Comparative proteomics of the toxigenic diazotroph Raphidiopsis raciborskii (cyanobacteria) in response to iron.

Raphidiopsis raciborskii is an invasive bloom-forming cyanobacteria with the flexibility to utilize atmospheric and fixed nitrogen. Since nitrogen-fixation has a high requirement for iron as an ezyme cofactor, we hypothesize that iron availability would determine the success of the species under nitrogen-fixing conditions. This study compares the proteomic response of cylindrospermopsin-producing and non-toxic strains of R. racibroskii to reduced iron concentrations, under nitrogen-fixing conditions, to examine any strain-specific adaptations that might increase fitness under these conditions. We also compared their proteomic responses at exponential and stationary growth phases to capture the changes throughout the growth cycle. Overall, the toxic strain was more competitive under Fe-starved conditions during exponential phase, with upregulated growth and transport-related proteins. The non-toxic strain showed reduced protein expression across multiple primary metabolism pathways. We propose that the increased expression of porin proteins during the exponential growth phase enables toxic strains to persist under Fe-starved conditions with this ability providing a potential explanation for the increased fitness of cylindrospermoipsin-producing strains during unfavourable environmental conditions.