseeing the study published in Nature. "This organism poses completely new biochemical questions. It looks like a parasite, and it's definitely not photosynthetic. But it still makes chlorophyll."

[...]

Chlorophyll is the green pigment found in plants and algae that allows them to absorb energy from sunlight during photosynthesis.

"Having chlorophyll without photosynthesis is actually very dangerous because chlorophyll is very good at capturing energy, but without photosynthesis to release the energy slowly it is like living with a bomb in your cells," Keeling says.

Question: Why is it that "Having chlorophyll without photosynthesis is actually very dangerous" and "like living with a bomb"?

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    $\begingroup$ While I lack detailed knowledge of this organism's internal structure, it's plausible that since it is still carrying out photocapture and acid pump that the acid pump directly powers ATP synthesis like it would normally do in mitochondria. It would take a link between the plastids and the mitochondira to do, but that's not ridiciulously implausible. $\endgroup$ – Joshua Apr 4 at 16:33
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    $\begingroup$ It is important to note, at least from my understanding of the article, that the organism doesn't actually produce chlorophyll; it merely has the genes to do so, but they seem to be inactive. $\endgroup$ – gardenhead Apr 4 at 21:10
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    $\begingroup$ They're referring to the fact that a bomb can release a lot of energy under the right circumstances. $\endgroup$ – Dmitry Grigoryev Apr 5 at 9:13
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Chlorophyll absorbs photons (light). The energy in the photon extracts an electron from a molecule of water. Electron transfer creates intermediate superoxide and hydroxyl radicals from the oxygen and hydrogen from the donor water molecule.

In normal photosynthesis, these radicals are quickly used to power the reduction of NADP to NADPH and the synthesis of ATP from ADP. NADPH and ATP in turn power the synthesis of sugars from carbon dioxide and water, via the Calvin cycle.

These radicals are highly reactive. If they hang around and don't get used properly in redox reactions, they will just as happily attack DNA, proteins, and structural lipids within the cell, and are therefore dangerous. In normal plant cells that get too much sun, for instance, free radicals can build up and cause cell damage.

My guess is that the author's "bomb" is made up of higher concentrations of these radicals within a cell with no apparent machinery to perform the downstream (photosynthetic) chemical reactions needed to consume them safely.

Edit

I skimmed the (sadly, paywalled) paper, and it sounds like my guess was right, that it is indeed these radicals that are the danger from having chlorophyll, with no light-independent (Calvin cycle) mediated reactions to safely consume the energy in them:

Chlorophyll itself has no natural biological function outside of photosynthesis, so if photosystems are indeed absent, corallicolids must have evolved a novel use for either chlorophyll or its closely related precursors or derivatives. However, these molecules generally function in light harvesting, which would be destructive to cellular integrity without the coupling of the resulting high-energy compounds to photosynthesis. Other possibilities are functions in light sensing, photo-quenching or the regulation of haem synthesis, but these too leave open the question of what the cell would do with the highenergy end products.

What's not clear to me is that the genes that help generate chlorophyll are expressed, but the cells are unpigmented. I don't see any explanation where the chlorophyll and associated proteins are localized in the cell — seems like a missing part of the paper, or I missed that part when skimming. Or perhaps the organism has evolved interesting and novel ways to manage the damage caused by these oxygen radicals, or has other mechanisms for consuming them, yet to be identified.

Should motivate further research, especially if these organisms share ancestry with malaria and toxoplasmosis — there might be something interesting to learn that would help with eliminating these diseases. I imagine a biochemical "radical bomb" could be very handy for destroying parasites, if there was a way to expose them to light or some other source of photons; perhaps some drug therapies could target the relevant genes and induce the parasite to destroy itself. Interesting paper.

