Biological dinitrogen (N2)-fixation is an important source of N in the environment and is catalyzed exclusively by prokaryotes, some of which are symbionts with multicellular or unicellular eukaryotes. The marine cyanobacteria UCYN-A are unusual N2-fixing endosymbionts of unicellular haptophyte algae closely related to Braarudosphaera bigelowii. They depend on each other’s carbon (C) and N2 fixation, establishing symbiotic pairs between different UCYN-A lineages and host species that can vary in cell volume up to 1000-fold. However, whether this natural size variation might inform about the metabolic interdependency of such symbiotic interactions remains unknown. We found that the size ratio between UCYN-A and their algal hosts is strikingly conserved across different lineages/species and metabolic modeling shows that this size relationship maximizes coordinated growth rate based on tradeoffs between nutrient acquisition and exchange. The data also show that size relationships in the UCYN-A symbiosis are similar to those between organelle and cell-size in the eukaryotic plankton, suggesting similar metabolic and growth constraints. These findings suggest that the UCYN-A symbiont is functioning similarly to an organelle, with implications for the evolution of organelles as well as the potential for bioengineering of N2-fixing plant cells

ASLO Aquatic Sciences Meeting, Resilience and Recovery in Aquatic Systems, 4-9 June 2023, Palma de Mallorca, Spain

Peer reviewed