Biochemical controls: photosynthesis from one photon (Introduction)

by David Turell , Wednesday, July 05, 2023, 16:10 (297 days ago) @ David Turell

Another view:

https://www.quantamagazine.org/microbes-gained-photosynthesis-superpowers-from-a-proton...

"Researchers knew that certain classes of phytoplankton — diatoms, dinoflagellates and coccolithophores — stand out for their exceptional photosynthetic abilities. Those cells are extremely proficient at absorbing carbon dioxide from their environment and directing it to their chloroplasts for photosynthesis, but the details of why they are so good at it haven’t been very clear. A feature unique to those three groups of phytoplankton, however, is that they have an extra membrane around their chloroplasts.

"Daniel Yee, the lead author on the new study, was studying diatoms for his doctorate at the Scripps Institution of Oceanography at the University of California, San Diego. Photosynthesis wasn’t his focus; he sought to understand how diatoms regulate their internal acidity to help with nutrient storage and to build their tough silica cell wall. But he kept noticing the unique additional membrane around their chloroplasts.

"He learned that the extra membrane was widely regarded by researchers as a remnant of an ancient, failed act of digestion. Scientists hypothesized that about 200 million years ago, a predatory protozoan tried to feast on a single-celled photosynthetic alga. It enveloped the resilient alga in a membrane structure called a food vacuole to digest it, but for unknown reasons, digestion did not occur. Instead, the alga survived and became a symbiotic partner to the protozoan, feeding it the fruits of its photosynthesis. This partnership deepened over the generations until the new two-in-one organism evolved into the diatoms we know today. But the extra layer of membrane that had been a food vacuole never disappeared.

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"Using a combination of molecular biology techniques, Yee and his team confirmed that the extra membrane around the phytoplankton chloroplast does contain an active, functional proton pump — one called VHA that often serves a digestive role in food vacuoles. They even fused the proton pump to a fluorescent protein so that they could watch it work in real time. Their observations supported the endosymbiotic theory of how the diatoms acquired the extra membrane around their chloroplasts.

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"Further work helped them understand that the pump enhanced photosynthesis by concentrating carbon near chloroplasts. The pump transferred protons from the cytoplasm to the compartment between the extra membrane and the chloroplast. The increased acidity in the compartment caused more carbon (in the form of bicarbonate ions) to diffuse into the compartment to neutralize it. Enzymes converted the bicarbonate back into carbon dioxide, which was then conveniently near the chloroplast’s carbon-fixing enzymes.

"Using statistics on the distribution of the diatoms and other phytoplankton with the extra membrane throughout the global ocean, the researchers extrapolated that this boost in efficiency from the VHA membrane protein accounts for almost 12% of Earth’s atmospheric oxygen. It also contributes between 7% and 25% of all the oceanic carbon fixed each year. That’s at least 3.5 billion tons of carbon — almost four times as much as the global aviation industry emits annually. At the high end of the researchers’ estimate, VHA could be responsible for tying up as much as 13.5 billion tons of carbon a year."

Comment: without this amazingly complex process we would not be here.