A diatom surprise could rewrite the global carbon cycle

When it comes to diatoms that live in the ocean, new research suggests that photosynthesis isn’t the only strategy for carbon accumulation. Instead, this single-celled plankton also creates biomass by feeding directly on organic carbon in broad swaths of the ocean.

These new findings could lead researchers to lower their estimate of how much carbon dioxide is pulled from the air by photosynthesis, which in turn could change our understanding of the global carbon cycle, which is especially important in light of a changing climate.

This research is being led by bioengineers, bioinformatics experts and other genomics researchers at the University of California, San Diego. The findings are published in Scientific advances on July 17, 2024. The paper is titled “Mixotropic growth of a ubiquitous marine diatom”.

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

In more than 70% 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 species of diatom can grow much faster when it consumes organic carbon in addition to photosynthesis.

In addition, the new research suggests the tantalizing possibility that specific species of bacteria supply organic carbon directly to a large percentage of these diatoms living throughout the global ocean.

This work 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 metabolic model at the genome level with global gene expression data obtained from 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 that suggest some diatoms may rely on strategies other than photosynthesis to obtain the carbon they need to survive, thrive and build biomass.

The UC San Diego-led team is in the process of scaling up the project to see how widespread this non-photosynthetic activity is among other diatom species.

Do marine bacteria feed on diatoms?

When the team looked at the physical and chemical parameters measured in their ocean water samples—including temperature, pH, salinity, light, nitrogen, and carbon availability—they found no correlation between those parameters and diatoms’ tendency to eschew a photosynthesis-only strategy.

However, the team found a clear signal when examining specific bacterial populations coexisting with the diatom Cylindrotheca closterium in ocean water samples. This finding suggests bacteria-diatom interactions that control the current mix of photosynthesis and direct consumption of organic carbon—a phenomenon known as “mixotropy.”

The team believes that specific bacteria can feed diatoms directly, helping these diatoms become one of the most successful and important microbes on the planet in terms of oxygen production, carbon sequestration, and as the basis of the food webs that support nearly all life in the ocean.

“Diatoms are major contributors to marine food chains and key drivers of the global carbon cycle. Previously, we estimated all carbon cycle models based on the assumption that the only role diatoms play is carbon dioxide fixation,” said Professor Karsten of UC San Diego. Zengler, a professor in the Department of Pediatrics and Bioengineering and a researcher in the Center for Microbiome Innovation at the Jacobs School of Engineering.

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

“While there have been curious observations in the lab about diatoms deviating from photosynthesis, it hasn’t been possible to test what kind of metabolism these diatoms do in the ocean – until now. That’s because there are lots and lots of genes involved in this process, and it’s very difficult determine which process is active from gene expression data alone. Our approach overcomes this challenge.”

The research team hopes that this work will stimulate interest in taking a much closer look at our understanding of the global carbon cycle, taking into account this new broader understanding of how oceanic diatoms acquire carbon.

What diatom-feeding bacteria can derive from the relationship is another question for further research.