INTERACTIONS BETWEEN PARTICULATE IRON AND CYANOBACTERIA: MULTIPLE FACETS OF IRON BIOAVAILABILITY

Cyanobacteria are a widespread and diverse group of photosynthetic, aquatic microorganisms that contribute significantly to global primary productivity. These organisms require large amounts of the essential micronutrient iron. Iron (Fe) bioavailability, as determined by its sources, sinks, solubility and speciation, places severe environmental constraints on microorganisms in aquatic environments. Cyanobacteria must contend with long periods of iron scarcity, and drastic, short-lived increases in iron supply from sediment re-suspension and atmospheric dust inputs. Using amorphous synthetic Fe-oxides (ferrihydrite) , we examined the bioavailability of particulate iron to the unicellular planktonic cyanobacterium, Synechocystis sp PCC 6803. Short term iron uptake assays with ⁵⁵ferrihydrite suggest that dissolution is a critical prerequisite for iron transport. Measurements of ferrihydrite dissolution rates demonstrate that Synechocystis 6803 can enhance ferrihydrite dissolution, exerting biological influence on ferrihydrite bioavailability. Furthermore, ferrihydrite dissolution experiments with conditioned medium suggest that bio-dissolution is mediated by a cell surface process. This was supported by confocal microscopy of cell-particle interactions that revealed Synechocystis 6803 cells securely attached to ferrihydrite aggregates. In addition, in testing bio-dissolution in a range of experimental conditions, we explored some of the mechanistic underpinnings of bio-dissolution by Synechocystis 6803, confirming that it is a biologically active process. Finally, long term physiological experiments and short term dissolution assays were applied in order to integrate both biological and chemical aspects of ferrihydrite bioavailability. Collectively, our findings provide insight into the multiple facets of particulate iron bioavailability to cyanobacteria.