Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Tuesday, August 24, 2010

Seeing red

Seventy percent of all energy consumed by life on Earth is in the form of solar photons. Interestingly this consumption is by a minority of organisms, those exploiting the mechanisms of photosynthesis. It amounts to a globally averaged energy intake of some 100 terawatts, but even this is peanuts compared to the solar input at the Earth's surface, which is about 90 petawatts. By comparison, all available geophysical energy - thermal and chemical - is a paltry 30 terawatts.

If you want energy, then photons are the way to go. Catching and transducing photons into chemical energy in biological systems is accomplished via the extraordinary chlorophyll pigments. In oxygen producing organisms four types of chlorophyll had been known, each tuned to slightly different wavelengths. For example, chlorophyll 'a' grabs photons at around 465 nanometers (bluish visible light) and at around 665 nanometers (reddish visible light) - leaving behind the familiar green photons that we enjoy in our foliage. The other, rarer, chlorophylls absorb at similar wavelengths. Now, in a neat paper that appeared in Science last week, Chen et al. have identified a fifth chlorophyll 'f' - extracted from organisms lurking in modern day stromatolite formations.

Why get excited about this? Chlorophyll 'f' does something not seen before in photosynthesis - it slurps up photons from the near-infrared. At about 706 nanometers we're into a regime just beyond the visible. Obviously other pigments and structures can, and do, absorb photons in this regime, but only 'f' is known to actually use the photons to split water molecules - the key step in oxygenic photosynthesis.

To my mind this raises a number of fascinating connections to questions of life on Earth and beyond. It suddenly connects the dots to earlier indications of organisms exploiting infra-red photons around deep-ocean hydrothermal vent systems - or at least offers a molecular solution. It also, and here I'm heading out on a limb, might connect to an issue that's long bothered me. We've talked before about how our Sun was as much as 30% fainter three or four billion years ago. With this faintness comes a small shift in the peak output of a star's spectrum (being a blackbody to first order) towards the red. For the young Sun this would have only been a few tens of nanometers - but it would have meant a slightly better flux of these near infrared photons. In addition, a different atmospheric composition and chemistry on a youthful Earth could have altered the typical range of photons making it down to the surface. It seems not unreasonable to suspect that chlorophyll 'f' could have given ancient microbial life a leg-up over the competition. Indeed, did chlorophyll 'f' come along first?

It's pretty startling evidence of nature's capacity to find molecular machinery to exploit even low energy infra-red photons. Now picture a world around a low-mass star, with a drastically redder spectrum. Chlorophyll 'f' points the way to how organisms there might manage all the great tricks of photosynthesis that we see here on Earth.


Eniac said...

It might be worth pointing out that there is a large class of organisms using an entirely different system of photosynthesis, based on bacteriorhodopsin, without chlorophyll. Its lower complexity and presence in archaea makes it a better candidate for the "first" photosynthetic mechanism than chlorophyll, 'f' or otherwise.

Also, the splitting of water is coupled so indirectly to photon absorption that it does not make sense to talk of it as a property of the chromophore. AFAIK splitting one water molecule requires more than one photon (two, I think?), even before we are talking infrared. I believe one photon translocates two protons and then 4 protons are returned to split one water molecule, but I may remember that wrong.

Caleb Scharf said...

Thanks for pointing out the function of bacteriorhodopsin, I'll confess to not having fully appreciated this. Very, very interesting.

Eniac said...

Looks like I got the part about water splitting wrong. The water splitting complex is directly coupled to the reaction center of photosystem II as an electron donor. Four photons are required for each water molecule split.


This brings up the question of how (or if?) bacteriorhodopsin based organisms split water, as there are no electrons transferred in this process. Hmm...

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