Although plants require carbon dioxide to make food, just as humans require “pre-made” organic carbon to eat, too much of a good thing can be, well, deadly. In high concentrations, CO2 is toxic for almost all life on earth, including plants. The reason has to do with the fact that our bodies are made mostly of water. When CO2 dissolves in water, it produces acid. Most of our cellular processes occur at a near-neutral pH. Our enzymes quickly lose functionality when cellular pH becomes too basic or too acidic. Overexposure to CO2 in humans leads to a painful and often deadly condition known as acidosis. Plants and most microorganisms, sharing much of the same basic biochemistry as us, are also sensitive to cellular acidification brought about by too much CO2.
For non-CO2 eaters, this situation does not really pose much of a problem. Concentrations of CO2 in our atmosphere are roughly 0.06%, low enough that we don’t need to worry about acidosis. However, modern atmospheric CO2 concentrations pose a different problem for plants: they are carbon-limited. Most plants would grow larger and faster in an atmosphere with more CO2. They use specialized enzymes to concentrate CO2 around their photosynthetic cells, so that a maximal amount of carbon fixation can be achieved. Clearly for plants, there is a tradeoff: greater CO2 concentrations would mean more fuel for photosynthesis, but too much causes acidosis.
Does this tradeoff have to exist? If plants could keep photosynthesizing under higher and higher CO2 concentrations that would certainly be a competitive advantage over less-CO2 tolerant species. For that matter, why are we even talking about plants? Isn’t this supposed to be a blog about microbes?
I think you can see where this is going.
As I’m discovering is often the case in nature, a select group of microorganisms are the exception to the rule. Microalgae, to be precise: tiny, green photosynthetic organisms, thought to be the evolutionary precursor to higher plants. It turns out several species of microalgae are “hyper tolerators”, thriving in CO2 concentrations of up to 20% (Eudorina), 40% (Chlorella), 60%, (Chlorococcum littorale). At the top of the food chain is Cyanidum caldarium, thought to tolerate up to 100% CO2! Perhaps not surprisingly, most of these algae are found in extreme environments, such as highly acidic hot springs, where acid tolerance is essential for survival. Scientists believe these hypertolerators may be relics from hundreds of millions of years ago, when higher atmospheric CO2 concentrations necessitated tolerance.
How are these microalgae able to avoid the harmful acidification effects that all other life experiences under high CO2? When cellular pH begins to drop in high-CO2 environments, a complex series of biochemical adjustments take place in these organisms. They being to actively pump out of their cells the excess protons that cause acidification while simultaneously shutting of their “CO2 concentrating enzymes”. They also start producing more lipids: carbon and energy-rich molecules that store excess CO2 after it gets fixed during photosynthesis.
High CO2 tolerance is more than just an interesting natural phenomenon. There may be money (and carbon credits) in these little green microbes. The energy-dense lipids produced by algae can be extracted and burnt as a biofuel. If algae could be grown on an industrial scale in a high-CO2 environment, theoretically, more lipids could be produced given the same amount of biomass. We could get more bang for our buck out of them. One might even envision a biofuel operation working in tandem with a traditional coal or gasoline power plant, siphoning off CO2 that would otherwise end up in the atmosphere. Green microbes may end up becoming an essential component of a greener future for humanity.