Hetharua, B., Xu, M., Sun, S. et al. Temperature-driven nitrogen mixotrophy shapes marine cyanobacteria Prochlorococcus and Synechococcus latitudinal distribution pattern. Commun Earth Environ 6, 149 (2025). DOI: https://doi.org/10.1038/s43247-025-02102-w
Blasts from the past
If you’re like me, a few years removed from marine science education, you might’ve forgotten some details about ocean circulation and biogeochemical cycles. But I bet you remember Synechococcus and Prochlorococcus.
Both are tiny (~1μm) cyanobacteria, with Prochlorococcus earning the title of the smallest photosynthesizer on Earth. Together, they contribute ~25% of the ocean’s total net primary productivity, reinforcing the phrase, “Every other breath is from phytoplankton.” Their small size enhances their surface area-to-volume ratio, helping them thrive in nutrient-poor environments.
Despite their similarities, they have distinct biogeographic patterns. Prochlorococcus prefers warm waters (40°N–40°S), much like my grandparents, while Synechococcus is more adaptable, tolerating both warm and cold waters, peaking around 40°N and 40°S.
Why not both?
Recently, both species have gained attention as mixotrophy—the ability to take up organic compounds in addition to autotropy—becomes more recognized as widespread in ocean ecosystems. This flexibility likely helps them survive in low-nutrient environments, making it crucial to study their role in climate change responses.
Data is everything
With big-data tools and global datasets, researchers can now analyze species distributions in relation to nutrients and temperature. The Kao lab in China did just that, examining how these cyanobacteria choose their habitats based on nitrogen availability and sea surface temperature. Their findings confirmed that both species can use organic and inorganic nitrogen, allowing them to exist across diverse ocean regions. However, their strategies differ. Prochlorococcus has a streamlined genome, making it highly efficient at taking up specific nitrogen species, while Synechococcus has a more complex genome, giving it the flexibility to utilize whichever nitrogen sources are available.
Despite its reduced genome, Prochlorococcus compensates through ecotypic variation. Low-light (LL) ecotypes, found at greater depths, have more nitrogen transporters to capture available nitrogen sources, which multiply at depth. High-light (HL) ecotypes, near the surface, have fewer transporters but use them with remarkable efficiency. This variation helps Prochlorococcus optimize survival across light and nutrient gradients.
Some like it hot…
Temperature plays a defining role in shaping these species’ nitrogen preferences, with 15°C emerging as a key threshold. In warm waters, Prochlorococcus primarily relies on ammonium, an inorganic nitrogen compound abundant in ocean nutrient cycles. In contrast, Synechococcus, in cooler waters, including those below 15 °C, utilizes organic nitrogen, particularly the amino acid glutamine. This organic nitrogen source is crucial for cellular regulation and serves as a precursor to many inorganic nitrogen compounds. The ability to switch between organic and inorganic nitrogen sources in fluctuating environments is one advantage of the more complex genome of Synechococcus.
The temperature-driven shift in nitrogen preferences highlights how these cyanobacteria respond to environmental gradients, a key factor in their global success. However, it also suggests that changing ocean temperatures could alter local nutrient cycles, impacting ecosystems in unpredictable ways. This could be particularly relevant in coastal environments, where agricultural runoff introduces specific nitrogen types, potentially fueling the dominance of certain algal species and contributing to harmful algal blooms.
As global sea temperatures rise, the insights from this study can help refine climate models and guide policy decisions. Since temperature dictates everything—from species distribution to nitrogen utilization—keeping an eye on the global thermometer will be essential for understanding and managing the future of these microscopic but mighty cyanobacteria.
Cover photo from ChatGPT 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.
