In the Depths, Antennas for Light

Coccolithophores are beautiful, inside and out.

As we pump out ever more carbon, burned from fossil sources, we are relying on the great geochemical and biological cycles to take in this waste and clear the air. At the same time, we are impairing these cycles by chopping down forests and acidifying the oceans. Half of photosynthetic productivity happens in the oceans, thanks to phytoplankton. While plants on land grow large and store carbon in their vast root and branch systems, plants in the ocean- the diatoms, algae, and of interest this week, coccolithophores- are all small and short-lived. They sequester carbon as a rain of detritus that falls into the deep ocean from the upper, planktonic regions. Coccolithophores, which are related to red algae, particularly make calcium carbonate shells that fix and remove a billion tons of carbon each year. Chalk landscapes like the cliffs of Dover- those are piles of coccolithophores from ancient seas.

These photosynthetic protists, which adopted a red algal symbiont somewhere in the mists of the past, are beautiful on the outside, with tough shells that are, ingeniously, transparent to the light they live from, just as are the silicate shells of their cousins the diatoms. These shells protect them from the physical buffeting of the ocean, from viruses and other pathogens, and the shell also buffers them from harsh chemicals and toxins. 

Coccolithophores make up roughly ten percent of oceanic phytoplankton, and specialize in more nutrient poor areas, relative to diatoms. One such area is deeper water, where light is relatively dim. A recent paper revealed structures for the light antenna complexes that allow these organisms to maximize their energy collection. All photosynthetic organisms have photosynthetic reaction centers where the key reactions happen- using light energy to crack the bonds of water molecules to liberate the hydrogens that are later stored in carbohydrates and fats. Complementary to that process is the re-union of two oxygen molecules into the waste product, oxygen gas, and also the fixation of carbon molecules from carbon dioxide, using those hydrogens to displace additional oxygen molecules.

But photosynthetic reaction centers are expensive to make, with requirements for obscure elements like manganese, and are highly reactive. Solar energy is diffuse, as we know from the somewhat painful, landscape-hogging process we use to collect it for our own electrical needs, so it turns out to be helpful to surround these reaction centers with light antennas that funnel energy in from a larger area. This only works because of quantum mechanics- the ability of electronic excitations to travel through a properly structured and electronically tuned pathway by Förster resonance energy transfer.

Full light harvesting complex and photoreaction center, of Emiliania huxleyi. A is a view from the top or outside of the (thylakoid) membrane, while C shows edge-on views. There are 38 antenna subunits in all.

All eukaryotic photosynthetic organisms use light antennas, but the ones developed by coccolithophores turn out to be particularly large and ornate. They form beautiful pinwheels of linked proteins, up to six per spoke, adding up to 38 antenna proteins in all, that conduct light from the outside in. They are packed with chlorophyl molecules, which are the pigments that receive light energy, and also with caroteniods, which are additional pigments that receive light and transfer it to nearby chlorophylls. They measured the time it took to transfer energy from an incoming light pulse to the reaction center at about 100 picoseconds. And this happens with an efficiency of 95%- amazing! An image of the antenna complex emphasizing the chlorophylls shows that they are lines up quite precisely from one unit to the next, making for an easy, even beautiful, path from the outer reaches into the center.

Each of the light harvesting proteins is loaded with chlorophylls, (blue, green), and with carotenoids (yellow).

Whereas land plants have only about five antenna complexes associated with their reaction centers, this coccolithophore has 38, making it the largest light harvesting complex ever found. This represents either a vast increase in light gathering capacity, or else an admission of a special cost of some kind in making or operating reaction centers, compared with antenna complexes. The authors do not delve into this issue, but one can imagine several constraints, such as a design optimum for keeping a reaction center busy at a certain high rate, or for minimizing the oxidative off-reactions that can occur in these centers. At any rate, the design, refined over hundreds of millions of years, is impressive indeed.

Emphasizing the chlorophylls in the light harvesting units shows how they are lined up, forming quantum wires for the conduction of excitation.

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• And abroad.
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• Cuba is in the dark.
• Surrender, at the end of a disastrous war.
• Novel solar financing.