Diatom surprise could rewrite global carbon cycle

When it comes to ocean-dwelling diatoms, new research suggests that photosynthesis is not the only strategy for accumulating carbon. Instead, these single-celled plankton also build biomass by feeding directly on organic carbon across large swaths of the ocean.

These new findings could lead researchers to narrow their estimate of how much carbon dioxide diatoms extract from the air through photosynthesis, which in turn could change our understanding of the global carbon cycle, which is particularly relevant given climate change.

The research is being conducted by bioengineers, bioinformatics experts and other genomics researchers at the University of California, San Diego. The results are published in Scientific progress July 17, 2024. The article is titled “Mixotrophic growth of a ubiquitous marine diatom.”

The team showed that the diatom Cylindrotheca closterium, found in oceans around the world, regularly performs a simultaneous mix of photosynthesis and direct consumption of carbon from organic sources such as plankton.

In more than 70 percent of the water samples the researchers analyzed from oceans around the world, the team found evidence of simultaneous photosynthesis and direct consumption of organic carbon by Cylindrotheca closterium.

The team also showed that this diatom species can grow much faster by consuming organic carbon in addition to photosynthesis.

Moreover, the new research raises the tantalizing possibility that specific species of bacteria directly supply organic carbon to a significant percentage of these diatoms living throughout the global ocean.

This work builds on a genome-scale metabolic modeling approach that the team used to understand the metabolism of the diatom Cylindrotheca closterium. The researchers constrained their genome-scale 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.

The team’s new metabolic modeling data 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 grow their biomass.

The UC San Diego-led team is expanding the scope of the project to determine how widespread this non-photosynthetic activity is among other diatom species.

Do ocean bacteria feed diatoms?

When the team looked at physical and chemical parameters measured in their seawater samples, including temperature, pH, salinity, light, nitrogen and carbon availability, they found no correlation between these parameters and a trend in diatoms away from photosynthesis-only strategies.

However, the team found a clear signal when exploring specific bacterial populations coexisting with the diatom Cylindrotheca closterium in seawater samples. This finding suggests bacteria-diatom interactions that result in the simultaneous mixing of photosynthesis and direct consumption of organic carbon, a phenomenon known as “mixotrophy.”

The team believes that specific bacteria could directly feed diatoms, helping them become some of the most successful and important microbes on the planet, producing oxygen, sequestering carbon and serving as the foundation of the food webs that support nearly all life in the ocean.

“Diatoms are important contributors to marine food chains and are key drivers of the global carbon cycle. Previously, we estimated all carbon cycle models assuming that the only role diatoms play is carbon dioxide fixation,” said UC San Diego Professor Karsten Zengler, professor in the departments of pediatrics and bioengineering and a researcher in the Jacobs School of Engineering’s Center for Microbiome Innovation.

“Our results demonstrate that this is not the case, but that diatoms also consume organic carbon. In other words, we have shown that diatoms do not rely exclusively on carbon dioxide fixation for their growth and biomass production. We believe that these results will have major implications for our understanding of the global carbon cycle.”

“Although there have been some intriguing observations in the lab of diatoms deviating from photosynthesis, it has been impossible to test what kind of metabolism these diatoms are performing in the ocean – until now. That’s because there are so many genes involved in this process, and it’s very difficult to determine which process is active from gene expression data alone. Our approach circumvents this challenge.”

The research team hopes that this work will spark interest in taking a closer look at our understanding of the global carbon cycle, taking into account this new, broader understanding of how ocean diatoms obtain their carbon.

What the bacteria that feed on diatoms can learn from this relationship is another question that will require further research.