66 million years ago, an asteroid impact caused the mass extinction of three quarters of life on Earth. The rest of life had to fight for its survival and research on these tenacious individuals can give us valuable information on how organisms survive in hostile environments. Some species of algae survived this impact, a finding that could change our view of global ocean processes.
Coccolithophores (Coccolithophoridae) are photosynthetic and use the sun’s energy to produce their food, like the vast majority of algae, but with great influence on the oceans. One aftermath of the asteroid impact was that the planet would be covered for months, in darkness that would be the death knell for many photosynthetic organisms on the planet. If we add to this the radioactive fallout, one of the consequences was the annihilation of more than 90 percent of all coccolithophore species. But others endured this whole series of natural cataclysms.
The research has been published in the journal New Phytologist, in an article entitled Osmotrophy of dissolved organic carbon by coccolithophores in darkness .
In the investigation, the team conducted laboratory experiments that demonstrated the possibility that some coccolithophores could survive without light. From this circumstance it follows that these organisms must have another way of producing energy and obtaining the carbon they need to make their nutrients.
“We have been stuck in the paradigm that algae are only photosynthetic organisms, and their ability to feed has been ignored for a long time,” says Jelena Godrijan, first author of the paper and part of the Bigelow Laboratory for Ocean Sciences team. “Getting coccolithophores to grow and survive in the dark seems amazing to me, especially when you think about how they managed to survive when animals like dinosaurs didn’t.”
The study has revealed how some species of coccolithophores could use organic compounds that have not yet been recognized as a carbon source, instead of the carbon dioxide used by plants. They have the ability to process organic compounds dissolved in water and use them immediately in a process called osmotrophy. Osmotrophy is the ability of some organisms (also cells) to obtain nutrients through osmotic absorption of dissolved substances. These findings may explain how coccolithophores survive in dark conditions—in which the light threshold for photosynthesis has been exceeded—such as after an asteroid impact or in the deep sea. Somehow coccolithophores can behave like heterotrophs and until now we didn’t know that.
Coccolithophores are an integral part of the processes that control the global ocean and atmosphere, including the carbon cycle itself. They receive carbon dioxide from the atmosphere, and are transported to the bottom of the ocean when they die.
“This is very important for the production of carbon dioxide for the Earth,” says William Balch, another co-author of the study. “If we didn’t have this carbon bomb, the carbon dioxide in our atmosphere would be much higher than it is now, probably more than double.”
Related to the above, coccolithophores play an important role in reducing ocean acidity, which can have a very negative effect on organisms such as corals or molluscs. These single-celled algae are covered with protective plates of calcium carbonate, which are called coccoliths. To make them, they extract carbon from the water, and when they die, they sink to the bottom. In this process, the acidity is relieved by sending the carbon to the deeper areas. It ends up becoming the seafloor and, over time, sedimentary rocks, thus becoming part of the carbon cycle. Therefore, coccolithophores are said to influence the carbon cycle through two “pumps”: the biological pump (heavy coccoliths end up escaping) and the alkalinity pump (decrease in alkalinity through calcification).
“Cocolithophores are integrated into global cycles in a way that we had never imagined,” says Balch. “This research really changes my thinking about food webs in dark regions where photosynthesis is clearly not happening.”
Given their microscopic size and wide distribution on the ocean floors, coccoliths are very important as trace fossils to resolve stratigraphic questions.
They used strains of Cruciplacolithus neohelis and Chrysotila Carterae . In the experiments carried out, radiolabels were used to study absorption kinetics, as well as pulse chasing to examine carbon uptake. They managed to keep the metabolism of individuals active for up to 30 days, with at least one cycle of cell division. They studied the absorption of three organic compounds: acetate, mannitol and glycerol, compared to a control sample. They chose these three compounds among a wide variety of organic compounds as possible carbon sources for coccolithophores and that had already been studied by the authors. Acetate is an organic salt with two carbon atoms and is very common in marine ecosystems, as discussed in the article. “Mannitol, a sugar alcohol with six carbon molecules, has been found to be absorbed by some microalgae and has growth-stimulating effects,” the study continues to justify the choice. Regarding the latter, “several strains of algae can absorb and grow rapidly with glycerol.” It is a polyol compound with three carbon atoms.
But they want to go further. The researchers want to conduct experiments in the oceans to observe how coccolithophores absorb nutrients in their natural environment, especially in the dark. They hope that their work will help reveal more about these organisms, their importance and their complex role on our planet.
“Cocolithophores are tiny creatures, but they have such a huge impact on all life that most people aren’t aware of it,” Godrijan said. “It gives me hope for our own lives to see how such small things can have such an influence on the planet.”
