Molecular insights into nutrient uptake mechanisms in marine <i>Synechococcus</i> spp.

<p dir="ltr">Marine picocyanobacteria are essential drivers of global biogeochemical cycles, playing a crucial role in ecosystem functioning. Their significant phylogenetic and functional diversity allow them to thrive across various marine regions. However, the mechanisms underpinning their adaptation to diverse oceanic niches remain poorly understood, especially with changing nutrient landscapes driven by climate change and anthropogenic activities. A critical factor in their success are nutrient uptake systems, particularly ATP-binding cassette (ABC) transporters, which account for over half of their transport capabilities. This thesis explores the substrate specificity, and ecological roles of substrate-binding proteins (SBPs) associated with these transporters, highlighting how these systems aid niche adaptation in marine picocyanobacteria. SBPs are key to ABC transporter function, binding substrates with high specificity and affinity, facilitating their cellular uptake. This study integrates structural biology, genomics, biophysical studies, and environmental genomics to elucidate the functions and ecological roles of these proteins, providing insights into the adaptive strategies of marine picocyanobacteria.</p><p dir="ltr">Functional characterisation of three phosphate binding protein homologs (PstS), three urea binding protein homologs (UrtA) and two glycine-betaine binding protein homologs (ProX) from <i>Synechococcus </i>strains inhabiting distinct oceanic niches was conducted. The study of PstS homologs in WH8102 revealed that PstS1b, unique to <i>Synechococcus </i>clade III strains, is the highest affinity binding protein, providing a competitive advantage in ultraoligotrophic environments. Analysis of UrtA homologs in CC9311 and WH8102 revealed UrtA from CC9311 had higher binding affinity to urea compared to both UrtA proteins from WH8102. This is intriguing as CC9311 originates from a high-nutrient mesotrophic environment, contrasting with WH8102’s habitat in a nutrient-poor oligotrophic region. Investigation of ProX homologs from MITS9220 and WH8102 revealed a functional distinction between the two proteins, with MITS9220_ProX likely behaving as a generalist solute-binding protein, while WH8102_ProX acts as a specialist binding protein for higher salinity environments.</p><p dir="ltr">The findings of this thesis highlight the versatility and adaptability of SBP-dependent ABC transporters in marine picocyanobacteria. The study demonstrates that these transport systems modulate their substrate affinities and specificities in response to environmental conditions, facilitating niche adaptation. This research is part of a larger project aimed at characterising the full repertoire of SBPs in marine picocyanobacteria, enhancing our understanding of nutrient acquisition mechanisms in these important microorganisms. The integration of various scientific approaches in this study provides a comprehensive framework for investigating the physiological and ecological significance of ABC uptake systems, offering valuable insights into the adaptive strategies of marine microbes in the face of changing oceanic conditions.</p>