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

Question

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"?

Answer 1

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.

Answer 2

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.