Have you ever wondered about the unsung heroes of our oceans? Enter the tiny yet powerful Prochlorococcus marinus, a vibrant green microbe that outshines all others in terms of abundance. Smaller than a human red blood cell, these single-celled “picoplankton” thrive in ocean surface waters, making Prochlorococcus the most prolific photosynthesizing organism on Earth. In fact, this remarkable microbe captures as much carbon dioxide as all the world’s crops combined, playing a crucial role in the global carbon cycle.
In a groundbreaking study featured in Science Advances, MIT researchers unveiled an astonishing new capability of this microscopic powerhouse: the cross-feeding of essential DNA building blocks. The team discovered that Prochlorococcus releases these compounds into the surrounding environment, where they’re absorbed by other ocean organisms, serving as vital nutrients, energy sources, and regulators of metabolism. Essentially, what Prochlorococcus discards becomes a resource for other microbes.
What’s fascinating is that this cross-feeding phenomenon follows a predictable rhythm. Typically, Prochlorococcus sheds its extra compounds at night, allowing opportunistic microbes, like the abundant SAR11, to consume these nighttime offerings. Researchers found that these “snacks” help SAR11 slow down its metabolic rate, effectively recharging for the day ahead.
This dynamic exchange suggests that Prochlorococcus enables microbial communities to flourish sustainably by sharing its excess, and it might even influence the daily rhythms of microbial life across the globe.
“The interactions between these two dominant microbial groups have captivated oceanographers for decades,” states co-author Sallie “Penny” Chisholm, who co-discovered Prochlorococcus in 1986. “This study reveals an elegant choreography that supports their growth and stability across vast ocean regions.”
Given the widespread presence of Prochlorococcus and SAR11, the researchers believe that their molecular exchange constitutes one of the ocean’s primary cross-feeding relationships, significantly regulating the carbon cycle.
“By delving into the richness of these cross-feeding processes, we can begin to uncover the critical factors shaping the carbon cycle,” adds Rogier Braakman, lead author and research scientist at MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS).
The Art of Sharing
While cross-feeding is a common phenomenon in various microbial ecosystems, it has typically been studied within tightly-knit communities. For example, microbes in the human gut can readily exchange resources due to their close proximity. In stark contrast, Prochlorococcus are nomadic organisms, often suspended amidst dynamic oceanic layers. Although scientists suspected that they engage in some form of cross-feeding, understanding how this works and identifying beneficiaries has been a challenge due to the minute concentrations of compounds involved.
In a recent 2023 collaboration, Braakman and his team at Woods Hole Oceanographic Institution (WHOI) pioneered techniques to measure small organic compounds in seawater. They cultivated various strains of Prochlorococcus under differing conditions to identify what these microbes exude. Among key released molecules were purines and pyridines, which serve as vital building blocks for DNA. This discovery raised an intriguing question: Why would Prochlorococcus, found in nitrogen-poor regions, rid itself of nitrogen-rich compounds?
A Global Connection
The research team meticulously examined how Prochlorococcus utilizes purines and pyridines before releasing them. By comparing the genomes of different microbes, they traced genes responsible for purine and pyridine metabolism. The data revealed that Prochlorococcus produces these compounds for DNA synthesis, recycling leftovers while discharging a portion into the ocean.
The team also studied gene expression patterns, observing spikes in recycling genes after their genome replication peaks at dusk. This led to further inquiries about which microorganisms might benefit from this nightly release.
To answer this, the scientists explored the genomes of over 300 heterotrophic microbes—which consume organic carbon rather than produce it through photosynthesis. The majority contained genes capable of assimilating purines or pyridines, indicating that microbes have developed diverse strategies for cross-feeding.
They zoomed in on SAR11, the most prevalent heterotrophic microbe in the ocean. By dissecting various strains, they found that different forms of SAR11 deploy purines in unique ways, ranging from simple uptake to catabolism for energy. How do location and environmental conditions influence these behaviors? Braakman and his team conducted a metagenome analysis of over 600 seawater samples worldwide, linking the distribution of SAR11 to local nutrient conditions. Results showed that SAR11 favors purines for nitrogen when it’s scarce in seawater, and shifts to using them for energy when nitrogen is abundant, illustrating how local ecosystems shape community interactions.
“Our findings suggest that oceanic microbes have forged unexpected relationships that amplify their growth potential,” notes co-author Elizabeth Kujawinski.
Moreover, lab experiments revealed that purines can induce a metabolic slowdown in SAR11. Yet, under stressful conditions, these same cells remained robust, as if the metabolic pause prepares them for future growth—helping them thrive amid adversity.
“As we consider the ocean’s daily cycle of purine release from Prochlorococcus, it appears to offer a signaling mechanism that tempers SAR11 metabolism, priming them for the day’s challenges,” explains Braakman. “This dynamic interplay showcases how Prochlorococcus serves as a conductor in the intricate symphony of oceanic metabolism, orchestrating a global synchronization among microbial life.”
This pivotal research received support from the Simons Foundation and the National Science Foundation.
Photo credit & article inspired by: Massachusetts Institute of Technology
