Calfee BC, Bowden EC, Zinser ER. Rival phytoplankton contribute to the cross protection of Prochlorococcus from oxidative stress. Appl Environ Microbiol 0:e01128-24. https://doi.org/10.1128/aem.01128-24
Getting to the top
“Everything is everywhere, but the environment selects.” That quote—originally by Lourens Baas Becking—was one of the first things on the slides in my biological oceanography class. It captures how microbial life in the oceans is both widespread and shaped by local environmental conditions. Crucially, it implies that no single species dominates in isolation—there’s value in diversity. But what tradeoffs do these species make to coexist?
Take Prochlorococcus, for example—the most abundant photosynthetic organism on Earth. Part of its success comes from an ultra-streamlined genome, which makes it highly efficient at growing—but less equipped to handle stress. One major weakness? It can’t degrade reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂), which naturally build up near the ocean surface.
Help from microbial friends
Instead of handling ROS itself, Prochlorococcus depends on other microbes—“helper” species like heterotrophs, other cyanobacteria, and some tiny (pico-) eukaryotes—to clean up the mess. Together, this microbial consortium acts as a crucial sink for ROSs in the marine environment. While earlier studies showed that heterotrophs can degrade H₂O₂, researchers in Tennessee wanted to know if other photosynthetic microbes like Synechococcus (the second-most abundant cyanobacterium) and picoeukaryotes also help.
Turns out—they do. In lab experiments, Prochlorococcus survived significantly better under oxidative stress when co-cultured with Synechococcus, picoeukaryotes, or a combination of both. Importantly, the helping species had to be alive for the benefit to kick in—it wasn’t just about physical presence. Further, the relative composition of each determined who handled most of the detox work. While heterotrophs dominated in some instances, Synechococcus and the picoeukaryotes took the reins in others. Another instance of the dynamic nature of the ocean environment.
This held true under both sudden (acute) H₂O₂ spikes—like after rainfall—and slower (chronic) increases that happen over a typical day from normal metabolic activity. In both cases, community mattered.
A royal hand
This dynamic echoes the Black Queen Hypothesis: the idea that organisms can lose genes for certain functions when others in the community take over. Rather than redundancy, it’s efficiency by delegation. In this case, Prochlorococcus may have dropped its ROS defense because its neighbors had it covered—and it ended up thriving because of it. A risky gamble to be sure, but it’s paid off as Prochlorococcus has become the dominant photosynthetic species.
In other words, survival isn’t just about the environment—it’s also about who’s around you.
Cover photo from Grok3 Image Generator.
I’m a former oceanographer with an MSc in Biological Oceanography from UConn where I studied mixotrophy in marine ciliates. After a year in Poland (studying freshwater critters) I moved to California. I currently work as a lab technician at Stanford. Outside of science, I enjoy a good book, a long run, and frozen fruit.