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A surprise discovery could change the global carbon cycle

  • July 19, 2024
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Ocean diatoms such as Cylindrotheca closterium build biomass both through photosynthesis and by consuming organic carbon, a discovery that could change our understanding of the global carbon cycle.


Ocean diatoms such as Cylindrotheca closterium build biomass both through photosynthesis and by consuming organic carbon, a discovery that could change our understanding of the global carbon cycle.


Recent research suggests that ocean-dwelling diatoms have multiple ways to store carbon. In addition to photosynthesis, these single-celled plankton increase their biomass by consuming organic carbon directly from the ocean. This discovery could prompt scientists to revise their estimates of the amount of carbon dioxide diatoms absorb from the atmosphere through photosynthesis. As a result, it could change our understanding of the global carbon cycle, which is particularly important in the context of climate change.

The study was led by bioengineers, bioinformaticians and other genomics researchers at the University of California, San Diego. The new results were published July 17, 2024, in the journal Science Advances.

The team showed that the diatom Cylindrotheca closterium, found in oceans around the world, regularly performs a simultaneous combination of both photosynthesis and direct carbon absorption from organic sources such as plankton. The researchers found evidence of simultaneous photosynthesis and direct organic carbon consumption by Cylindrotheca closterium in more than 70% of water samples analyzed from oceans around the world.

The team also showed that this type of diatom can grow much faster when it consumes organic carbon in addition to photosynthesis.

What’s more, the new study points to the tantalizing possibility that certain types of bacteria directly provide organic carbon to a large percentage of these diatoms that live in the world’s oceans.

The study is based on a genome-wide metabolic modeling approach that the team used to unravel the metabolism of the diatom Cylindrotheca closterium. The researchers constrained their genome-wide metabolic model with global gene expression data obtained during the TARA Ocean Expedition. The researchers believe this is the first time genome-scale models have been used on a global scale.

New data from metabolic modeling supports recent laboratory experiments suggesting that some diatoms may rely on strategies other than photosynthesis to absorb the carbon they need to survive, thrive and build biomass. The UC San Diego-led team is expanding the scope of the project to determine how common this non-photosynthetic activity is among other diatom species.

Do diatoms feed ocean bacteria?

When the research team analyzed physical and chemical parameters measured in ocean water samples, including temperature, pH, salinity, light, nitrogen and carbon availability, they found no correlation between these parameters and the diatoms’ tendency to shift away from strategies that focus solely on photosynthesis.

But by examining specific bacterial populations coexisting with the diatom Cylindrotheca closterium in ocean water samples, the team found a clear signal. This suggests an interaction between the bacteria and the diatoms that results in the simultaneous combination of photosynthesis and direct consumption of organic carbon—a phenomenon known as mixotrophy.

The team believes that some bacteria may feed diatoms directly, helping them become among the most successful and important microbes on the planet, producing oxygen, absorbing carbon and acting as the backbone of food webs that support nearly all life in the ocean.

“Diatoms are important participants in marine food chains and key drivers of the global carbon cycle. Previously, we evaluated all models of the carbon cycle based on the assumption that diatoms’ sole role was to fix carbon dioxide. Our results show that this is not the case, and that diatoms simultaneously absorb organic carbon. In other words, we have shown that diatoms do not rely solely on carbon dioxide fixation for their growth and biomass production. We believe these results will have important implications for our understanding of the global carbon cycle,” said Carsten Zengler, professor in the Departments of Pediatrics and Bioengineering at San Diego State University and researcher in the Microbiome Innovation Center in the Jacobs School of Engineering.

“While we have had interesting observations of diatoms deviating from photosynthesis in the lab, it has been impossible until now to test what kind of metabolism these diatoms are performing in the ocean. This is because so many genes are involved in this process, and it is very difficult to determine which process is active based on gene expression data alone. Our approach solves this problem.”

The research team hopes that this study will stimulate interest in taking a closer look at our understanding of the global carbon cycle, given this new and broader understanding of how ocean diatoms obtain their carbon. Another question that needs further investigation is what bacteria that feed on diatoms might gain from this connection.

Source: Port Altele

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