Planktonic microorganisms form the foundation of global ocean ecosystems vital to the survival of life on Earth. Representing 90% of biomass within our oceans, the diverse functional roles played by this collection of organisms in nutrient production and cycling are unknown. The streamlined genomes of several cyanobacteria display a prevalence of ABC transporters for nutrient acquisition, which may explain the success of these organisms in colonising diverse environments. Substrate-binding proteins (SBPs) interact with ABC transporters in prokaryotes to mediate nutrient uptake. The SBP family displays a highly conserved structural fold despite little (or no) sequence similarity across its many members, making sequence-based assessment of likely ligand-binding chemistries unreliable.
We are integrating structural genomics, evolutionary studies, and biophysical analyses with metagenomics and ecological data to better annotate the repertoire of nutrient uptake in marine cyanobacteria, using the ubiquitous Synechococcusas a model. We have identified 1064 SBPs originating from Synechococcus ecotypes across 353 ocean metagenome samples and 51 available sequenced genomes. From these, 110 SBPs from 8 cultivatable strains were selected for high-throughput production and characterisation, with ligand screening by fluorometric and calorimetric techniques revealing significantly different ligand partners than predicted by annotations.