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    $\begingroup$ It would be great if you could provide some references. $\endgroup$ – WYSIWYG Apr 4 at 11:07
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    $\begingroup$ Chlorophyll (and more commonly porphyrins) are in fact used to kill cells by light exposure in photodynamic therapy (Song et al., 2014) $\endgroup$ – WYSIWYG Apr 4 at 14:34
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    $\begingroup$ @elzell I would say that's usual dehydration/UV induced damage. Usually chlorophyll synthesis is regulated and there are anti-oxidants to prevent oxidative damage. Note that excessive sunlight also burns animals. $\endgroup$ – WYSIWYG Apr 4 at 14:55
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    $\begingroup$ Well now this just leaves me wondering if these organisms are doing anything with this chlorophyll. Are they just in some really weird evolutionary niche where they manage to produce chlorophyll they don't need, without it being a large enough burden to stop the species from surviving? $\endgroup$ – JMac Apr 4 at 16:05
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    $\begingroup$ @mbrig The algae Dunaliella "accumulates massive amounts of beta- carotene when cultivated under high light intensity" and the absorbtion of light is thought to be protective against the deleterious effects of sunlight (ref) and of course D. may cause lakes to turn pink $\endgroup$ – user1136 Apr 4 at 21:15
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Alex Reynolds has explained why “Having chlorophyll without photosynthesis is actually very dangerous”, but I think that the original sensational statement by Phys.org is unhelpful as it distracts from the real question. However that’s what you get with so-called ‘free’ journalism.

Obviously organisms don’t synthesize complex molecules without a reason. If the corals don’t perform photosynthesis, they use the high-energy electrons produced from the light photons for something else. To quote from the paper:

Chlorophyll itself has no natural biological function outside of photosynthesis, so if photosystems are indeed absent, corallicolids must have evolved a novel use for either chlorophyll or its closely related precursors or derivatives. However, these molecules generally function in light harvesting, which would be destructive to cellular integrity without the coupling of the resulting high-energy compounds to photosynthesis. Other possibilities are functions in light sensing, photo-quenching or the regulation of haem synthesis, but these too leave open the question of what the cell would do with the high- energy end products. Moreover, we detected corallicolids in sun coral (Tubastrea sp.) and black coral (order Antipatharia), both of which are considered to be non-photosynthetic corals, which further suggests that corallicolids deviate from classical modes of light harvesting.

The interesting question is what they use it for. Bombs are for writers that don’t do biochemistry.

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    $\begingroup$ You probably didn't notice but the article is directly quoting one of the authors. The reference to "bomb" is in quotation marks, with the author's name following immediately. Your bone to pick is with the author himself, not with Phys.org or "free journalism" (which it isn't, Phys.org has advertisements). I also felt it was strange that an author of the paper would use this wording, You may want to fix this glaring error in the beginning of your otherwise helpful answer. Thanks! $\endgroup$ – uhoh Apr 4 at 23:47
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    $\begingroup$ (-1) because first and final sentences are inaccurate and should be corrected. You're claiming that the author of the Nature paper doesn't do biochemistry. Yikes! $\endgroup$ – uhoh Apr 5 at 0:10
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    $\begingroup$ "Obviously organisms don’t synthesize complex molecules without a reason." I don't see this as obvious; vestigial structures are a counterexample. The reason could simply be random chance that hasn't been selected against. $\endgroup$ – Bob Apr 5 at 4:30
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    $\begingroup$ @uhoh On your 'yikes!' comment (agreed), to quote Sidney Brenner "... two things disappeared in 1990: one was communism, the other was biochemistry and [that] only one of these should be allowed to come back. Of course, biochemistry never really went away but continued to flourish in the thousands of unread pages of biochemical journals" Biochemistry Strikes Back. $\endgroup$ – user1136 Apr 5 at 10:22
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    $\begingroup$ @user1136 that's an interesting editorial. The paragraph immediately before the one to which you refer also has significance here with respect to this organism producing but not using chlorophyl; "...or indeed the very presence, of a protein may be very significant or totally irrelevant depending on whether it is following a ‘don’t care’ condition." $\endgroup$ – uhoh Apr 5 at 10:30

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