The world’s oceans are changing, becoming warmer, more acidic, and increasingly polluted by excess fertilisers and anthropogenic inputs. These shifts are increasing the availability of organic and inorganic nutrients in marine environments and are predicted to affect key organisms such as marine cyanobacteria Prochlorococcus and Synechococcus. Together, these microorganisms profoundly influence Earth’s climate and ecosystems, driving global carbon fixation by sequestering ~12 gigatonnes of CO₂ each year, and sustaining marine nitrogen and phosphorus cycles.
These organisms dominate nutrient-poor open ocean environments, surviving by potentially scavenging for nutrients, dedicating ~50% of their uptake capacity to highly conserved ATP-binding cassette (ABC) transporters that are predicted to move both inorganic and organic nutrients into the cell. As photosynthetic organisms thought to require only CO2 and sunlight for growth, the presence of organic transporters is surprising. Their occurrence challenges the longstanding notion that these organisms are strictly photoautotrophic. While some functional annotations of these ABC transporters have been verified, experimental studies reveal several misannotations, highlighting the need for experimental validation of these nutrient uptake systems.
Core to these systems are high-specificity substrate binding proteins (SBPs), capturing and delivering nutrients to ABC transporters, serving as proxies for understanding transport function. Here, we provide the first functional characterisation of a predicted amino acid SBP from marine picocyanobacteria. Using a combination of bioinformatics, biochemistry and cell physiology, we show that the respective SBP is specific only for the charged amino acids, glutamate and aspartate, with strong binding affinity (nanomolar). Our analysis reveals strong evolutionary conservation and a widespread distribution across metagenomic and metatranscriptomic datasets, suggesting a selective advantage across diverse ecological conditions. These findings highlight additional metabolic flexibility in the world's most abundant photosynthetic organisms, providing new insight into nutrient acquisition strategies that underpin the global distribution and ecological success of marine picocyanobacteria.